Table of contents

Using particle-in-cell simulations, we investigate the onset of magnetic reconnection from a quiescent Harris current sheet in collisionless plasmas. After the current sheet is destabilized by the collisionless tearing mode instability, it proceeds to onset of reconnection, which manifests spontaneous thinning of current sheet and pileup of upstream magnetic flux. Once the current sheet thins to a critical thickness, about two electron inertial lengths, reconnection begins to grow explosively in this electron current sheet. This study shows that the spontaneous onset of collisionless magnetic reconnection is controlled by electron kinetics.

It is shown that a gravitationally bound system with a one-dimensional velocity dispersion σ can at most dissipate a fraction $\sim 36{\left(\sigma /c\right)}^{3}$ of the gravitational wave (GW) energy propagating through it, even if their dynamical time is shorter than the wave period. The limit is saturated for low-frequency waves propagating through a system of particles with a mean-free-path equal to the size of the system, such as hot protons in galaxy clusters, strongly interacting dark matter particles in halos, or massive black holes in clusters. For such systems with random motions and no resonances, the dissipated fraction, $\lesssim {10}^{-6}$, does not degrade the use of GWs as cosmological probes. At high-wave frequencies, the dissipated fraction is additionally suppressed by the square of the ratio between the collision frequency and the wave frequency. The electromagnetic counterparts that result from the dissipation are too faint to be detectable at cosmological distances.

To determine the epoch of reionization precisely and to reveal the property of inhomogeneous reionization are some of the most important topics of modern cosmology. Existing methods to investigate reionization that use cosmic microwave background, Lyα emitters, quasars, or gamma-ray bursts have difficulties in terms of accuracy or event rate. We propose that recently discovered fast luminous blue transients (FLBTs) have potential as a novel probe of reionization. We study the detectability of FLBTs at the epoch of reionization with upcoming WFIRST Wide-Field Instrument, using a star formation rate (SFR) derived from galaxy observations and an event rate of FLBTs proportional to the SFR. We find that if FLBTs occur at a rate of 1% of the core-collapse supernova rate, 2 (0.3) FLBTs per year per deg² at z > 6 (z > 8) can be detected by a survey with a limiting magnitude of 26.5 mag in the near-infrared band and a cadence of 10 days. We conclude that the WFIRST supernova deep survey can detect ∼20 FLBTs at the epoch of reionization in the near future.

Recent observations with the MeerKAT radio telescope reveal a unique population of faint nonthermal filaments pervading the central molecular zone, a region rich in molecular gas near the Galactic center. Some of those filaments are organized into groups of almost parallel filaments, seemingly sorted by their length, so that their morphology resembles a harp with radio-emitting "strings." We argue that the synchrotron-emitting GeV electrons of these radio harps have been consecutively injected by the same source (a massive star or pulsar) into spatially intermittent magnetic fiber bundles within a magnetic flux tube or via time-dependent injection events. After escaping from this source, the propagation of cosmic-ray (CR) electrons inside a flux tube is governed by the theory of CR transport. We propose to use observations of radio harp filaments to gain insight into the specifics of CR propagation along magnetic fields of which there are two principle modes: CRs could either stream with self-excited magnetohydrodynamic waves or diffuse along the magnetic field. To disentangle these possibilities, we conduct hydrodynamical simulations of either purely diffusing or streaming CR electrons and compare the resulting brightness distributions to the observed synchrotron profiles of the radio harps. We find compelling evidence that CR streaming is the dominant propagation mode for GeV CRs in one of the radio harps. Observations at higher angular resolution should detect more radio harps and may help to disentangle projection effects of the possibly three-dimensional flux-tube structure of the other radio harps.

The recent detection of circularly polarized, long-duration (>8 hr), low-frequency (∼150 MHz) radio emission from the M4.5 dwarf GJ 1151 has been interpreted as arising from a star–planet interaction via the electron cyclotron maser instability. The existence or parameters of the proposed planets have not been determined. Using 20 new HARPS-N observations, we put 99th-percentile upper limits on the mass of any close companion to GJ 1151 at $M\sin i\lt 5.6\,{M}_{\oplus }$. With no stellar, brown dwarf, or giant planet companion likely in a close orbit, our data are consistent with detected radio emission emerging from a magnetic interaction between a short-period terrestrial-mass planet and GJ 1151 (https://github.com/benjaminpope/video).

