Table of contents

We quantify the inside-out growth of the Milky Way's low-α stellar disk, modeling the ages, metallicities, and Galactocentric radii of APOGEE red clump stars with 6 kpc < R < 13 kpc. The current stellar distribution differs significantly from that expected from the star formation history due to the redistribution of stars through radial orbit mixing. We propose and fit a global model for the Milky Way disk, specified by an inside-out star formation history, radial orbit mixing, and an empirical, parametric model for its chemical evolution. We account for the spatially complex survey selection function, and find that the model fits all data well. We find distinct inside-out growth of the Milky Way disk; the best-fit model implies that the half-mass radius of the Milky Way disk has grown by 43% over the last 7 Gyr. Yet, such inside-out growth still results in a present-day age gradient weaker than 0.1 Gyr kpc⁻¹. Our model predicts the half-mass and half-light sizes of the Galactic disk at earlier epochs, which can be compared to the observed redshift–size relations of disk galaxies. We show that radial orbit migration can reconcile the distinct disk-size evolution with redshift, also expected from cosmological simulations, with the modest present-day age gradients seen in the Milky Way and other galaxies.

We report an astrochemical study on the evolution of interstellar molecular clouds and consequent star formation in the center of the barred spiral galaxy M83. We used the Atacama Large Millimeter/submillimeter Array (ALMA) to image molecular species indicative of shocks (SiO and CH₃OH), dense cores (N₂H⁺), and photodissociation regions (CN and CCH), as well as a radio recombination line (H41α) tracing active star-forming regions. M83 has a circumnuclear gas ring that is joined at two intersections by gas streams from the leading-edge gas lanes on the bar. We found elevated abundances of the shock and dense-core tracers in one of the orbit-intersecting areas, and found peaks of CN and H41α downstream. In the other orbit-intersection area, we found a similar enhancement of the shock tracers, but less variation of other tracers, and no sign of active star formation in the stream. We propose that the observed chemical variation or lack of it is due to the presence or absence of collision-induced evolution of molecular clouds and induced star formation. This work presents the clearest case of the chemical evolution in the circumnuclear rings of barred galaxies thanks to the ALMA resolution and sensitivity.

The effects of inhomogeneous magnetic fields on the propagation of magnetohydrodynamical (MHD) laminar flame fronts are investigated. This investigation is motivated by the occurrence of magnetized thermonuclear combustion in astrophysical systems. Magnetized thermonuclear burning occurs on the surfaces of neutron stars during Type I X-ray bursts, within the interiors of white dwarfs during SNe Ia, and during classical novae. Thermonuclear flames that propagate in these systems are expected to travel through inhomogeneous magnetic fields. We present the results of a series of 1.5-dimensional numerical simulations of magnetized flame propagation. A simplified flame model is used with one-step Arrhenius kinetics, an ideal gas equation of state, and constant thermal conductivity coefficients. Although idealized, the model allows for the opportunity to study the physics of the problem without the complexities of the nuclear kinetics of thermonuclear burning. We simulate the propagation of laminar flames through inhomogeneous magnetic media. A changing magnetic medium significantly alters the structure of the flame through the generation of an electric current. The electric current rotates the direction of the magnetic field within the flame and produces strong shear flows. Furthermore, for flames that conduct heat anisotropically and that propagate at an angle 0 < ψ ≲ π/2 to the magnetic field, the flame speed increases due to the nonuniform magnetic field. Naturally occurring flames in astrophysical systems may experience similar changes to their structure and speed that would influence the observational properties of these systems.

By assuming that the solar wind flow is spherically symmetric and that the flow speed becomes constant beyond some critical distance r = R₀ (neglecting solar gravitation and acceleration by high coronal temperature), the large-scale solar wind magnetic field lines are distorted into a Parker spiral configuration, which is usually simplified to an Archimedes spiral. Using magnetic field observations near Mercury, Venus, and Earth during solar maximum of Solar Cycle 24, we statistically surveyed the Parker spiral angles and obtained the empirical equations of the Archimedes and Parker spirals by fitting the multiple-point results. We found that the solar wind magnetic field configurations are slightly different during different years. Archimedes and Parker spiral configurations are quite different from each other within 1 au. Our results provide empirical Archimedes and Parker spiral equations that depend on the solar wind velocity and the critical distance (R₀). It is inferred that R₀ is much larger than that previously assumed. In the near future, the statistical survey of the near-Sun solar wind velocity by Parker Solar Probe can help verify this result.

Reverberation mapping (RM) is one of the most efficient ways to investigate the broad-line region around the central supermassive black holes of active galactic nuclei (AGNs). A common way of performing the RM is to perform a long term spectroscopic monitoring of AGNs, but the spectroscopic monitoring campaign of a large number of AGNs requires an extensive amount of observing time of medium to large size telescopes. As an alternative way, we present the results of photometric RM with medium-band photometry. As the widths of medium-band filters match well with the widths of AGN broad emission lines, the medium-band observation with small telescopes can be a cost-effective way to perform RM. We monitored five nearby AGNs with available spectroscopic RM results showing days to weeks scale variability. Observations were performed for ∼3 months with an average of 3 days cadence using three medium-band filters on a 0.43 m telescope. The time lags between the continuum and the Hα emission line light curves are calculated using the JAVELIN software and the discrete correlation function. We find time lags of 1.5–15.9 days for these AGNs, which are consistent with the time lags derived from previous spectroscopic RM measurements. This result demonstrates that even a 0.5 m class telescope can perform RM with medium-bands. Furthermore, we show that RM for tens of thousands AGNs is possible with a dedicated 1 m class telescope.

In our recent report, observational evidence supports that the rotational direction of a galaxy tends to be coherent with the average motion of its nearby neighbors within 1 Mpc. We extend the investigation to neighbors at farther distances in order to examine if such dynamical coherence is found even in large scales. The Calar Alto Legacy Integral Field Area (CALIFA) survey data and the NASA-Sloan Atlas (NSA) catalog are used. From the composite map of velocity distribution of "neighbor" galaxies within 15 Mpc from the CALIFA galaxies, the composite radial profiles of the luminosity-weighted mean velocity of neighbors are derived. These profiles show unexpectedly strong evidence of the dynamical coherence between the rotation of the CALIFA galaxies and the average line-of-sight motion of their neighbors within several-megaparsec distances. Such a signal is particularly strong when the neighbors are limited to red ones: the luminosity-weighted mean velocity at 1 < D ≤ 6 Mpc is as large as 30.6 ± 10.9 km s⁻¹ (2.8σ significance to random spin-axis uncertainty) for central rotation (R ≤ R_e). In the comparison of several subsamples, the dynamical coherence tends to be marginally stronger for the diffuse or kinematically well-aligned CALIFA galaxies. For this mysterious coherence in large scales, we cautiously suggest a scenario in which it results from a possible relationship between the long-term motion of a large-scale structure and the rotations of galaxies in it.

We study the behavior of internal signal chain reflections and antenna cross coupling as systematics for 21 cm cosmological surveys. We outline the mathematics for how these systematics appear in interferometric visibilities and describe their phenomenology. We then describe techniques for modeling and removing these systematics without attenuating the 21 cm signal in the data. This has critical implications for low-frequency radio surveys aiming to characterize the 21 cm signal from the Epoch of Reionization (EoR) and Cosmic Dawn, as systematics can cause bright foreground emission to contaminate the EoR window and prohibit a robust detection. We also quantify the signal loss properties of the systematic modeling algorithms, and show that our techniques demonstrate resistance against EoR signal loss. In a companion paper, we demonstrate these methods on data from the Hydrogen Epoch of Reionization Array as a proof-of-concept.

We present an analysis of the time-averaged spectrum of the Seyfert-2 active galaxy NGC 4388, obtained by NICER. The intrinsic strength of the reflection spectrum in NGC 4388, the large collecting area and favorable passband of NICER, and a net exposure of 105.6 ks yielded an exceptionally sensitive spectrum. Using two independent families of models, the intrinsic spectrum from the central engine is found to be highly obscured but not Compton-thick. Enforcing physical self-consistency within each model, the independent treatments give formally consistent results: ${N}_{{\rm{H}}}={2.67}_{-0.03}^{+0.02}\times {10}^{23}\,{\mathrm{cm}}^{-2}$ or ${N}_{{\rm{H}}}={2.64}_{-0.03}^{+0.03}\times {10}^{23}\,{\mathrm{cm}}^{-2}$. Past measurements made with Suzaku and XMM-Newton are in broad agreement with these column density values. A more recent measurement with NuSTAR (in late 2013) recorded a column density about twice as large; the robustness of this variability is reinforced by the use of consistent models and procedures. The neutral Fe Kα line in the NICER spectrum is nominally resolved and consistent with an origin in the optical broad-line region). The data also require ionized absorption in the Fe K band, similar to the "warm absorbers" detected in Seyfert-1 active galactic nuclei. The low-energy spectrum is consistent with a set of ionized plasma components. We discuss these findings and note that the geometric inferences that derive from this analysis can be tested with XRISM and Athena.

We present the Bayesian Asteroseismology data Modeling (BAM) pipeline, an automated asteroseismology pipeline that returns global oscillation parameters and granulation parameters from the analysis of photometric time series. BAM also determines whether a star is likely to be a solar-like oscillator. We have designed BAM to specially process K2 light curves, which suffer from unique noise signatures that can confuse asteroseismic analysis, though it may be used on any photometric time series—including those from Kepler and TESS. We demonstrate that the BAM oscillation parameters are consistent within ∼1.53% (random) ± 0.2% (systematic) and 1.51% (random) ± 0.6% (systematic) for ${\nu }_{\max }$ and ${\rm{\Delta }}\nu$ with benchmark results for typical K2 red giant stars in the K2 Galactic Archaeology Program's (GAP) Campaign 1 sample. Application of BAM to 13,016 K2 Campaign 1 targets not in the GAP sample yields 104 red giant solar-like oscillators. Based on the number of serendipitous giants we find, we estimate an upper limit on the average purity in dwarf selection among C1 proposals of ≈99%, which could be lower when considering incompleteness in BAM detection efficiency and proper-motion cuts specific to C1 Guest Observer proposals.

Previous observations revealed the existence of CO gas at nearly protoplanetary level in several dust-rich debris disks around young A-type stars. Here we used the Atacama Large Millimeter/submillimeter Array (ALMA) 7 m Array to measure ¹³CO and C¹⁸O emission toward two debris disks, 49 Cet and HD 32297, and detected similarly high CO content (>0.01 M_⊕). These high CO masses imply a highly efficient shielding of CO molecules against stellar and interstellar ultraviolet photons. Adapting a recent secondary gas disk model that considers both shielding by carbon atoms and self-shielding of CO, we can explain the observed CO level in both systems. Based on the derived gas densities we suggest that, in the HD 32297 disk, dust and gas are coupled and the dynamics of small grains is affected by the gaseous component. For 49 Cet, the question of coupling remains undecided. We found that the main stellar and disk properties of 49 Cet and HD 32297 are very similar to those of previously identified debris disks with high CO content. These objects constitute together the first known representatives of shielded debris disks.

The characteristics of the spectral evolution of the prompt emission of gamma-ray bursts (GRBs), which are closely related to the radiation mechanism (synchrotron or photosphere), are still an unsolved subject. Here, by performing the detailed time-resolved spectral fitting of GRB 131231A, which has a very bright and well-defined single pulse, some interesting spectral evolution features have been found. (i) Both the low-energy spectral index α and the peak energy E_p exhibit the "flux-tracking" pattern ("double-tracking" characteristics). (ii) The parameter relations, i.e., F (the energy flux)-α, F–E_p, and E_p–α, along with the analogous Yonetoku E_p–L_γ,iso relation for the different time-resolved spectra, show strong monotonous (positive) correlations, both in the rising and the decaying phases. (iii) The values of α do not exceed the synchrotron limit (α = −2/3) in all slices across the pulse, favoring the synchrotron origin. We argue that the one-zone synchrotron emission model with the emitter streaming away at a large distance from the central engine can explain all of these special spectral evolution characteristics.

