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During a pointed 2018 NuSTAR observation, we detected a flare with a 2.2 hr duration from the magnetar 1RXS J170849.0−400910. The flare, which rose in ∼25 s to a maximum flux 6 times larger than the persistent emission, is highly pulsed with an rms pulsed fraction of 53%. The pulse profile shape consists of two peaks separated by half a rotational cycle, with a peak flux ratio of ∼2. The flare spectrum is thermal with an average temperature of 2.1 keV. Phase-resolved spectroscopy shows that the two peaks possess the same temperature, but differ in size. These observational results, along with simple light curve modeling, indicate that two identical antipodal spots, likely the magnetic poles, are heated simultaneously at the onset of the flare and for its full duration. Hence, the origin of the flare has to be connected to the global dipolar structure of the magnetar. This might best be achieved externally, via twists to closed magnetospheric dipolar field lines seeding bombardment of polar footpoint locales with energetic pairs. Approximately 1.86 hr following the onset of the flare, a short burst with its own 3 minutes thermal tail occurred. The burst tail is also pulsating at the spin period of the source and phase-aligned with the flare profile, implying an intimate connection between the two phenomena. The burst may have been caused by a magnetic reconnection event in the same twisted dipolar field lines anchored to the surface hot spots, with subsequent return currents supplying extra heat to these polar caps.

We derive the longest uniform record of rotational intensities solar coronal magnetic field since 1968 and compare it with the heliospheric magnetic field (HMF) observed at the Earth. We scale the Mount Wilson Observatory and Wilcox Solar Observatory observations of the photospheric magnetic field to the level of the Synoptic Optical Long-term Investigations of the Sun/Vector Spectro Magnetograph and apply the potential field source surface model to calculate the coronal magnetic field. We find that the evolution of the coronal magnetic field during the last 50 yr agrees with the HMF observed at the Earth only if the effective coronal size, the distance of the coronal source surface of the HMF, is allowed to change in time. We calculate the optimum source surface distance for each rotation and find that it experienced an abrupt decrease in the late 1990s. The effective volume of the solar corona shrunk to less than one half during a short period of only a few years. We note that this abrupt shrinking coincides with other changes in solar magnetic fields that are likely related to the decrease of the overall solar activity, i.e., the demise of the Grand Modern Maximum.

To gain insights into long-term active galactic nuclei (AGN) variability, we analyze an AGN sample from the Sloan Digital Sky Survey (SDSS) and compare their photometry with observations from the Hyper Suprime-Cam survey (HSC) observed $\langle 14.85\rangle$ yr after SDSS. On average, the AGN are fainter in HSC than SDSS. We demonstrate that the difference is not due to subtle differences in the SDSS versus HSC filters or photometry. The decrease in mean brightness is redshift dependent, consistent with expectations for a change that is a function of the rest-frame time separation between observations. At a given redshift, the mean decrease in brightness is stronger for more luminous AGN and for objects with longer time separation between measurements. We demonstrate that the dependence on redshift and luminosity of measured mean brightness decrease is consistent with simple models of Eddington ratio variability in AGN on long (Myr, Gyr) timescales. We show how our results can be used to constrain the variability and demographic properties of AGN populations.

2I/Borisov is the first-ever observed interstellar comet (and the second detected interstellar object (ISO)). It was discovered on 2019 August 30 and has a heliocentric orbital eccentricity of ∼3.35, corresponding to a hyperbolic orbit that is unbound to the Sun. Given that it is an ISO, it is of interest to compare its properties—such as composition and activity—with the comets in our solar system. This study reports low-resolution optical spectra of 2I/Borisov. The spectra were obtained by the MDM Observatory Hiltner 2.4 m telescope/Ohio State Multi-Object Spectrograph (on 2019 October 31.5 and November 4.5, UT). The wavelength coverage spanned from 3700  to 9200 Å. The dust continuum reflectance spectra of 2I/Borisov show that the spectral slope is steeper in the blue end of the spectrum (compared to the red). The spectra of 2I/Borisov clearly show CN emission at 3880 Å, as well as C₂ emission at both 4750  and 5150 Å. Using a Haser model to covert the observed fluxes into estimates for the molecular production rates, we find Q(CN) = 2.4 ± 0.2 × 10²⁴ s⁻¹, and Q(C₂) = (5.5 ± 0.4) × 10²³ s⁻¹ at the heliocentric distance of 2.145 au. Our Q(CN) estimate is consistent with contemporaneous observations, and the Q(C₂) estimate is generally below the upper limits of previous studies. We derived the ratio Q(C₂)/Q(CN) = 0.2 ± 0.1, which indicates that 2I/Borisov is depleted in carbon-chain species, but is not empty. This feature is not rare for the comets in our solar system, especially in the class of Jupiter-family comets.