Despite the rapidly growing number of stellar-mass binary black hole mergers discovered through gravitational waves, the origin of these binaries is still not known. In galactic centers, black holes can be brought to each others' proximity by dynamical processes, resulting in mergers. It is also possible that black holes formed in previous mergers encounter new black holes, resulting in so-called hierarchical mergers. Hierarchical events carry signatures such as higher-than-usual black hole mass and spin. Here we show that the recently reported gravitational-wave candidate, GW170817A, could be the result of such a hierarchical merger. In particular, its chirp mass ∼40 M_⊙ and effective spin of χ_eff ∼ 0.5 are the typically expected values from hierarchical mergers within the disks of active galactic nuclei. We find that the reconstructed parameters of GW170817A strongly favor a hierarchical merger origin over having been produced by an isolated binary origin (with an odds ratio of > 10³).

An investigation into the excitation sources of oscillations detected in a coronal loop structure is carried out using the images obtained with Interface Region Imaging Spectrometer (IRIS) and the Atmospheric Imaging Assembly (AIA) instrument on board the Solar Dynamics Observatory (SDO). A loop structure in the active region AR 11967 on 2014 January 28, oscillating in the vicinity of a strong eruption and an M3.6 class flare site, is clearly noticeable in SDO/AIA 171 Å images. We study in detail, the oscillations with detected periods between 4 and 13 minutes and their connection in IRIS SJI 1330 Å and SDO/AIA 1700 Å images; both of these wavelengths sample the lower parts of the solar atmosphere. The simultaneous presence of many oscillations in the region of interest in all three wavelength passbands suggest that these oscillations were excited in the lower-chromosphere–photosphere plasma connected to the loop structure and then propagated at higher heights. We further investigate the Doppler velocity measurements from the spectrograph snapshots in IRIS C ii 1336 Å, Si iv 1403 Å and Mg ii k 2796 Å. These show signatures of upflows in the vicinity of the loop structure's endpoints estimated from 171 Å images. We suggest that some of the oscillations observed in AIA 171 Å have been triggered by plasma ejections and perturbations seen in the lower layers of the solar atmosphere. Based on the estimated phase speeds, the oscillations are likely to be slow magnetoacoustic in nature.

We report the first detection of isotopic methanol (¹³CH₃OH) maser emission in interstellar space. The emission was detected toward the high-mass young stellar object G358.93-0.03 during monitoring of a flare in the 6.7 GHz methanol (CH₃OH) maser emission in this source. We find that the spectral and spatial distribution of the ¹³CH₃OH masers differs from the CH₃OH masers imaged at the same epoch, contrary to expectations from similarity of their pumping. This conclusively demonstrates that isotopic methanol masers are bright under different physical conditions and suggests that they can provide additional, complementary information to the CH₃OH masers from the same source. We detect a rapid decay of the ¹³CH₃OH maser lines suggesting that they are transient phenomena (masing for only a few months), likely associated with rapid changes in radiation field due to an accretion burst induced by massive disk fragmentation. Changes in the line flux density are faster than required to achieve equilibrium in the energy level populations, indicating that the pumping of these masers is likely variable.

Micrometeoroid bombardments are one of the causes of space weathering on airless bodies. We have simulated micrometeoroid bombardments on the surfaces of C-type asteroids by pulse-laser irradiation experiments on Murchison CM2 chondrite samples. In this Letter, we focus in particular on the effect of lower-energy irradiation compared to our previous study, where the laser energy range was set to 5–15 mJ, causing spectral flattening and water absorption band suppression. Murchison powder samples were irradiated with pulse lasers of various laser intensities (0.7, 1, 2, and 5 mJ). The irradiation energies are equivalent to micrometeoroid bombardments on the main-belt asteroids for ∼5.7 × 10⁷ yr for 5 mJ and ∼7.9 × 10⁶ yr for 0.7 mJ, respectively. We measured reflectance spectra and analyzed chemical compositions and microstructures of the surface of the laser-irradiated Murchison samples. Laser-irradiated Murchison spectra show flattening and darkening in the ultraviolet (UV)–visible (Vis)–infrared (IR) range. As the laser energy was increased up to 5 mJ, the 3 and 0.7 μm band depths decreased by 12% and 50%, respectively. The particle surface in the 5 mJ irradiated area shows melted and vesiculated structures, indicating high-temperature heating by laser irradiation followed by rapid cooling. The chemical composition of the melted and bubbled portions is similar to FeS-rich amorphous silicate particles observed in the high-energy laser irradiation case. Each mineralogical change of Murchison due to short-duration heating would cause spectral bluing, darkening, and absorption band suppression.