Type I X-ray bursts are thermonuclear burning events that occur on the surfaces of accreting neutron stars. Burning begins in a localized spot in the star's ocean layer before propagating across the entire surface as a deflagration. On the scale of the entire star, the burning front can be thought of as discontinuity. To model this, we investigated the reactive Riemann problem for relativistic deflagrations and detonations and developed a numerical solver. Unlike for the Newtonian Riemann problem, where only the velocity perpendicular to the interface is relevant, in the relativistic case the tangential velocity becomes coupled through the Lorentz factor and can alter the waves present in the solution. We investigated whether a fast tangential velocity may be able to cause a deflagration wave to transition to a detonation. We found that such a transition is possible, but only for tangential velocities that are a significant fraction of the speed of light or for systems already on the verge of transitioning. Consequently, it is highly unlikely that this transition would occur for a burning front in a neutron star ocean without significant contributions from additional multidimensional effects.

Warm absorbers and related phenomena are some of the observable manifestations of outflows or winds from active galactic nuclei (AGNs). Warm absorbers are common in low-luminosity AGNs. They have been extensively studied observationally and are well described by simple phenomenological models. However, major open questions remain. What is the driving mechanism? What is the density and geometrical distribution? How much associated fully ionized gas is there? What is the relation to the quasi-relativistic "ultrafast outflows"? In this paper we present synthetic spectra for the observable properties of warm absorber flows and associated quantities. We use ab initio dynamical models, i.e., solutions of the equations of motion for gas in finite difference form. The models employ various plausible assumptions for the origin of the warm absorber gas and the physical mechanisms affecting its motion. The synthetic spectra are presented as an observational test of these models. In this way we explore various scenarios for warm absorber dynamics. We show that observed spectra place certain requirements on the geometrical distribution of the warm absorber gas, and that not all dynamical scenarios are equally successful at producing spectra similar to what is observed.

We present results from a joint ALMA/HST study of the nearby spiral galaxy NGC 628. We combine the Hubble Space Telescope (HST) Legacy ExtraGalactic UV Survey (LEGUS) database of over 1000 stellar clusters in NGC 628 with ALMA Cycle 4 mm/submillimeter observations of the cold dust continuum that span ∼15 kpc² including the nuclear region and western portions of the galaxy's disk. The resolution—11 or approximately 50 pc at the distance of NGC 628—allows us to constrain the spatial variations in the slope of the millimeter dust continuum as a function of the ages and masses of the nearby stellar clusters. Our results indicate an excess of dust emission in the millimeter, assuming a typical cold dust model for a normal star-forming galaxy, but little correlation of the dust continuum slope with stellar cluster age or mass. For the depth and spatial coverage of these observations, we cannot substantiate the millimeter/submillimeter excess arising from the processing of dust grains by the local interstellar radiation field. We detect a bright unknown source in NGC 628 in ALMA bands 4 and 7 with no counterparts at other wavelengths from ancillary data. We speculate this is possibly a dust-obscured supernova.

Recent hydrodynamical models of supernova remnants (SNRs) demonstrate that their evolution depends heavily on the inhomogeneities of the surrounding medium. As SNRs expand, their morphologies are influenced by the nonuniform and turbulent structure of their environments, as reflected in their radio continuum emission. In this paper, we measure the asymmetries of 96 SNRs in radio continuum images from three surveys of the Galactic plane and compare these results to the SNRs' radii, which we use as a proxy for their age. We find that larger (older) SNRs are more elliptical/elongated and more mirror asymmetric than smaller (younger) SNRs, though the latter vary in their degrees of asymmetry. This result suggests that SNR shells become more asymmetric as they sweep up the interstellar medium (ISM), as predicted in hydrodynamical models of SNRs expanding in a multiphase or turbulent ISM.

Precision measurements of cosmic microwave background (CMB) polarization require extreme control of instrumental systematics. In a companion paper we have presented cosmological constraints from observations with the BICEP2 and Keck Array experiments up to and including the 2015 observing season (BK15), resulting in the deepest CMB polarization maps to date and a statistical sensitivity to the tensor-to-scalar ratio of σ(r) = 0.020. In this work we characterize the beams and constrain potential systematic contamination from main beam shape mismatch at the three BK15 frequencies (95, 150, and 220 GHz). Far-field maps of 7360 distinct beam patterns taken from 2010–2015 are used to measure differential beam parameters and predict the contribution of temperature-to-polarization leakage to the BK15 B-mode maps. In the multifrequency, multicomponent likelihood analysis that uses BK15, Planck, and Wilkinson Microwave Anisotropy Probe maps to separate sky components, we find that adding this predicted leakage to simulations induces a bias of Δr = 0.0027 ± 0.0019. Future results using higher-quality beam maps and improved techniques to detect such leakage in CMB data will substantially reduce this uncertainty, enabling the levels of systematics control needed for BICEP Array and other experiments that plan to definitively probe large-field inflation.

We use imaging and spectroscopy from the Hubble Space Telescope (HST) to examine the properties of a bright planetary nebula (PN) projected within M31's young open cluster B477-D075. We show that the probability of a chance superposition of the PN on the cluster is small, ≲2%. Moreover, the radial velocity of the PN is the same as that of the cluster within the measurement error of ∼10 km s⁻¹. Given the expected ∼70 km s⁻¹ velocity dispersion in this region, ∼8 kpc from M31's nucleus, the velocity data again make it extremely likely that the PN belongs to the cluster. Applying isochrone fitting to archival color–magnitude photometric data from the HST Advanced Camera for Surveys, we determine the cluster age and metallicity to be 290 Myr and Z = 0.0071, respectively, implying an initial mass of ${3.38}_{-0.02}^{+0.03}\,{M}_{\odot }$ for any PN produced by the cluster. From HST's Space Telescope Imaging Spectrograph observations and Cloudy photoionization modeling, we find that the PN is likely a Type I planetary, with a nitrogen abundance that is enhanced by ∼5–6 times over the solar value scaled to the cluster metallicity. If the PN is indeed a cluster member, these data present strong empirical evidence that hot-bottom burning occurs in asymptotic giant branch stars with initial masses as low as 3.4 M_⊙.

The broadband spectrum from the 2013 December 20 $\gamma$-ray flare from 3C 279 is analyzed with our previously developed one-zone blazar jet model. We are able to reproduce two spectral energy distributions (SEDs), a quiescent and flaring state, the latter of which had an unusual SED, with hard $\gamma$-ray spectrum, high Compton dominance, and short duration. Our model suggests that there is insufficient energy for a comparable X-ray flare to have occurred simultaneously, which is an important constraint given the lack of X-ray data. We show that first- and second-order Fermi acceleration are sufficient to explain the flare, and that magnetic reconnection is not needed. The model includes particle acceleration, escape, and adiabatic and radiative energy losses, including the full Compton cross section, and emission from the synchrotron, synchrotron self-Compton, and external Compton processes. We provide a simple analytic approximation to the electron distribution solution to the transport equation that may be useful for simplified modeling in the future.

Recently, very high-energy photons above 100 GeV were reported to be detected from GRB 190114C and GRB 180720B at, respectively, 100–1000 s and 10 hr after the burst. We model the available broadband data of both GRBs with the synchrotron plus synchrotron self-Compton (SSC) emission of the afterglow shocks. We find that the sub-TeV emission of GRB 180720B can be interpreted as the SSC emission from afterglow shocks expanding in a constant-density circumburst medium. The SSC emission of GRB 190114C dominates over the synchrotron component from GeV energies at ∼100 s, which can explain the possible hard spectrum of the GeV emission at this time. The extrapolated flux of this SSC component to sub-TeV energies can explain the high-significance detection of GRB 190114C by the MAGIC telescope. The parameter values (such as the circumburst density and shock microphysical parameters) in the modeling are not unusual for both gamma-ray bursts, implying that the detection of sub-TeV photons from these two bursts should be attributed to their large burst energies and low redshifts.

While observations have suggested that power-law electron energy spectra are a common outcome of strong energy release during magnetic reconnection, e.g., in solar flares, kinetic simulations have not been able to provide definite evidence of power-laws in energy spectra of nonrelativistic reconnection. By means of 3D large-scale fully kinetic simulations, we study the formation of power-law electron energy spectra in nonrelativistic low-β reconnection. We find that both the global spectrum integrated over the entire domain and local spectra within individual regions of the reconnection layer have power-law tails with a spectral index p ∼ 4 in the 3D simulation, which persist throughout the nonlinear reconnection phase until saturation. In contrast, the spectrum in the 2D simulation rapidly evolves and quickly becomes soft. We show that 3D effects such as self-generated turbulence and chaotic magnetic field lines enable the transport of high-energy electrons across the reconnection layer and allow them to access several main acceleration regions. This leads to a sustained and nearly constant acceleration rate for electrons at different energies. We construct a model that explains the observed power-law spectral index in terms of the dynamical balance between particle acceleration and escape from main acceleration regions, which are defined based upon a threshold for the curvature drift acceleration term. This result could be important for explaining the formation of power-law energy spectrum in solar flares.

We investigate the performance of different methodologies that measure the time lag between broad-line and continuum variations in reverberation mapping data using simulated light curves that probe a range of cadence, time baseline, and signal-to-noise ratio in the flux measurements. We compare three widely adopted lag-measuring methods: the interpolated cross-correlation function (ICCF), the z-transformed discrete correlation function (ZDCF), and the Markov chain Monte Carlo code JAVELIN, for mock data with qualities typical of multiobject spectroscopic reverberation mapping (MOS-RM) surveys that simultaneously monitor hundreds of quasars. We quantify the overall lag-detection efficiency, the rate of false detections, and the quality of lag measurements for each of these methods and under different survey designs (e.g., observing cadence and depth) using mock quasar light curves. Overall JAVELIN and ICCF outperform ZDCF in essentially all tests performed. Compared with ICCF, JAVELIN produces higher quality lag measurements, is capable of measuring more lags with timescales shorter than the observing cadence, is less susceptible to seasonal gaps and signal-to-noise ratio degradation in the light curves, and produces more accurate lag uncertainties. We measure the Hβ broad-line region size–luminosity (R–L) relation with each method using the simulated light curves to assess the impact of selection effects of the design of MOS-RM surveys. The slope of the R–L relation measured by JAVELIN is the least biased among the three methods and is consistent across different survey designs. These results demonstrate a clear preference for JAVELIN over the other two nonparametric methods for MOS-RM programs, particularly in the regime of limited light-curve quality as expected from most MOS-RM programs.

We present here a combined analysis of four high spectral resolution observations of the Diffuse X-ray Background, made using the University of Wisconsin-Madison/Goddard Space Flight Center X-ray Quantum Calorimeter sounding rocket payload. The observed spectra support the existence of a ∼0.1 keV Local Hot Bubble and a ∼0.2 keV Hot Halo, with discrepancies between repeated observations compatible with expected contributions of time-variable emission from Solar Wind Charge Exchange. An additional component of ∼0.9 keV emission observed only at low galactic latitudes can be consistently explained by unresolved dwarf M stars.