The well-known age–metallicity-attenuation degeneracy does not permit unique and good estimates of basic parameters of stars and stellar populations. The effects of dust can be avoided using spectral line indices, but current methods have not been able to break the age–metallicity degeneracy. Here we show that using at least two new spectral line indices defined and measured on high-resolution (R = 6000) spectra of a signal-to-noise ratio (S/N) ≥ 10, one gets unambiguous estimates of the age and metallicity of intermediate to old stellar populations. Spectroscopic data retrieved with new astronomical facilities, e.g., X-shooter, MEGARA, and MOSAIC, can be employed to infer the physical parameters of the emitting source by means of spectral line index and index–index diagram analysis.

A hoop force driven magnetic Rayleigh–Taylor instability (MRTI) is observed in a laboratory experiment that simulates a solar coronal loop. Increase of the axial wavelength λ is observed when the axial magnetic field increases. This scaling is consistent with the theoretical MRTI growth rate ${\gamma }^{2}={gk}-2{\left({\boldsymbol{k}}\cdot {{\boldsymbol{B}}}_{0}\right)}^{2}/{\mu }_{0}\rho$, which implies that if ${\boldsymbol{k}}$ is parallel to ${{\boldsymbol{B}}}_{0}$ (i.e., undular mode), the fastest-growing mode has $\lambda =2\pi /k=8\pi {B}_{0}^{2}/{\mu }_{0}\rho g$.

The cometary mission Rosetta has shown the presence of higher-than-expected suprathermal electron fluxes. In this study, using 3D fully kinetic electromagnetic simulations of the interaction of the solar wind with a comet, we constrain the kinetic mechanism that is responsible for the bulk electron energization that creates the suprathermal distribution from the warm background of solar wind electrons. We identify and characterize the magnetic field-aligned ambipolar electric field that ensures quasi-neutrality and traps warm electrons. Solar wind electrons are accelerated to energies as high as 50–70 eV close to the comet nucleus without the need for wave–particle or turbulent heating mechanisms. We find that the accelerating potential controls the parallel electron temperature, total density, and (to a lesser degree) the perpendicular electron temperature and the magnetic field magnitude. Our self-consistent approach enables us to better understand the underlying plasma processes that govern the near-comet plasma environment.

Since the onset of the "space revolution" of high-precision high-cadence photometry, asteroseismology has been demonstrated as a powerful tool for informing Galactic archeology investigations. The launch of the NASA Transiting Exoplanet Survey Satellite (TESS) mission has enabled seismic-based inferences to go full sky—providing a clear advantage for large ensemble studies of the different Milky Way components. Here we demonstrate its potential for investigating the Galaxy by carrying out the first asteroseismic ensemble study of red giant stars observed by TESS. We use a sample of 25 stars for which we measure their global asteroseimic observables and estimate their fundamental stellar properties, such as radius, mass, and age. Significant improvements are seen in the uncertainties of our estimates when combining seismic observables from TESS with astrometric measurements from the Gaia mission compared to when the seismology and astrometry are applied separately. Specifically, when combined we show that stellar radii can be determined to a precision of a few percent, masses to 5%–10%, and ages to the 20% level. This is comparable to the precision typically obtained using end-of-mission Kepler data.