Recent observations of repeating fast radio bursts (FRBs) suggest that some FRBs reside in an environment consistent with that of binary neutron star (BNS) mergers. The bursting rate for repeaters could be very high and the emission site is likely from a magnetosphere. We discuss a hypothesis of producing abundant repeating FRBs in BNS systems. Decades to centuries before a BNS system coalesces, the magnetospheres of the two neutron stars start to interact relentlessly. Abrupt magnetic reconnection accelerates particles, which emit coherent radio waves in bunches via curvature radiation. FRBs are detected as these bright radiation beams point toward Earth. This model predicts quasi-periodicity of the bursts at the rotation periods of the two merging neutron stars (tens of milliseconds and seconds, respectively) as well as the period of orbital motion (of the order of 100 s). The bursting activities are expected to elevate with time as the two neutron stars get closer. The repeating FRB sources should be gravitational-wave (GW) sources for space-borne detectors such as Laser Interferometer Space Antenna (LISA), and eventually could be detected by ground-based detectors when the two neutron stars coalesce.

In this Letter, we investigate the impact of environment on integrated and spatially resolved stellar kinematics of a sample of massive, quiescent galaxies at intermediate redshift (0.6 < z < 1.0). For this analysis, we combine photometric and spectroscopic parameters from the UltraVISTA and Large Early Galaxy Astrophysics Census surveys in the COSMOS field and environmental measurements. We analyze the trends with overdensity (1+δ) on the rotational support of quiescent galaxies and find no universal trends at either fixed mass or fixed stellar velocity dispersion. This is consistent with previous studies of the local universe; rotational support of massive galaxies depends primarily on stellar mass. We highlight two populations of massive galaxies ($\mathrm{log}\,{M}_{\star }/{M}_{\odot }\geqslant 11$) that deviate from the average mass relation. First, the most massive galaxies in the most underdense regions ((1 + δ) ≤ 1) exhibit elevated rotational support. Similarly, at the highest masses ($\mathrm{log}\,{M}_{\star }/{M}_{\odot }\geqslant 11.25$) the range in rotational support is significant in all but the densest regions. This corresponds to an increasing slow-rotator fraction such that only galaxies in the densest environments ((1 + δ) ≥ 3.5) are primarily (90% ± 10%) slow rotators. This effect is not seen at fixed velocity dispersion, suggesting minor merging as the driving mechanism: only in the densest regions have the most massive galaxies experienced significant minor merging, building stellar mass and diminishing rotation without significantly affecting the central stellar velocity dispersion. In the local universe, most massive galaxies are slow rotators, regardless of environment, suggesting minor merging occurs at later cosmic times (z ≲ 0.6) in all but the most dense environments.

We study the emission properties of thermonuclear explosions in a helium white dwarf (WD) tidal disruption event (TDE). We consider a TDE where a 0.2 M_⊙ helium WD is disrupted by a 10^2.5M_⊙ intermediate-mass black hole (IMBH). The helium WD is not only tidally disrupted but is also detonated by the tidal compression and by succeeding shocks. We focus on the emission powered by radioactive nuclei in the unbound TDE ejecta. We perform hydrodynamic simulations coupled with nuclear reactions, post-process detailed nucleosynthesis calculations, and radiative transfer simulations. We thus derive multi-band light curves and spectra. The helium WD TDE shows rapid (Δt_1mag ≃ 5–10 days) and relatively faint (${L}_{\mathrm{peak}}\simeq {10}^{42}\,\mathrm{erg}\,{{\rm{s}}}^{-1}$) light curves, because the ejecta mass and ⁵⁶Ni mass are low (0.12 M_⊙ and 0.03 M_⊙, respectively). The spectra show strong calcium and Fe-peak features and very weak silicon features, reflecting the peculiar elemental abundance. The key feature is the Doppler shift of the spectral lines up to ≃ ±12,000 km s⁻¹, depending on the viewing angle, due to the bulk motion of the ejecta. Our model matches well with two rapid and faint transients reported in Pursiainen et al. The particular model presented here does not match with observed SNe Iax, calcium-rich transients, or .Ia explosion candidates, either in the spectra or light curves. However, we expect a large variety of observational signatures once a wide range of the WD/black hole masses and orbital parameters are considered. This study contributes to the search for WD TDEs with current and upcoming surveys, and to the identification of IMBHs as disrupters in TDEs.