We present comprehensive multiwavelength radio to X-ray observations of GRB 181201A spanning from ≈150 s to ≈163 days after the burst, comprising the first joint ALMA–VLA–GMRT observations of a gamma-ray burst (GRB) afterglow. The radio and millimeter-band data reveal a distinct signature at ≈3.9 days, which we interpret as reverse-shock (RS) emission. Our observations present the first time that a single radio-frequency spectral energy distribution can be decomposed directly into RS and forward shock (FS) components. We perform detailed modeling of the full multiwavelength data set, using Markov Chain Monte Carlo sampling to construct the joint posterior density function of the underlying physical parameters describing the RS and FS synchrotron emission. We uncover and account for all discovered degeneracies in the model parameters. The joint RS–FS modeling reveals a weakly magnetized (σ ≈ 3 × 10⁻³), mildly relativistic RS, from which we derive an initial bulk Lorentz factor of Γ₀ ≈ 103 for the GRB jet. Our results support the hypothesis that low-density environments are conducive to the observability of RS emission. We compare our observations to other events with strong RS detections and find a likely observational bias selecting for longer lasting, nonrelativistic RSs. We present and begin to address new challenges in modeling posed by the present generation of comprehensive, multifrequency data sets.

The observed properties (i.e., source size, source position, time duration, and decay time) of solar radio emission produced through plasma processes near the local plasma frequency, and hence the interpretation of solar radio bursts, are strongly influenced by propagation effects in the inhomogeneous turbulent solar corona. In this work, a 3D stochastic description of the propagation process is presented, based on the Fokker–Planck and Langevin equations of radio-wave transport in a medium containing anisotropic electron density fluctuations. Using a numerical treatment based on this model, we investigate the characteristic source sizes and burst decay times for Type III solar radio bursts. Comparison of the simulations with the observations of solar radio bursts shows that predominantly perpendicular density fluctuations in the solar corona are required, with an anisotropy factor of ∼0.3 for sources observed at around 30 MHz. The simulations also demonstrate that the photons are isotropized near the region of primary emission, but the waves are then focused by large-scale refraction, leading to plasma radio emission directivity that is characterized by a half width at half maximum of about 40° near 30 MHz. The results are applicable to various solar radio bursts produced via plasma emission.

X-ray and gamma-ray polarization measurements of the prompt emission of gamma-ray bursts (GRBs) are believed to be extremely important for testing various models of GRBs. So far, the available measurements of hard X-ray polarization of GRB prompt emission have not significantly constrained the GRB models, particularly because of the difficulty of measuring polarization in these bands. The CZT Imager (CZTI) on board AstroSat is primarily an X-ray spectroscopic instrument that also works as a wide-angle GRB monitor due to the transparency of its support structure above 100 keV. It also has experimentally verified polarization measurement capability in the energy range 100–300 keV and thus provides a unique opportunity to attempt spectropolarimetric studies of GRBs. Here we present the polarization data for the brightest 11 GRBs detected by CZTI during its first year of operation. Among these, five GRBs show polarization signatures with ⪆3σ, and one GRB shows 2σ detection significance. We place upper limits for the remaining five GRBs. We provide details of the various tests performed to validate our polarization measurements. While it is difficult yet to discriminate between various emission models with the current sample alone, the large number of polarization measurements that CZTI expects to gather in its minimum lifetime of five years should help to significantly improve our understanding of the prompt emission.

The slow diffusion of cosmic rays could be common around pulsars as indicated by the recent observations of HAWC, which can significantly change the pulsar interpretation of the well-known positron excess. Meanwhile, the latest measurement by AMS-02 shows a clear high-energy cutoff in the positron spectrum. Here, we check all the identified pulsars under the two-zone diffusion model to explain the new AMS-02 data. We find that the candidates must be nearby and middle-aged. Geminga, which was generally believed to be a very likely candidate, has recently been disfavored by Fermi-LAT observations of the GeV γ-ray flux. Following recent studies which indicate that PSR B1055−52 is much closer to the Earth than previously assumed, we propose for the first time that PSR B1055−52 is the most promising source of the positron excess. PSR B1055−52 can well reproduce both the intensity and the high-energy cutoff of the AMS-02 positron spectrum.

We present X-ray flux and spectral analyses of the three pointed Suzaku observations of the TeV high synchrotron peak blazar Mrk 421 taken throughout its complete operational duration. The observation taken on 2008 May 5 is, at 364.6 ks (i.e., 101.3 hr), the longest and most evenly sampled continuous observation of this source, or any blazar, in the X-ray energy 0.8–60 keV until now. We found large amplitude intraday variability in all soft and hard bands in all the light curves. The discrete correction function analysis of the light curves in soft and hard bands peaks on zero lag, showing that the emission in hard and soft bands are cospatial and emitted from the same population of leptons. The hardness ratio plots imply that the source is more variable in the harder bands compared to the softer bands. The source is harder when brighter, following the general behavior of high synchrotron peak blazars. Power spectral densities of all three light curves are red noise dominated, with a range of power spectra slopes. If one assumes that the emission originates very close to the central super massive black hole, a crude estimate for its mass, of ∼4 × 10⁸M_⊙, can be made; but if the variability is due to perturbations arising there that are advected into the jet and are thus Doppler boosted, substantially higher masses are consistent with the quickest seen variations. We briefly discuss the possible physical mechanisms most likely responsible for the observed flux and spectral variability.

The Kepler mission has provided a treasure trove of eclipsing binaries (EBs), observed at extremely high photometric precision, nearly continuously for several years. We are carrying out a survey of ∼100 of these EBs to derive dynamical masses and radii with precisions of 3% or better. We use multiplexed near-infrared H-band spectroscopy from the Sloan Digital Sky Survey-III and -IV APOGEE instrument and optical spectroscopy from the Hobby–Eberly Telescope High-resolution Spectrograph to derive double-lined spectroscopic orbits and dynamical mass ratios (q) for the EB sample, two of which we showcase in this paper. This orbital information is combined with Kepler photometry to derive orbital inclination, dynamical masses of the system components, radii, and temperatures. These measurements are directly applicable for benchmarking stellar models that are integrating the next generation of improvements, such as the magnetic suppression of convection efficiency, updated opacity tables, and fine-tuned equations of state. We selected our EB sample to include systems with low-mass ($M\lesssim 0.8\,\,{M}_{\odot }$) primary or secondary components, as well as many EBs expected to populate the relatively sparse parameter space below ∼0.5 M_⊙. In this paper, we describe our EB sample and the analytical techniques we are utilizing, and also present masses and radii for two systems that inhabit particularly underpopulated regions of mass–radius–period space: KIC 2445134 and KIC 3003991. Our joint spectroscopic and photometric analysis of KIC 2445134 ($q=0.411\pm 0.001$) yields masses and radii of ${\text{}}{M}_{A}=1.29\pm 0.03\,{M}_{\odot }$, ${\text{}}{M}_{B}=0.53\pm 0.01\,{M}_{\odot }$, ${\text{}}{R}_{A}=1.42\pm 0.01\,{R}_{\odot }$, ${\text{}}{R}_{B}=0.510\pm 0.004\,{R}_{\odot }$, and a temperature ratio of ${T}_{B}/{T}_{A}=0.635\pm 0.001;$ our analysis of KIC 3003991 ($q=0.298\pm 0.006$) yields ${\text{}}{M}_{A}=0.74\pm 0.04\,{M}_{\odot }$, ${\text{}}{M}_{B}=0.222\pm 0.007\,{M}_{\odot }$, ${\text{}}{R}_{A}=0.84\pm 0.01\,{R}_{\odot }$, ${\text{}}{R}_{B}=0.250\pm 0.004\,{R}_{\odot }$, and a temperature ratio of ${T}_{B}/{T}_{A}=0.662\pm 0.001$.

With the use of our JOANNA code, which solves radiative equations for ion + electron and neutral fluids, we perform realistic 2.5D numerical simulations of plasma outflows associated with the solar granulation. These outflows exhibit physical quantities that are consistent, to the order of magnitude, with the observational findings for mass and energy losses in the upper chromosphere, transition region, and inner corona, and they may originate the fast solar wind.

We use Hyper Suprime-Cam on the Subaru Telescope to investigate the structural and photometric properties of early-type dwarf galaxies and young stellar systems at the center of the M81 Group. We have mapped resolved stars to ∼2 mag below the tip of the red giant branch (RGB) over almost 6.5 square degrees, corresponding to a projected area of 160 × 160 kpc at the distance of M81. The resulting stellar catalog enables a homogeneous analysis of the member galaxies with unprecedented sensitivity to low surface brightness emission. The radial profiles of the dwarf galaxies are well described by Sérsic and King profiles, and show no obvious signatures of tidal disruption. The measured radii for most of these systems are larger than the existing literature values, and we find the total luminosity of IKN (M_V,0 = −14.29) to be almost 3 mag brighter than previously thought. We identify new dwarf satellite candidates, d1006+69 and d1009+68, which we estimate to lie at a distance of 4.3 ± 0.2 Mpc and 3.5 ± 0.5 Mpc. With M_V,0 = −8.91 ± 0.40 and [M/H] = −1.83 ± 0.28, d1006+69 is one of the faintest and most metal-poor dwarf satellites currently known in the M81 Group. The luminosity functions of young stellar systems in the outlying tidal neutral hydrogen (H i) debris imply continuous star formation in the recent past and the existence of populations as young as 30 Myr old. We find no evidence for old RGB stars coincident with the young MS/cHeB stars that define these objects, supporting the idea that they are genuinely new stellar systems resulting from triggered star formation in gaseous tidal debris.

Galaxies host a wide array of internal stellar components, which need to be decomposed accurately in order to understand their formation and evolution. While significant progress has been made with recent integral-field spectroscopic surveys of nearby galaxies, much can be learned from analyzing the large sets of realistic galaxies now available through state-of-the-art hydrodynamical cosmological simulations. We present an unsupervised machine-learning algorithm, named auto-GMM, based on Gaussian mixture models, to isolate intrinsic structures in simulated galaxies based on their kinematic phase space. For each galaxy, the number of Gaussian components allowed by the data is determined through a modified Bayesian information criterion. We test our method by applying it to prototype galaxies selected from the cosmological simulation IllustrisTNG. Our method can effectively decompose most galactic structures. The intrinsic structures of simulated galaxies can be inferred statistically by non-human supervised identification of galaxy structures. We successfully identify four kinds of intrinsic structures: cold disks, warm disks, bulges, and halos. Our method fails for barred galaxies because of the complex kinematics of particles moving on bar orbits.

We report a Fermi-LAT γ-ray analysis for the Chamaeleon molecular cloud complex using a total column density (${N}_{{\rm{H}}}$) model based on the dust optical depth at 353 GHz (${\tau }_{353}$) with the Planck thermal dust emission model. Gamma rays with energy from 250 MeV to 100 GeV are fitted with the ${N}_{{\rm{H}}}$ model as a function of ${\tau }_{353}$, ${N}_{{\rm{H}}}\propto {\tau }_{353}^{1/\alpha }$ (α ≥ 1.0), to explicitly take into account a possible nonlinear ${\tau }_{353}$/${N}_{{\rm{H}}}$ ratio. We found that a nonlinear relation, α ∼ 1.4, gives the best fit to the γ-ray data. This nonlinear relation may indicate dust evolution effects across the different gas phases. Using the best-fit ${N}_{{\rm{H}}}$ model, we derived the CO-to-${{\rm{H}}}_{2}$ conversion factor (${X}_{\mathrm{CO}}$) and gas mass, taking into account the uncertainties of the ${N}_{{\rm{H}}}$ model. The value of ${X}_{\mathrm{CO}}$ is found to be (0.63–0.76) ×10²⁰ cm⁻² K⁻¹ km⁻¹ s, which is consistent with that of a recent γ-ray study of the Chamaeleon region. The total gas mass is estimated to be (6.0–7.3) × 10⁴${M}_{\odot }$, of which the mass of additional gas not traced by standard ${\rm{H}}\,{\rm{I}}$ or CO line surveys is 20%–40%. The additional gas amounts to 30%–60% of the gas mass estimated in the case of optically thin ${\rm{H}}\,{\rm{I}}$ and has five to seven times greater mass than the molecular gas traced by CO. Possible origins of the additional gas are discussed based on scenarios of optically thick ${\rm{H}}\,{\rm{I}}$ and CO-dark ${{\rm{H}}}_{2}$. We also derived the γ-ray emissivity spectrum, which is consistent with the local ${\rm{H}}\,{\rm{I}}$ emissivity derived from Fermi-LAT data within the systematic uncertainty of ∼20%.