In an effort to measure the Rossiter–McLaughlin effect for the TRAPPIST-1 system, we performed high-resolution spectroscopy during transits of planets e, f, and b. The spectra were obtained with the InfraRed Doppler spectrograph on the Subaru 8.2 m telescope, and were supplemented with simultaneous photometry obtained with a 1 m telescope of the Las Cumbres Observatory Global Telescope. By analyzing the anomalous radial velocities, we found the projected stellar obliquity to be λ = 1 ± 28° under the assumption that the three planets have coplanar orbits, although we caution that the radial-velocity data show correlated noise of unknown origin. We also sought evidence for the expected deformations of the stellar absorption lines, and thereby detected the "Doppler shadow" of planet b with a false-alarm probability of 1.7%. The joint analysis of the observed residual cross-correlation map including the three transits gave $\lambda ={19}_{-15}^{+13}$°. These results indicate that the the TRAPPIST-1 star is not strongly misaligned with the common orbital plane of the planets, although further observations are encouraged to verify this conclusion.

Galactic-scale winds driven by active galactic nuclei (AGN) are often invoked to suppress star formation in galaxy evolution models, but the mechanisms driving these outflows are hotly debated. Two key AGN feedback models are (1) radiation pressure accelerating cool gas and (2) a hot outflowing wind entraining the interstellar medium (ISM). Highly ionized emission-line diagnostics represent a powerful means of differentiating these scenarios because of their sensitivity to the expected compression of the ISM clouds by the hot wind. Here, we report the first spatially resolved UV emission spectroscopy of a prototypical (radio-quiet) quasar-driven superwind around the obscured quasar SDSS J1356+1026 at z = 0.123. We observe ratios of O vi/C iv, N v/C iv, and C iv/He ii that are remarkably similar for outflowing gas clouds ≲100 pc and ≈10 kpc from the nucleus. Such similarity is expected for clouds with AGN radiation-pressure-dominated dynamics. Comparing the observed line emission to models of clouds in balance with radiation pressure and/or a hot wind, we rule out the presence of a dynamically important hot wind and constrain the ratio of hot gas pressure to radiation pressure to P_hot/P_rad ≲ 0.25 both at ≲100 pc and ≈10 kpc from the nucleus. Moreover, the predictions of the radiation pressure confined cloud models that best fit observed UV line ratios are consistent with the observed diffuse X-ray spectrum. These results indicate that this AGN superwind is driven by radiation pressure or was driven by a hot wind that has since dissipated despite ongoing AGN activity.

We analyze the X-ray, optical, and mid-infrared data of a "changing-look" Seyfert galaxy SDSS J155258+273728 at z ≃ 0.086. Over a period of one decade (2009–2018), its broad Hα line intensity increased by a factor of ∼4. Meanwhile, the X-ray emission in 2014 as observed by Chandra was about five times brighter than that in 2010 by Suzaku, and the corresponding emissions in the V-band, mid-infrared W1 band brighten by ∼0.18, 0.32 mag, respectively. Moreover, the absorption in X-rays is moderate and stable, i.e., ${N}_{{\rm{H}}}\sim {10}^{21}\ {\mathrm{cm}}^{-2}$, but the X-ray spectrum becomes harder in the 2014 Chandra bright state (i.e., photon index ${\rm{\Gamma }}={1.52}_{-0.06}^{+0.06}$) than that of the 2010 Suzaku low state (${\rm{\Gamma }}={2.03}_{-0.21}^{+0.22}$). With an Eddington ratio being lower than a few percent, the inner region of the accretion disk in SDSS J155258+273728 is likely a hot accretion flow. We then compile from literature the X-ray data of "changing-look" active galactic nuclei (AGNs), and find that they generally follow the well-established "V"-shaped correlation in AGNs, that is, above a critical turnover luminosity the X-ray spectra soften with the increasing luminosity, and below that luminosity the trend is reversed in the way of "harder when brighter." This presents direct evidence that CL-AGNs have distinctive changes in not only the optical spectral type, but also the X-ray spectral shape. The similarity in the X-ray spectral evolution between CL-AGNs and black hole X-ray binaries indicates that the observed CL-AGNs phenomena may relate to the state transition in accretion physics.