Short heat pulses can trigger plasma pressure fronts inside closed magnetic tubes in the corona. The alternation of condensations and rarefactions from the pressure modes drive large-amplitude pulsations in the plasma emission. Here we show the detection of such pulsations along magnetic tubes that brighten transiently in the hot 94 Å EUV channel of the Solar Dynamics Observatory/AIA. The pulsations are consistent with those predicted by hydrodynamic loop modeling, and confirm pulsed heating in the loop system. The comparison of observations and model provides constraints on the heat deposition: a good agreement requires loop twisting and pulses deposited close to the footpoints with a duration of 0.5 minutes in one loop, and deposited in the corona with a duration of 2.5 minutes in another loop of the same loop system.

We investigate the precise location of the radio core in the nearby blazar Mrk 501 for the first time during its X-ray and TeV γ-ray active state in 2012 by revisiting from the perspective of astrometry the six-epoch observations with the Very Long Baseline Array at 43 GHz reported by Koyama et al. We find that the position of the radio core seen at 43 GHz remained stable during our observations from 2012 to 2013 February within 42 μas in the southeast jet direction and 56 μas along the northeast jet direction. This implies that the location of the 43 GHz radio-emitting core was limited within the deprojected scale of 4.6 × 10³ Schwarzschild radii (R_s) during the high-energy active state. This result is a contrast to another case of the astrometric observation of the famous nearby TeV blazar Mrk 421, which showed a clear radio core position change soon after the large X-ray flare in 2011, reported by Niinuma et al. We compare the two cases and discuss possible origins of the different results of the radio core astrometry in the high-energy active states between the nearby blazars. Based on the internal shock model for blazars, the Lorentz factors of the ejecta explaining the stability of the radio core in Mrk 501 are expected to be a few times smaller than those for the wandering core in Mrk 421.

The redshift range z = 4–6 marks a transition phase between primordial and mature galaxy formation in which galaxies considerably increase their stellar mass, metallicity, and dust content. The study of galaxies in this redshift range is therefore important to understanding early galaxy formation and the fate of galaxies at later times. Here, we investigate the burstiness of the recent star formation history (SFH) of 221z ∼ 4.5 main-sequence galaxies at $\mathrm{log}(M/{M}_{\odot })\gt 9.7$ by comparing their ultra-violet (UV) continuum, Hα luminosity, and Hα equivalent-width (EW). The Hα properties are derived from the Spitzer [3.6 μm]−[4.5 μm] broadband color, thereby properly taking into account model and photometric uncertainties. We find a significant scatter between Hα- and UV-derived luminosities and star formation rates (SFRs). About half of the galaxies show a significant excess in Hα compared to expectations from a constant smooth SFH. We also find a tentative anticorrelation between Hα EW and stellar mass, ranging from 1000 Å at $\mathrm{log}(M/{M}_{\odot })\lt 10$ to below 100 Å at $\mathrm{log}(M/{M}_{\odot })\gt 11$. Consulting models suggests that most z ∼ 4.5 galaxies had a burst of star formation within the last 50 Myr, increasing their SFRs by a factor of >5. The most massive galaxies on the other hand might decrease their SFRs and may be transitioning to a quiescent stage by z = 4. We identify differential dust attenuation (f) between stars and nebular regions as the main contributor to the uncertainty. With local galaxies selected by increasing Hα EW (reaching values similar to high-z galaxies), we predict that f approaches unity at z > 4, consistent with the extrapolation of measurements out to z = 2.

The central star of NGC 2392 shows the hardest X-ray emission among central stars of planetary nebulae (CSPNe). The recent discovery of a spectroscopic companion with an orbital period of 1.9 days could provide an explanation for its hard X-ray emission, as well as for the collimation of its fast outflow. Here, we analyze the available Chandra and XMM-Newton X-ray observations to determine accurately the spectral and temporal variation properties of the CSPN of NGC 2392. The X-ray emission can be described by an absorbed thermal plasma model with temperature ${26}_{-5}^{+8}$ MK and X-ray luminosity (8.7 ± 1.0) × 10³⁰ erg s⁻¹. No long-term variability is detected in the X-ray emission level, but the Chandra light curve is suggestive of short-term variations with a period ∼0.26 days. The possible origins of this X-ray emission are discussed. X-ray emission from the coronal activity of a companion or shocks in the stellar wind can be ruled out. Accretion of material from an unseen main-sequence companion onto the CSPN or from the CSPN wind onto a white dwarf companion are the most plausible origins for its hard X-ray emission, although the mismatch between the rotational period of the CSPN and the modulation timescale of the X-ray emission seems to preclude the former possibility.

We investigate the three-dimensional asymmetrical kinematics and present time stamps of the Milky Way disk between Galactocentric distances of R = 12 and 15 kpc, using red clump stars selected from the LAMOST Galactic survey, also with proper motion measurements provided by the Gaia DR2. We discover velocity substructure above the Galactic plane corresponding to a density dip found recently ("South-middle opposite" density structure [R ∼ 12–15 kpc, Z ∼ 1.5 kpc] discovered in Wang et al.) in the radial and azimuthal velocity. For the vertical velocity, we detect clear vertical bulk motions or bending mode motions, which has no clear North–South asymmetry corresponding to the in-plane asymmetrical features. In the subsample of stars with different ages, we find that there is little temporal evolution of the in-plane asymmetry from 0 to 14 Gyr, which means the structure is possibly sensitive to the perturbations in almost cosmic time. We propose that the possible scenario of this asymmetric velocity structure is caused by the mechanisms generated in-plane, rather than vertical perturbations.

Polycyclic aromatic hydrocarbon (PAH) emission has long been proposed to be a potential star formation rate indicator, as it arises from the photodissociation region bordering the Strömgren sphere of young, massive stars. We apply a recently developed technique of mid-infrared spectral decomposition to obtain a uniform set of PAH measurements from Spitzer low-resolution spectra of a large sample of star-forming galaxies spanning a wide range in stellar mass (M_⋆ ≈ 10⁶–10^11.4M_⊙) and star formation rate (∼0.1–2000 M_⊙ yr⁻¹). High-resolution spectra are also analyzed to measure [Ne ii] 12.8 μm and [Ne iii] 15.6 μm, which effectively trace the Lyman continuum. We present a new relation between PAH luminosity and star formation rate based on the [Ne ii] and [Ne iii] lines. Calibrations are given for the integrated 5–15 μm PAH emission, the individual features at 6.2, 7.7, 8.6, and 11.3 μm, as well as several mid-infrared bandpasses sensitive to PAH. We confirm that PAH emission is suppressed in low-mass dwarf galaxies, and we discuss the possible physical origin of this effect.

Probing magnetic fields in giant molecular clouds is often challenging. Fortunately, recent simulations show that analysis of velocity gradients (the velocity gradient technique; VGT) can be used to map out the magnetic field morphology of different physical layers within molecular clouds when applied to CO isotopologues with different optical depths. Here, we test the effectiveness of the VGT in reconstructing the magnetic field structure of the molecular cloud Vela C, employing seven chemical tracers that have different optical depths, i.e., ¹²CO, ¹³CO, C¹⁸O, CS, HNC, HCO⁺, and HCN. Our results show good correspondence between the magnetic field morphology inferred from velocity gradients using these different molecular tracers and the magnetic field morphology inferred from BLASTPol polarization observations. We also explore the possibility of using a combination of velocity gradients for multiple chemical tracers to explain the structure of the magnetic field in molecular clouds. We search for signatures of gravitational collapse in the alignment of the velocity gradients and magnetic field and conclude that collapsing regions constitute a small fraction of the cloud.

High obliquity planets represent potentially extreme limits of terrestrial climate, as they exhibit large seasonality, a reversed annual-mean pole-to-equator gradient of stellar heating, and novel cryospheres. A suite of 3D global climate model simulations is performed for low and high obliquity planets with various stellar fluxes, CO₂ concentrations, and initial conditions to explore the propensity for high obliquity climates to undergo global glaciation. We also simulate planets with thick CO₂ or H₂ atmospheres, such as those expected to develop near or beyond the outer edge of the habitable zone. We show that high obliquity planets are hotter than their low obliquity counterparts due to ice-albedo feedbacks for cold climates, and water vapor in warm climates. We suggest that the water vapor greenhouse trapping is greater on high obliquity bodies for a given global-mean temperature due to the different dynamical regimes that occur between the two states. While equatorial ice belts are stable at high obliquity in some climate regimes, it is substantially harder to achieve global glaciation than for a low obliquity planet. Temperate polar conditions can be present at high obliquity at forcings for which low obliquity planets would be in a hard snowball state. Furthermore, open ocean can persist even in the winter hemisphere and when global-mean temperatures are well below freezing. However, the influence of obliquity diminishes for dense atmospheres, in agreement with calculations from 1D energy balance models.

We search for potential galactic and extragalactic dust contamination in thermal Sunyaev–Zeldovich maps derived from the Planck data. To test for contamination, we apply a variety of galactic dust and cosmic infrared background (CIB) models to the data as part of the y map reconstruction process. We evaluate the level of contamination by cross-correlating these y maps with mass tracers based on weak lensing data. The lensing data we use are the convergence map, κ, from the Red Sequence Cluster Lensing survey, and the cosmic microwave background (CMB) lensing potential map, ϕ, from the Planck Collaboration. We make a CIB-subtracted y map and measure the cross-correlation between it and the lensing data. By comparing it with CIB-contaminated cross-correlation, we find that the cross-correlation between κ and y is only slightly contaminated by CIB signal, at the level of 6.8 ± 3.5%, which implies that previous detections of κ × y are robust to CIB contamination. However, we find that ϕ × y is more significantly contaminated, by 16.7 ± 3.5%, because the CMB lensing potential probes higher redshift sources that overlap more with the CIB sources. We find that Galactic dust does not significantly contaminate either cross-correlation signal.

The source G25.65+1.05 (RAFGL7009S, IRAS 18316-0602) is the least studied of the three regions of massive star formation known to show exceptionally powerful H₂O maser bursts. We report spectral line observations of the H₂O maser at 22 GHz, the methanol maser transitions at 6.7, 12.2, and 44 GHz, and the continuum in these same frequency bands with The Karl G. Jansky Very Large Array at the post-burst epoch of 2017. For the first time, maps of 22 GHz H₂O and 44 GHz CH₃OH maser spots are obtained and the absolute position of the 22 GHz H₂O bursting feature is determined with milliarcsecond precision. We detected four continuum components, three of which are closely spaced in a linear orientation, suggesting a physical link between them.

Using four years of full-disk-integrated coronal differential emission measures calculated in Schonfeld et al. (2017), we investigate the relative contribution of bremsstrahlung and gyroresonance emission in observations of F_10.7, the 10.7 cm (2.8 GHz) solar microwave spectral flux density and commonly used activity proxy. We determine that the majority of coronal F_10.7 is produced by the bremsstrahlung mechanism, but the variability observed over individual solar rotations is often driven by gyroresonance sources rotating across the disk. Our analysis suggests that the chromosphere may contribute significantly to F_10.7 variability and that coronal bremsstrahlung emission accounts for 14.2 ± 2.1 sfu (∼20%) of the observed solar minimum level. The bremsstrahlung emission has a power-law relationship to the total F_10.7 at high activity levels, and this combined with the observed linearity during low activity yields a continuously differentiable piecewise fit for the bremsstrahlung component as a function of F_10.7. We find that the bremsstrahlung component fit, along with the Mg ii index, correlates better with the observed 5–37 nm spectrum than the common 81 day averaged F_10.7 proxy. The bremsstrahlung component of F_10.7 is also well approximated by the moderate-strength photospheric magnetic field parameterization from Henney et al. (2012), suggesting that it could be forecast for use in both atmospheric research and operational models.

Gravitational interactions between a protoplanetary disk and its embedded planet are one of the formation mechanisms of gaps and rings found in recent ALMA observations. To quantify the gap properties measured in not only surface density but also rotational velocity profiles, we run two-dimensional hydrodynamic simulations of protoplanetary disks by varying three parameters: the mass ratio q of a planet to a central star, the ratio of the disk scale height h_p to the orbital radius r_p of the planet, and the viscosity parameter α. We find that the gap depth δ_Σ in the gas surface density depends on a single dimensionless parameter $K\equiv {q}^{2}{({h}_{p}/{r}_{p})}^{-5}{\alpha }^{-1}$ as ${\delta }_{{\rm{\Sigma }}}={(1+0.046K)}^{-1}$, consistent with the previous results of Kanagawa et al. The gap depth δ_V in the rotational velocity is given by ${\delta }_{V}=0.007({h}_{p}/{r}_{p}){K}^{1.38}/(1+0.06{K}^{1.03})$. The gap width, in both surface density and rotational velocity, has a minimum of about 4.7h_p when the planet mass M_p is around the disk thermal mass M_th, while it increases in a power-law fashion as M_p/M_th increases or decreases from unity. This minimum in the gap width arises because spirals from sub-thermal planets have to propagate before they shock the disk gas and open a gap. We compare our relations for the gap depth and width with the previous results, and discuss their applicability to observations.

Impulsive solar energetic particle events are widely believed to be due to the prompt escape into the interplanetary medium of flare-accelerated particles produced by solar eruptive events. According to the standard model for such events, however, particles accelerated by the flare reconnection should remain trapped in the flux rope comprising the coronal mass ejection. The particles should reach the Earth only much later, along with the bulk ejecta. To resolve this paradox, we have extended our previous axisymmetric model for the escape of flare-accelerated particles to fully three-dimensional (3D) geometries. We report the results of magnetohydrodynamic simulations of a coronal system that consists of a bipolar active region embedded in a background global dipole field structured by solar wind. Our simulations show that multiple magnetic reconnection episodes occur prior to and during the coronal mass ejection (CME) eruption and its interplanetary propagation. In addition to the episodes that build up the flux rope, reconnection between the open field and the CME couples the closed corona to the open interplanetary field. Flare-accelerated particles initially trapped in the CME thereby gain access to the open interplanetary field along a trail blazed by magnetic reconnection. A key difference between these 3D results and our previous calculations is that the interchange reconnection allows accelerated particles to escape from deep within the CME flux rope. We estimate the spatial extent of the particle-escape channels. The relative timings between flare acceleration and release of the energetic particles through CME/open-field coupling are also determined. All our results compare favorably with observations.

Transitional millisecond pulsars provide a unique set of observational data for understanding accretion at low rates onto magnetized neutron stars. In particular, PSR J1023+0038 exhibits a remarkable bimodality of the X-ray luminosity (low and high modes), pulsations extending from the X-ray to the optical band, GeV emission, and occasional X-ray flares. We discuss a scenario for the pulsar interaction with the accretion disk capable of explaining the observed behavior. We suggest that during the high mode the disk is truncated outside the light cylinder, allowing the pulsar wind to develop near the equatorial plane and strike the disk. The dissipative wind–disk collision energizes the disk particles and generates synchrotron emission, which peaks in the X-ray band and extends down to the optical band. The emission is modulated by the pulsar wind rotation, resulting in a pulse profile with two peaks 180° apart. This picture explains the high mode luminosity, spectrum, and pulse profile (X-ray and optical) of PSR J1023+0038. It may also explain the X-ray flares as events of sudden increase in the effective disk cross section intercepting the wind. In contrast to previously proposed models, we suggest that the disk penetrates the light cylinder only during the low X-ray mode. This penetration suppresses the dissipation caused by the pulsar wind–disk collision, and the system enters the propeller regime. The small duty cycle of the propeller explains the low spin-down rate of the pulsar.

Our heliosphere's innermost boundary—the termination shock—slows and heats the supersonic solar wind and energizes anomalous cosmic rays (ACRs). We show that in addition to their termination shock crossings, the Voyager 1 and 2 spacecraft measurements identify additional points on the termination shock when they magnetically disconnect from the ACR source. These four points define a spherical approximation of the termination shock with radius 117 au, offset ∼32 au tailward, ∼27 au north, and ∼12 au to the port side of the Sun. Interstellar Boundary Explorer (IBEX) spacecraft observations independently confirm these general offsets, with the closest region of the termination shock ∼ 20° south of the interstellar inflow direction and a minimum distance ∼74 au. The maximum distance is ∼161 au, consistent with required ACR acceleration times. Thus, Voyager and IBEX spacecraft observations have directly revealed the global size and location of our heliosphere's termination shock for the first time.

We present ALMA observations of ¹²CO, ¹³CO, and C¹⁸O J = 2–1 lines and the 230 GHz continuum for the FU Ori–type object (FUor) V900 Mon (d ∼ 1.5 kpc), for which the accretion burst was triggered between 1953 and 2009. We identified CO emission associated with a molecular bipolar outflow extending up to an ∼10⁴ au scale and a rotating molecular envelope extending over >10⁴ au. The interaction with the hot energetic FUor wind, which was observed using optical spectroscopy, appears limited to a region within ∼400 au of the star. The envelope mass and collimation of the extended CO outflow suggest that the progenitor of this FUor is a low-mass Class I young stellar object (YSO). These parameters for V900 Mon, another FUor, and a few FUor-like stars are consistent with the idea that FUor outbursts are associated with normal YSOs. The continuum emission is marginally resolved in our observations with a 02 × 015 (∼300 × 225 au) beam, and a Gaussian model provides a deconvolved FWHM of ∼90 au. The emission is presumably associated with a dusty circumstellar disk, plus a possible contribution from a wind or wind cavity close to the star. The warm compact nature of the disk continuum emission could be explained with viscous heating of the disk, while gravitational fragmentation in the outer disk and/or a combination of grain growth and their inward drift may also contribute to its compact nature.

In a previous paper, we tried to test the Kerr nature of the stellar-mass black hole in GRS 1915+105 by analyzing NuSTAR data of 2012 with our reflection model relxill_nk. We found that the choice of the intensity profile of the reflection component is crucial and eventually we were not able to get any constraint on the spacetime metric around the black hole in GRS 1915+105. In the present paper, we study the same source with Suzaku data of 2007. We confirm that the intensity profile plays an important role, but now we find quite stringent constraints consistent with the Kerr hypothesis. The key differences with respect to our previous study are likely the lower disk temperature in the Suzaku observation and the higher energy resolution near the iron line of the Suzaku data. We also apply different relxill flavors (different descriptions of the coronal spectrum and variable disk electron density) obtaining essentially the same results. We thus conclude that this choice is not very important for our tests of the Kerr hypothesis while the intensity profile does play an important role, and that with high-quality data it is possible to measure both the spacetime metric and the intensity profile.

An unusual object, G2, had its pericenter passage around Sgr A*, the 4 × 10⁶M_⊙ supermassive black hole in the Galactic Center, in Summer 2014. Several research teams have reported evidence that, following G2's pericenter encounter, the rate of Sgr A*'s bright X-ray flares increased significantly. Our analysis carefully treats varying flux contamination from a nearby magnetic neutron star and is free from complications induced by using data from multiple X-ray observatories with different spatial resolutions. We test the scenario of an increased bright X-ray flaring rate using a massive data set from the Chandra X-ray Observatory, the only X-ray instrument that can spatially distinguish between Sgr A* and the nearby Galactic Center magnetar throughout the full extended period encompassing G2's encounter with Sgr A*. We use X-ray data from the 3 Ms observations of the Chandra X-ray Visionary Program (XVP) in 2012, as well as an additional 1.5 Ms of observations up to 2018. We use detected flares to make distributions of flare properties. Using simulations of X-ray flares accounting for important factors such as the different Chandra instrument modes, we test the null hypothesis on Sgr A*'s bright (or any flare category) X-ray flaring rate around different potential change points. In contrast to previous studies, our results are consistent with the null hypothesis; the same model parameters produce distributions consistent with the observed ones around any plausible change point.

Episodic accretion has been used to explain the wide range of protostellar luminosities, but its origin and influence on the star-forming process are not yet fully understood. We present an ALMA survey of N₂H⁺ (1−0) and HCO⁺ (3−2) toward 39 Class 0 and Class I sources in the Perseus molecular cloud. N₂H⁺ and HCO⁺ are destroyed via gas-phase reactions with CO and H₂O, respectively, thus tracing the CO and H₂O snowline locations. A snowline location at a much larger radius than that expected from the current luminosity suggests that an accretion burst has occurred in the past that has shifted the snowline outward. We identified 18/18 Class 0 and 9/10 Class I post-burst sources from N₂H⁺ and 7/17 Class 0 and 1/8 Class I post-burst sources from HCO⁺. The accretion luminosities during the past bursts are found to be ∼10–100 L_⊙. This result can be interpreted as either evolution of burst frequency or disk evolution. In the former case, assuming that refreeze-out timescales are 1000 yr for H₂O and 10,000 yr for CO, we found that the intervals between bursts increase from 2400 yr in the Class 0 stage to 8000 yr in the Class I stage. This decrease in the burst frequency may reflect that fragmentation is more likely to occur at an earlier evolutionary stage when the young stellar object is more prone to instability.

We present a model atom for C i–C ii–C iii–C iv using the most up-to-date atomic data and evaluated the non-local thermodynamic equilibrium line formation in classical 1D atmospheric models of O-B-type stars. Our models predict the emission lines of C ii 9903 Å and 18,535 Å to appear at effective temperature T_eff ≥ 17,500 K, those of C ii 6151 Å and 6461 Å to appear at T_eff > 25,000 K, and those of C iii 5695, 6728–44 and 9701–17 Å to appear at T_eff ≥ 35,000 K (log g = 4.0). Emission occurs in the lines of minority species, where the photoionization-recombination mechanism provides a depopulation of the lower levels to a greater extent than the upper levels. For C ii 9903 and 18535 Å, the upper levels are mainly populated from the C iii reservoir through the Rydberg states. For C iii 5695 and 6728–44 Å, the lower levels are depopulated due to photon losses in UV transitions at 885, 1308, and 1426–28 Å, which become optically thin in the photosphere. We analyzed the lines of C i, C ii, C iii, and C iv for twenty-two O-B-type stars with temperature range between 15,800 ≤T_eff ≤ 38,000 K. Abundances from emission lines of C i, C ii, and C iii are in agreement with those from absorption ones for most of the stars. We obtained log _C = 8.36 ± 0.08 from twenty B-type stars, that is in line with the present-day Cosmic Abundance Standard. The obtained carbon abundances in 15 Mon and HD 42088 from emission and absorption lines are 8.27 ± 0.11 and 8.31 ± 0.11, respectively.