Supernovae (SNe) drive multiphase galactic outflows, impacting galaxy formation; however, cosmological simulations mostly use ad hoc feedback models for outflows, making outflow-related predictions from first principles problematic. Recent small-box simulations resolve individual SNe remnants in the interstellar medium (ISM), naturally driving outflows and permitting a determination of the wind loading factors of energy η_E, mass ${\eta }_{m}$, and metals ${\eta }_{Z}$. In this Letter, we compile small-box results, and find consensus that the hot outflows are much more powerful than the cool outflows: (i) hot outflows generally dominate the energy flux, and (ii) their specific energy e_s,h is 10–1000 times higher than cool outflows. Moreover, the properties of hot outflows are remarkably simple: ${e}_{s,h}\propto {\eta }_{E,h}/{\eta }_{m,h}$ is almost invariant over four orders of magnitude of star formation surface density. Also, we find tentatively that ${\eta }_{E,h}/{\eta }_{Z,h}\sim$ 0.5. If corroborated by more simulation data, these correlations reduce the three hot phase loading factors into one. Finally, this one parameter is closely related to whether the ISM has a "breakout" condition. The narrow range of ${e}_{s,h}$ indicates that hot outflows cannot escape dark matter halos with log ${M}_{\mathrm{halo}}\ [{M}_{\odot }]\gtrsim 12$. This mass is also where the galaxy mass–metallicity relation reaches its plateau, implying a deep connection between hot outflows and galaxy formation. We argue that hot outflows should be included explicitly in cosmological simulations and (semi-)analytic modeling of galaxy formation.

Ultra-short-period planets (USPs) provide important clues to planetary formation and migration. It was recently found that the mutual inclinations of the planetary systems are larger if the inner orbits are closer (≲5R_*) and if the planetary period ratios are larger (P₂/P₁ ≳ 5). This suggests that the USPs experienced both inclination excitation and orbital shrinkage. Here we investigate the increase in the mutual inclination due to stellar oblateness. We find that the stellar oblateness (within ∼1 Gyr) is sufficient to enhance the mutual inclination to explain the observed signatures. This suggests that the USPs can migrate closer to the host star in a near coplanar configuration with their planetary companions (e.g., disk migration+tides or in situ+tides), before mutual inclination gets excited due to stellar oblateness.

We focus on two repeating fast radio bursts (FRBs) recently detected by the CHIME/FRB experiment in 2018–2019 (Source 1: 180916.J0158+65, and Source 2: 181030.J1054+73). These sources have low excess dispersion measures ($\lt 100\,\mathrm{pc}\,{\mathrm{cm}}^{-3}$ and $\lt 20\,\mathrm{pc}\,{\mathrm{cm}}^{-3}$, respectively), implying relatively small maximal distances. They were repeatedly observed by AGILE in the MeV–GeV energy range. We do not detect prompt emission simultaneously with these repeating events. This search is particularly significant for the submillisecond and millisecond integrations obtainable by AGILE. The sources are constrained to emit a MeV-fluence in the millisecond range below $F{{\prime} }_{\mathrm{MeV}}={10}^{-8}\,\mathrm{erg}\,{\mathrm{cm}}^{-2}$ corresponding to an isotropic energy near ${E}_{\mathrm{MeV},\mathrm{UL}}\simeq 2\times {10}^{46}$ erg for a distance of 150 Mpc (applicable to Source 1). We also searched for γ-ray emission for time intervals up to 100 days, obtaining 3σ upper limits (ULs) for the average isotropic luminosity above 50 MeV, ${L}_{\gamma ,\mathrm{UL}}\,\simeq$ (5–10)$\,\times \,{10}^{43}\,\mathrm{erg}\,{{\rm{s}}}^{-1}$. For a source distance near 100 kpc (possibly applicable to Source 2), our ULs imply ${E}_{\mathrm{MeV},\mathrm{UL}}\simeq {10}^{40}\,\mathrm{erg}$, and ${L}_{\gamma ,\mathrm{UL}}\,\simeq$ 2 $\times {10}^{37}\,\mathrm{erg}\,{{\rm{s}}}^{-1}$. Our results are significant in constraining the high-energy emission of underlying sources such as magnetars, or other phenomena related to extragalactic compact objects, and show the prompt emission to be lower than the peak of the 2004 magnetar outburst of SGR 1806-20 for source distances less than about 100 Mpc.