We have used spectra of 181 projected quasar pairs at separations ≤1.5' from the Sloan Digital Sky Survey Data Release 12 in the redshift range of 2.5–3.5 to probe the proximity regions of the foreground quasars. We study the proximity effect both in the longitudinal and in the transverse directions, by carrying out a comparison of the Lyα  absorption lines originating from the vicinity of quasars to those originating from the general intergalactic medium at the same redshift. We found an enhancement in the transmitted flux within 4 Mpc to the quasar in the longitudinal direction. However, the trend is found to be reversed in the transverse direction. In the longitudinal direction, we derived an excess overdensity profile showing an excess up to r ≤ 5 Mpc after correcting for the quasar's ionization, taking into account the effect of low spectral resolution. This excess overdensity profile matches with the average overdensity profile in the transverse direction without applying any correction for the effect of the quasar's ionization. Among various possible interpretations, we found that the anisotropic obscuration of the quasar's ionization seems to be the most probable explanation. This is also supported by the fact that all of our foreground quasars happen to be type 1 AGNs. Finally, we constrain the average quasar's illumination along the transverse direction as compared to that along the longitudinal direction to be ≤27% (3σ confidence level).

We have conducted a highly sensitive census of the evolved-star population in the metal-poor dwarf galaxy Leo P and detected four asymptotic giant branch (AGB) star candidates. Leo P is one of the best examples of a nearby analog of high-redshift galaxies because of its primitive metal content (2% of the solar value), proximity, and isolated nature, ensuring a less complicated history. Using medium-band optical photometry from the Hubble Space Telescope (HST), we have classified the AGB candidates by their chemical type. We have identified one oxygen-rich source which appears to be dusty in both the HST and Spitzer observations. Its brightness, however, suggests it may be a planetary nebula or post-AGB object. We have also identified three carbon-rich candidates, one of which may be dusty. Follow-up observations are needed to confirm the nature of these sources and to study the composition of any dust that they produce. If dust is confirmed, these stars would likely be among the most metal-poor examples of dust-producing stars known and will provide valuable insight into our understanding of dust formation at high redshift.

The diffuse hard X-ray emission that fills the Galactic center, bulge, and ridge is believed to arise from unresolved populations of X-ray binary systems. However, the identity of the dominant class of accreting objects in each region remains unclear. Recent studies of Fe line properties and the low-energy (<10 keV) X-ray continuum of the bulge indicate a major population fraction of nonmagnetic cataclysmic variables (CVs), in particular quiescent dwarf novae (DNe). This is in contrast to previous high-energy (>10 keV) X-ray measurements of the bulge and ridge, which indicate a dominant population of magnetic CVs, in particular intermediate polars. In addition, NuSTAR broadband measurements have uncovered a much heavier intermediate polar population in the central ∼100 pc than previously assumed, raising the possibility that some fraction of this population extends further from the center. Here we use NuSTAR's large aperture for unfocused photons and its broadband X-ray range to probe the diffuse continuum of the inner ∼1°–3° of the Galactic bulge. This allows us to constrain possible multitemperature components of the spectrum, such as could indicate a mixture of soft and hard populations. Our emissivity is consistent with previous hard X-ray measurements in the bulge and ridge, with the diffuse X-ray luminosity tracing the stellar mass. The spectrum is well described by a single-temperature thermal plasma with kT ≈ 8 keV, with no significant emission above 20 keV. This supports that the bulge is dominated by quiescent DNe; we find no evidence of a significant intermediate polar population in the hard X-ray band.

We report the serendipitous detection of two 3 mm continuum sources found in deep ALMA Band 3 observations to study intermediate-redshift galaxies in the COSMOS field. One is near a foreground galaxy at 13, but is a previously unknown dust-obscured star-forming galaxy (DSFG) at probable z_CO = 3.329, illustrating the risk of misidentifying shorter wavelength counterparts. The optical-to-millimeter spectral energy distribution (SED) favors a gray λ^−0.4 attenuation curve and results in significantly larger stellar mass and SFR compared to a Calzetti starburst law, suggesting caution when relating progenitors and descendants based on these quantities. The other source is missing from all previous optical/near-infrared/submillimeter/radio catalogs ("ALMA-only"), and remains undetected even in stacked ultradeep optical (>29.6 AB) and near-infrared (>27.9 AB) images. Using the ALMA position as a prior reveals faint signal-to-noise ratio ∼ 3 measurements in stacked IRAC 3.6+4.5, ultradeep SCUBA2 850 μm, and VLA 3 GHz, indicating the source is real. The SED is robustly reproduced by a massive M* = 10^10.8M_⊙ and M_gas = 10¹¹M_⊙, highly obscured A_V ∼ 4, star-forming SFR ∼ 300 M_⊙ yr⁻¹ galaxy at redshift z = 5.5 ± 1.1. The ultrasmall 8 arcmin² survey area implies a large yet uncertain contribution to the cosmic star formation rate density CSFRD(z = 5) ∼ 0.9 × 10⁻²M_⊙ yr⁻¹ Mpc⁻³, comparable to all ultraviolet-selected galaxies combined. These results indicate the existence of a prominent population of DSFGs at z > 4, below the typical detection limit of bright galaxies found in single-dish submillimeter surveys, but with larger space densities ∼3 × 10⁻⁵ Mpc⁻³, higher duty cycles of 50%–100%, contributing more to the CSFRD, and potentially dominating the high-mass galaxy stellar mass function.

We present calculated cross sections and rate coefficients for the formation of the dicarbon cation (${{\rm{C}}}_{2}^{+}$) by the radiative association process in collisions of a ${\rm{C}}{(}^{3}P)$ atom and a ${{\rm{C}}}^{+}{(}^{2}{P}^{o})$ ion. Molecular structure calculations for a number of low-lying doublet and quartet states of ${{\rm{C}}}_{2}^{+}$ are used to obtain the potential energy surfaces and transition dipole moments coupling the states of interest, substantially increasing the available molecular data for ${{\rm{C}}}_{2}^{+}$. Using a quantum-mechanical method, we explore a number of allowed transitions and determine those contributing to the radiative association process. The calculations extend the available data for this process down to the temperature of 100 K, where the rate coefficient is found to be about $2\times {10}^{-18}\,{\mathrm{cm}}^{3}\,{{\rm{s}}}^{-1}$. We provide analytical fits suitable for incorporation into astrochemical reaction databases.

We investigate the environmental dependence of the local gas-phase metallicity in a sample of star-forming galaxies from the MaNGA survey. Satellite galaxies with stellar masses in the range $9\lt \mathrm{log}({M}_{* }/{M}_{\odot })\lt 10$ are found to be ∼0.05 dex higher in metallicity than centrals of similar stellar mass. Within the low-mass satellite population, we find that the interstellar medium (ISM) metallicity depends most strongly on the stellar mass of the galaxy that is central to the halo, though there is no obvious difference in the metallicity gradients. At fixed total stellar mass, the satellites of high-mass (M_* > 10^10.5M_⊙) centrals are ∼0.1 dex more metal-rich than the satellites of low-mass (M_* < 10¹⁰M_⊙) centrals, controlling for local stellar mass surface density and gas fraction. Fitting a gas regulator model to the spaxel data, we are able to account for variations in the local gas fraction, stellar mass surface density, and local escape velocity–dependent outflows. We find that the best explanation for the metallicity differences is the variation in the average metallicity of accreted gas between different environments that depends on the stellar mass of the dominant galaxies in each halo. This is interpreted as evidence for the exchange of enriched gas between galaxies in dense environments that is predicted by recent simulations.

Coronal mass ejections (CMEs) play a decisive role in driving space weather, especially the fast ones (e.g., with speeds above 800 km s⁻¹). Understanding the trigger mechanisms of fast CMEs can help us gain important information in forecasting them. The filament eruptions accompanied with CMEs provide a good tracer in studying the early evolution of CMEs. Here we surveyed 66 filament-accompanied fast CMEs to analyze the correlation between the trigger mechanisms, namely either magnetic reconnection or ideal MHD process, associated flares, and CME speeds. Based on the data gathered from SDO, GONG, and STEREO, we find that (1) active region (AR) filament and intermediate filament (IF) eruptions show a higher probability for producing fast CMEs than quiet Sun (QS) filaments, while the probability of polar crown (PC) filament eruptions is zero in our statistic; (2) AR filament eruptions that produce fast CMEs are more likely to be triggered by magnetic reconnection, while QS filaments and IFs are more likely to be triggered by an ideal MHD process; (3) for AR filaments and IFs, it seems that the specific trigger mechanism does not have a significant influence on the resulting CME speeds, while for the QS filaments, the ideal MHD mechanism can more likely generate a faster CME; (4) comparing with previous statistical studies, the onset heights of filament eruptions and the decay indexes of the overlying field show some differences: for AR filaments and IFs, the decay indexes are larger and much closer to the theoretical threshold, while for QS filaments, the onset heights are higher than those obtained in previous results.

Measurements of elemental abundances hold important clues to how mass and energy flow through the solar atmosphere. Variations in abundances are organized by an element's first ionization potential (FIP), and many previous studies have assumed that low FIP (less than 10 eV) elements are enriched by a factor of 3–4 in the corona. In this paper, we use spatially resolved observations from the Extreme-ultraviolet Imaging Telescope on board the Hinode spacecraft to examine the spatial variability of elemental abundance in and around active regions. We find substantial variations within some active regions. In general, however, we find that the enrichment of low FIP elements is limited to bright, active region structures. In faint active region structures and in the dark, quiet regions around active regions, the measured abundances are close to photospheric. These measurements use the ratio of low FIP Si to high FIP S. Similar conclusions concerning quiet Sun regions have been reached recently by Del Zanna using full-Sun spectra. He has found that the coronal quiet Sun (at temperatures greater than 1 MK) has photospheric abundances. Transition region abundances (at temperatures less than 1 MK in the solar atmosphere) have been found to be photospheric. These results and results from this paper suggest that a coronal composition is not a general property of million-degree plasma, but is limited to bright active region loops, and is variable.

We present results from a survey searching for spatially resolved near-infrared line emission from molecular hydrogen gas in the circumstellar environments of nine young stars: AA Tau, AB Aur, DoAr 21, GG Tau, GM Aur, LkCa 15, LkHα 264, UY Aur, and V773 Tau. Prior high-resolution spectra of these stars showed the presence of rovibrational H₂ line emission at 2.12 μm with characteristics more typical of gas located in protoplanetary disks rather than outflows. In this study, we spatially resolve the H₂ emission in the eight stars for which it is detected. LkCa 15 is the only target that exhibits no appreciable H₂ despite a prior detection. We find an anticorrelation between H₂ and X-ray luminosities, likely indicating that the X-ray ionization process is not the dominant H₂ excitation mechanism in these systems. AA Tau, UY Aur, and V773 Tau show discrete knots of H₂, as typically associated with shocks in outflowing gas. UY Aur and V773 Tau exhibit spatially resolved velocity structures, while the other systems have spectrally unresolved emission consistent with systemic velocities. V773 Tau exhibits a complex line morphology indicating the presence of multiple excitation mechanisms, including red- and blueshifted bipolar knots of shock-excited outflowing gas. AB Aur, GM Aur, and LkHα 264 have centralized yet spatially resolved H₂ emission consistent with a disk origin. The H₂ images of AB Aur reveal spiral structures within the disk, matching those observed in ALMA CO maps. This survey reveals new insights into the structure and excitation of warm gas in the circumstellar environments of these young stars.

We analyze the light curve of the M5.5 dwarf Proxima Centauri obtained by the Transiting Exoplanet Survey Satellite (TESS) in Sectors 11 and 12. In the ≈50 day long light curve we identified and analyzed 72 flare events. The flare rate was 1.49 events per day; in total, 7.2% of the observing time was classified as flaring. The estimated flare energies were on the order of 10³⁰–10³² erg in the TESS passband (≈4.8× higher in bolometric energies, but on the same order of magnitude). Most of the eruptions appeared in groups. Two events showed quasiperiodic oscillations during their decay phase with a timescale of a few hours, which could be caused by quasiperiodic motions of the emitting plasma or oscillatory reconnection. From the cumulative flare frequency distribution we estimate that superflares with energy output of 10³³ erg are expected to occur three times per year, while magnitude larger events (with 10³⁴ erg) can occur every second year. This reduces the chances of habitability of Proxima Cen b, although earlier numerical models did not rule out the existence of liquid water on the planetary surface. We did not find any obvious signs of planetary transit in the light curve.

Two-dimensional magnetohydrodynamics (MHD) simulations, treating plasma and neutral populations (hereafter, neutrals) as two separate components of the magneto-fluid, are performed in order to investigate the effect of ionization and recombination (or I/R) on the time evolution of the Harris-type current sheet in partially ionized plasmas. Our MHD simulations, including the effect of ambipolar diffusion (arising due to ion-neutral interactions) along with the I/R, show that the current sheet thinning occurs due to the diffusion of neutral particles from the current sheet. In addition to ambipolar diffusion, frictional heating also appears and affects the evolution of the current sheet. In a current sheet that is formed in a partially ionized plasma, the neutral population tries to spread outward and the plasma population tries to converge toward the center of the current sheet, and the overall process is influenced by the I/R. One of the important feature that is captured in our 2D simulations is that the escape of neutrals from the current sheet is sometimes suppressed due to the increase in ionization rate at the center of the current sheet, for the case of collisional I/R. As long as the ionization degree is kept low inside the current sheet, the current sheet thinning and elongation takes place and the current sheet becomes unstable due to the tearing-mode and plasmoid formation. The ion-neutral interactions coupled with I/R and the dynamics of the magnetic reconnection play an important role in plasmoid-mediated reconnection, therefore, the present study on the current sheet thinning and plasmoid formation could serve as a key for understanding bursty and intermittent plasma ejections observed in the solar chromosphere.

HaloSat is a small satellite (CubeSat) designed to map soft X-ray oxygen line emission across the sky in order to constrain the mass and spatial distribution of hot gas in the Milky Way. The goal of HaloSat is to help determine if hot gas gravitationally bound to individual galaxies makes a significant contribution to the cosmological baryon budget. HaloSat was deployed from the International Space Station in 2018 July and began routine science operations in 2018 October. We describe the goals and design of the mission, the on-orbit performance of the science instrument, and initial observations.

We report the detection in Chandra Advanced CCD Imaging Spectrometer archival data of an elongated soft (<3 keV) X-ray feature to the south of the Compton-thick active galactic nucleus (AGN) galaxy IC 2497, coincident with the emission-line feature known as Hanny's Voorwerp. The data are consistent with the spatial correspondence between X-ray, optical emission-line, and radio features detected in nearby obscured AGNs (e.g., ESO 428-G014). The X-ray luminosity of the (0.3–3.0 keV) soft feature is ∼1.2 × 10⁴⁰ erg s⁻¹. We infer an [O iii]/soft-X-ray ratio in the range of ∼200, consistent with the highest values measured in some of the clouds of NGC 4151. Overall, given the uncertainties, Hanny's Voorwerp appears to be a feature consistent with the ionization cone emission of nearby AGNs. We estimate an X-ray recombination time of ∼2 × 10⁷ yr, longer than the [O iii] recombination time (∼8000 yr). This suggests that extended soft X-ray components may be a better diagnostic of overall long-term activity, while detection of an [O iii] HV would point to a time-limited activity burst.

Weak lensing peak abundance analyses have been applied in different surveys and demonstrated to be a powerful statistic in extracting cosmological information complementary to cosmic shear two-point correlation studies. Future large surveys with high number densities of galaxies will enable tomographic peak analyses. Focusing on high peaks, we investigate quantitatively how the tomographic redshift binning can enhance the cosmological gains. We also perform detailed studies about the degradation of cosmological information due to photometric redshift (photo-z) errors. We show that for surveys with a number density of galaxies of ∼40 arcmin⁻², a median redshift of ∼1, and a survey area of ∼15,000 deg², the four-bin tomographic peak analyses can reduce the error contours of (Ω_m, σ₈) by a factor of 5 compared to 2D peak analyses in the ideal case of the photo-z error being absent. More redshift bins can hardly lead to significantly better constraints. The photo-z error model here is parameterized by z_bias and σ_ph and the fiducial values of z_bias = 0.003 and σ_ph = 0.02 are taken. We find that using tomographic peak analyses can constrain the photo-z errors simultaneously with cosmological parameters. For four-bin analyses, we can obtain σ(z_bias)/z_bias ∼ 10% and σ(σ_ph)/σ_ph ∼ 5% without assuming priors on them. Accordingly, the cosmological constraints on Ω_m and σ₈ degrade by factors of ∼2.2 and ∼1.8, respectively, with respect to zero uncertainties on photo-z parameters. We find that the uncertainty of z_bias plays a more significant role in degrading the cosmological constraints than that of σ_ph.

We report the discovery of an EL CVn-type binary consisting of a low-mass, pre-helium white dwarf and the first detection of hybrid δ Sct-γ Dor pulsations in such binary systems. Based on the four years Kepler data, we determined comprehensive photometric solution of the eclipsing binary KIC 8113154. The light-curve modeling reveals that it is a detached system containing a thermally bloated, low-mass, pre-He white dwarf with the mass of 0.26 ± 0.02 M_⊙ and the radius of 0.13 ± 0.01 R_⊙. After removal of the binary model from the observed Kepler data, multiple frequency analysis is applied to the light residuals. The Fourier spectrum shows low-order p-modes and high-order g-mode pulsations that very likely stem from the F-type primary component star, which could be classified as a new δ Sct-γ Dor hybrid. We detected 111 frequencies with signal-to-noise amplitude ratios larger than 4.0. From these frequencies, we identified 17 high-order quadrupole (l = 2) g modes, including eight zonal (m = 0) and nine prograde (m = 2) ones, on the basis of which we derived the asymptotic period spacing of the g modes and the internal rotation rate of the convective core. This is significantly smaller than the orbital frequency, indicating that the core and envelope of the primary star in KIC 8113154 rotate differentially.

Compact symmetric objects (CSOs) have been observed with Chandra and XMM-Newton to gain insights into the initial stages of a radio source evolution and to probe the black hole activity at the time of relativistic outflow formation. However, there have been no CSO observations to date at the hard X-ray energies (>10 keV), impeding our ability to robustly constrain the properties of the intrinsic X-ray emission and of the medium surrounding the young expanding jets. We present the first hard X-ray observation of a CSO performed with the Nuclear Spectroscopic Telescope Array (NuSTAR). Our target, OQ +208, is detected up to 30 keV, and thus we establish CSOs as a new class of the NuSTAR sources. We analyze the NuSTAR data jointly with our new Chandra and archival XMM-Newton data and find that a young (∼250 yr old) radio jet spanning the length of 10 pc coexists with cold obscuring matter, consistent with a dusty torus, with an equivalent hydrogen column density of N_H = 10²³–10²⁴ cm⁻². The primary X-ray emission is characterized by a photon index of Γ ∼ 1.45 and an intrinsic 0.5–30 keV luminosity of L ≃ 10⁴³ erg s⁻¹. The results of our spectral modeling and broad-line optical classification of the source suggest a porous structure of the obscuring torus. Alternatively, the source may belong to the class of optically unobscured/X-ray-obscured active galactic nucleus. The observed X-ray emission is too weak compared to that predicted by the expanding radio lobes model, leaving an accretion disk corona or jets as the possible origins of the X-ray emission from this young radio galaxy.

We carried out observations of CCH and its two ¹³C isotopologues, ¹³CCH and C¹³CH, in the 84–88 GHz band toward two starless cores, L1521B and L134N (L183), using the Nobeyama 45 m radio telescope. We detected C¹³CH with a signal-to-noise ratio of 4, whereas no line of ¹³CCH was detected in either of the dark clouds. The column densities of the normal species were derived to be (1.66 ± 0.18) × 10¹⁴ cm⁻² and (7.3 ± 0.9) × 10¹³ cm⁻² (1σ) in L1521B and L134N, respectively. The column density ratios of N(C¹³CH)/N(¹³CCH) were calculated to be >1.1 and >1.4 in L1521B and L134N, respectively. The characteristic that ¹³CCH is less abundant than C¹³CH is likely common for dark clouds. Moreover, we find that the ¹²C/¹³C ratios of CCH are much higher than those of HC₃N in L1521B by more than a factor of 2, as well as in Taurus Molecular Cloud-1 (TMC-1). In L134N, the differences in the ¹²C/¹³C ratios between CCH and HC₃N seem to be smaller than those in L1521B and TMC-1. We discuss the origins of the ¹³C isotopic fractionation of CCH and investigate possible routes that cause the significantly high ¹²C/¹³C ratio of CCH especially in young dark clouds, with the help of chemical simulations. The high ¹²C/¹³C ratios of CCH seem to be caused by reactions between hydrocarbons (e.g., CCH, C₂H₂, l-C₃H and c-C₃H) and C⁺.

MAXI J1621–501 is the first Swift/XRT Deep Galactic Plane Survey transient that was followed up with a multitude of space missions (NuSTAR, Swift, Chandra, NICER, INTEGRAL, and MAXI) and ground-based observatories (Gemini, IRSF, and ATCA). The source was discovered with MAXI on 2017 October 19 as a new, unidentified transient. Further observations with NuSTAR revealed two Type I X-ray bursts, identifying MAXI J1621–501 as a low mass x-ray binary with a neutron star primary. Overall, 24 Type I bursts were detected from the source during a 15 month period. At energies below 10 keV, the source spectrum was best fit with three components: an absorbed blackbody with kT = 2.3 keV, a cutoff power law with index Γ = 0.7, and an emission line centered on 6.3 keV. Timing analysis of the X-ray persistent emission and burst data has not revealed coherent pulsations from the source or an orbital period. We identified, however, a super-orbital period ∼78 days in the source X-ray light curve. This period agrees very well with the theoretically predicted radiative precession period of ∼82 days. Thus, MAXI J1621–501 joins a small group of sources characterized with super-orbital periods.

We carry out a comprehensive Bayesian correlation analysis between hot halos and direct masses of supermassive black holes (SMBHs), by retrieving the X-ray plasma properties (temperature, luminosity, density, pressure, and masses) over galactic to cluster scales for 85 diverse systems. We find new key scalings, with the tightest relation being ${M}_{\bullet }$−${\text{}}{T}_{{\rm{x}}}$, followed by ${M}_{\bullet }$−${\text{}}{L}_{{\rm{x}}}$. The tighter scatter (down to 0.2 dex) and stronger correlation coefficient of all the X-ray halo scalings compared with the optical counterparts (as the ${M}_{\bullet }$−${\sigma }_{{\rm{e}}}$) suggest that plasma halos play a more central role than stars in tracing and growing SMBHs (especially those that are ultramassive). Moreover, ${M}_{\bullet }$ correlates better with the gas mass than dark matter mass. We show the important role of the environment, morphology, and relic galaxies/coronae, as well as the main departures from virialization/self-similarity via the optical/X-ray fundamental planes. We test the three major channels for SMBH growth: hot/Bondi-like models have inconsistent anticorrelation with X-ray halos and too low feeding; cosmological simulations find SMBH mergers as subdominant over most of cosmic time and too rare to induce a central-limit-theorem effect; the scalings are consistent with chaotic cold accretion, the rain of matter condensing out of the turbulent X-ray halos that sustains a long-term self-regulated feedback loop. The new correlations are major observational constraints for models of SMBH feeding/feedback in galaxies, groups, and clusters (e.g., to test cosmological hydrodynamical simulations), and enable the study of SMBHs not only through X-rays, but also via the Sunyaev–Zel'dovich effect (Compton parameter), lensing (total masses), and cosmology (gas fractions).

The Radio Arc is one of the brightest systems of nonthermal filaments (NTFs) in the Galactic Center, located near several prominent H ii regions (Sickle and Pistol) and the Quintuplet stellar cluster. We present observations of the Arc NTFs using the S, C, and X bands of the Very Large Array interferometer. Our images of total intensity reveal large-scale helical features that surround the Arc NTFs, very narrow subfilamentation, and compact sources along the NTFs. The distribution of polarized intensity is confined to a relatively small area along the NTFs. There are elongated polarized structures that appear to lack total intensity counterparts. We detect a range of rotation measure values from −1000 to −5800 rad m⁻², likely caused by external Faraday rotation along the line of sight. After correcting for Faraday rotation, the intrinsic magnetic field orientation is found to generally trace the extent of the NTFs. However, the intrinsic magnetic field in several regions of the Arc NTFs shows an ordered pattern that is rotated with respect to the extent of the NTFs. We suggest this changing pattern may be caused by an additional magnetized source along the line of sight, so that we observe two field systems superposed in our observations. We suggest that the large-scale helical segments near the Radio Arc could be components of such a source causing these changes in the intrinsic magnetic field, and some variations in the polarization and rotation measure values along the NTFs.

The advent of high-angular-resolution IR and submillimeter interferometry allows for spatially resolved observations of the parsec-scale environment of active galactic nuclei (AGNs), commonly referred to as the "torus." While molecular lines show the presence of large, massive disks, the IR observations appear to be dominated by a strong polar component that has been interpreted as a dusty wind. This paper aims at using characteristics shared by AGNs in each of the wavebands and a set of simple physical principles to form a unifying view of these seemingly contradictory observations: dusty molecular gas flows in from galactic scales of ∼100 pc to the subparsec environment via a disk with small to moderate scale height. The hot, inner part of the disk puffs up due to IR radiation pressure and unbinds a large amount of the inflowing gas from the black hole's gravitational potential, providing the conditions to launch a wind driven by the radiation pressure from the AGN. The dusty wind feeds back mass into the galaxy at a rate of the order of ∼0.1–100 M_⊙ yr⁻¹, depending on the AGN luminosity and Eddington ratio. Angle-dependent obscuration as required by AGN unification is provided by a combination of disk, wind, and wind-launching region.

To investigate the mass dependence of structural transformation and star formation quenching, we construct three galaxy samples using massive (M_* > 10¹⁰M_⊙) red, green, and blue galaxy populations at 0.5 < z < 2.5 in five 3D–HST/CANDELS fields. The structural parameters, including effective radius (r_e), galaxy compactness (Σ_1.5), and second-order moment of 20% brightest pixels (M₂₀), are found to be correlated with stellar mass. Sérsic index (n), concentration (C), and Gini coefficient (G) seem to be insensitive to stellar mass. The morphological distinction between blue and red galaxies is found at a fixed mass bin, suggesting that quenching processes should be accompanied with transformations of galaxy structure and morphology. Except for r_e and Σ_1.5 at the high-mass end, structural parameters of green galaxies are intermediate between red and blue galaxies in each stellar mass bin at z < 2, indicating that green galaxies are at a transitional phase when blue galaxies are being quenched into quiescent statuses. The similar sizes and compactness for the blue and green galaxies at the high-mass end imply that some of these galaxies will not appear to be significantly shrunk until they are completely quenched into red quiescent galaxies. For the green galaxies at 0.5 < z < 1.5, a morphological transformation sequence of bulge buildup can be seen as their star formation activities are gradually shut down, while a faster morphological transformation is verified for the green galaxies at 1.5 < z < 2.5.

The clustered nature of star formation should produce a high degree of structure in the combined phase and chemical space in the Galactic disk. To date, observed structure of this kind has been mostly limited to bound clusters and moving groups. In this paper, we present a new dynamical model of the Galactic disk that takes into account the clustered nature of star formation. This model predicts that the combined phase and chemical space is rich in substructure and that this structure is sensitive to both the precise nature of clustered star formation and the large-scale properties of the Galaxy. The model self-consistently evolves 4 billion stars over the last 5 Gyr in a realistic potential that includes an axisymmetric component, a bar, spiral arms, and giant molecular clouds. All stars are born in clusters with an observationally motivated range of initial conditions. As direct N-body calculations for billions of stars are computationally infeasible, we have developed a method of initializing star cluster particles to mimic the effects of direct N-body effects, while the actual orbit integrations are treated as test particles within the analytic potential. We demonstrate that the combination of chemical and phase space information is much more effective at identifying truly conatal populations than either chemical or phase space alone. Furthermore, we show that comoving pairs of stars are very likely to be conatal if their velocity separation is <2 km s⁻¹ and their metallicity separation is <0.05 dex. The results presented here bode well for harnessing the synergies between Gaia and spectroscopic surveys to reveal the assembly history of the Galactic disk.

Using photometry and proper motions from Gaia Data Release 2, we detect a 50° long stream of about 70 stars extending westward from the halo globular cluster M5. Based on the similarities in distance, proper motions, inferred color–magnitude distribution, and trajectory, we identify this stream as the trailing tidal tail of M5. While the surface density of stars is very low (≃1.5 star per square degree, or ≈35 mag per square arcsecond), selecting only stars with proper motions consistent with the orbit of the cluster yields a detection significance of ≈10σ. While we find a possible continuation of the stream to ≈85°, increasing foreground contamination combined with a greater predicted stream distance makes it difficult to detect with current data even if the stream continues unabated. The nonuniform distribution of stars in the stream appears to be consistent with episodic tidal stripping, with the most recently shed stars now trailing the cluster by tens of degrees. We provide a table of the highest-ranked candidate stream stars for ongoing and future spectroscopic surveys.

It is well established that solar flares and coronal mass ejections (CMEs) are powered by the free magnetic energy stored in volumetric electric currents in the corona, predominantly in active regions (ARs). Much effort has been made to search for eruption-related signatures from magnetic field observed mostly in the photosphere; and the signatures are further employed for predicting flares and CMEs. The parameters in the Space-weather HMI Active Region Patches (SHARP) data from the Solar Dynamics Observatory/HMI observation of vector magnetic field are designed and generated for this purpose. In this paper, we report research done on modification of these SHARP parameters with an attempt to improve flare prediction. The newly modified parameters are weighed heavily by magnetic polarity inversion lines (PIL) with high magnetic gradient, as suggested by Schrijver, by multiplying the parameters with a PIL mask. We demonstrate that the number of the parameters that can well discriminate erupted and nonerupted ARs increases significantly by a factor of two, in comparison with the original parameters. This improvement suggests that the high-gradient PILs are tightly related with solar eruption that agrees with previous studies. This also provides new data that possess potential to improve the machine-learning-based solar flare prediction models.

We develop a self-consistent model for the equilibrium gas temperature and size-dependent dust temperature in cold, dense, prestellar cores, assuming an arbitrary power-law size distribution of dust grains. Compact analytical expressions applicable to a broad range of physical parameters are derived and compared with predictions of the commonly used standard model. It is suggested that combining the theoretical results with observations should allow us to constrain the degree of dust evolution and the cosmic-ray ionization rate in dense cores, and to help with discriminating between different regimes of cosmic-ray transport in molecular clouds. In particular, assuming a canonical MRN distribution of grain sizes, our theory demonstrates that the gas-temperature measurements in the prestellar core L1544 are consistent with an ionization rate as high as ∼10⁻¹⁶ s⁻¹, an order of magnitude higher than previously thought.

We apply novel survival analysis techniques to investigate the relationship between a number of the properties of galaxies and their atomic (M_{H i}) and molecular (${M}_{{{\rm{H}}}_{2}}$) gas mass, with the aim of devising efficient, effective empirical estimators of the cold gas content in galaxies that can be applied to large optical galaxy surveys. We find that dust attenuation, A_V, of both the continuum and nebular emission, shows significant partial correlations with ${M}_{{{\rm{H}}}_{2}}$, after controlling for the effect of star formation rate (SFR). The partial correlation between A_V and M_{H i}, however, is weak. This is expected because in poorly dust-shielded regions molecular hydrogen is dissociated by far-ultraviolet photons. We also find that the stellar half-light radius, R₅₀, shows significant partial correlations with both ${M}_{{{\rm{H}}}_{2}}$ and M_{H i}. This hints at the importance of environment (e.g., galactocentric distance) on the gas content of galaxies and the interplay between gas and SFR. We fit multiple regression to summarize the median, mean, and the 0.15/0.85 quantile multivariate relationships among ${M}_{{{\rm{H}}}_{2}}$, A_V, metallicity, and/or R₅₀. A linear combination of A_V and metallicity (inferred from stellar mass) or A_V and R₅₀, can estimate molecular gas masses within ∼2.5–3 times the observed masses. If SFR is used in addition, ${M}_{{{\rm{H}}}_{2}}$ can be predicted to within a factor ≲2. In this case, A_V and R₅₀ are the two best secondary parameters that improve the primary relation between ${M}_{{{\rm{H}}}_{2}}$ and SFR. Likewise, M_{H i} can be predicted to within a factor ≲3 using R₅₀ and SFR.

The migration of planetary cores embedded in a protoplanetary disk is an important mechanism within planet-formation theory, relevant for the architecture of planetary systems. Consequently, planet migration is actively discussed, yet often results of independent theoretical or numerical studies are unconstrained due to the lack of observational diagnostics designed in light of planet migration. In this work we follow the idea of inferring the migration behavior of embedded planets by means of the characteristic radial structures that they imprint in the disk's dust density distribution. We run hydrodynamical multifluid simulations of gas and several dust species in a locally isothermal α-disk in the low-viscosity regime (α = 10⁻⁵) and investigate the obtained dust structures. In this framework, a planet of roughly Neptune mass can create three (or more) rings in which dust accumulates. We find that the relative spacing of these rings depends on the planet's migration speed and direction. By performing subsequent radiative transfer calculations and image synthesis we show that—always under the condition of a near-inviscid disk—different migration scenarios are, in principle, distinguishable by long-baseline, state-of-the-art Atacama Large Millimeter/submillimeter Array observations.

The evolution of the magnetic field and plasma quantities inside a coronal mass ejection (CME) with distance are known from statistical studies using data from 1 au monitors, planetary missions, Helios, and Ulysses. This does not cover the innermost heliosphere, below 0.29 au, where no data are yet publicly available. Here, we describe the evolution of the properties of simulated CMEs in the inner heliosphere using two different initiation mechanisms. We compare the radial evolution of these properties with that found from statistical studies based on observations in the inner heliosphere by Helios and MESSENGER. We find that the evolution of the radial size and magnetic field strength is nearly indistinguishable for twisted flux rope from that of writhed CMEs. The evolution of these properties is also consistent with past studies, primarily with recent statistical studies using in situ measurements and with studies using remote observations of CMEs.

We investigate the lowest-mass quiescent galaxies known to exist in isolated environments (${M}^{* }={10}^{9.0-9.5}\,{M}_{\odot }$; 1.5 Mpc from a more massive galaxy). This population may represent the lowest stellar mass galaxies in which internal feedback quenches galaxy-wide star formation. We present a Keck/Echelle Spectrograph and Imager long-slit spectroscopy for 27 isolated galaxies in this regime (20 quiescent galaxies and 7 star-forming galaxies). We measure emission line strengths as a function of radius and place galaxies on the Baldwin–Phillips–Terlevich (BPT) diagram. Remarkably, 16 of 20 quiescent galaxies in our sample host central active galactic nucleus (AGN)-like line ratios. Only five of these quiescent galaxies were identified as AGN-like in the Sloan Digital Sky Survey due to a lower spatial resolution and signal-to-noise ratio. We find that many of the quiescent galaxies in our sample have spatially extended emission across the non-star-forming regions of BPT-space. While quenched galaxies in denser environments in this mass range often show no evidence for AGN activity, a significant fraction of quiescent galaxies in isolation host AGNs despite their overall passive appearances.