The binary black hole mergers observed by Laser Interferometer Gravitational-Wave Observatory (LIGO)–Virgo gravitational-wave detectors pose two major challenges: (i) how to produce these massive black holes from stellar processes; and (ii) how to bring them close enough to merge within the age of the universe? We derive a fundamental constraint relating the binary separation and the available stellar budget in the universe to produce the observed black hole mergers. We find that ≲14% of the entire budget contributes to the observed merger rate of (30+30) M_⊙ black holes, if the separation is around the diameter of their progenitor stars. Furthermore, the upgraded LIGO detector and third-generation gravitational-wave detectors are not expected to find stellar-mass black hole mergers at high redshifts. From LIGO's strong constraints on the mergers of black holes in the pair-instability mass gap (60–120 M_⊙), we find that ≲0.8% of all massive stars contribute to a remnant black hole population in this gap. Our derived separation–budget constraint provides a robust framework for testing the formation scenarios of stellar binary black holes.

We have analyzed AstroSat observations of the galactic microquasar system GRS 1915+105, when the system exhibited C-type quasi-periodic oscillations (QPOs) in the frequency range of 3.4–5.4 Hz. The broadband spectra (1–50 keV) obtained simultaneously from the Large Area X-ray Proportional Counter and Soft X-ray Telescope can be well described by a dominant relativistic truncated accretion disk along with thermal Comptonization and reflection. We find that while the QPO frequency depends on the inner radii with a large scatter, a much tighter correlation is obtained when both the inner radii and accretion rate of the disk are taken into account. In fact, the frequency varies just as the dynamic frequency (i.e., the inverse of the sound crossing time) does as predicted decades ago by the relativistic standard accretion disk theory for a black hole with a spin parameter of ∼0.9. We show that this identification has been possible due to the simultaneous broadband spectral coverage with temporal information as obtained from AstroSat.

We report the discovery of an extreme X-ray flux rise (by a factor of ≳20) of the weak-line quasar Sloan Digital Sky Survey (SDSS) J153913.47+395423.4 (hereafter SDSS J1539+3954) at z = 1.935. SDSS J1539+3954 is the most-luminous object among radio-quiet type 1 active galactic nuclei (AGNs) where such dramatic X-ray variability has been observed. Before the X-ray flux rise, SDSS J1539+3954 appeared X-ray weak compared with the expectation from its ultraviolet (UV) flux; after the rise, the ratio of its X-ray flux and UV flux is consistent with the majority of the AGN population. We also present a contemporaneous HET spectrum of SDSS J1539+3954, which demonstrates that its UV continuum level remains generally unchanged despite the dramatic increase in the X-ray flux, and its C iv emission line remains weak. The dramatic change only observed in the X-ray flux is consistent with a shielding model, where a thick inner accretion disk can block our line of sight to the central X-ray source. This thick inner accretion disk can also block the nuclear ionizing photons from reaching the high-ionization broad emission-line region, so that weak high-ionization emission lines are observed. Under this scenario, the extreme X-ray variability event may be caused by slight variations in the thickness of the disk. This event might also be explained by gravitational light-bending effects in a reflection model.

The composition of comets in the solar system comes in multiple groups thought to encode information about their formation in different regions of the outer protosolar disk. The recent discovery of the second interstellar object, 2I/Borisov, allows for spectroscopic investigations into its gas content and a preliminary classification of it within the solar system comet taxonomies to test the applicability of planetesimal formation models to other stellar systems. We present spectroscopic and imaging observations from 2019 September 20 through October 26 from the Bok, MMT telescope (formerly the Multiple Mirror Telescope, Mount Hopkins, Arizona), and Large Binocular Telescopes. We identify CN in the comet's spectrum and set precise upper limits on the abundance of C₂ on all dates in October. We use a Haser model to convert our integrated fluxes to production rates and find Q(CN) = (1.1–1.9) ∗ 10²⁴ mols s⁻¹ increasing over 2019 October 1 to 26, consistent with contemporaneous observations. We set our lowest upper limit on a C₂ production rate, Q(C₂) < 1.6 ∗ 10²³ mols s⁻¹ on 2019 October 10. The measured upper limit ratio for that date Q(C₂)/Q(CN) < 0.1 indicates that 2I/Borisov is strongly in the (carbon-chain) "depleted" taxonomic group if there is any C₂ production at all. Most "depleted" comets are Jupiter-family comets (JFCs), perhaps indicating a similarity in formation conditions between the most depleted of the JFCs and 2I/Borisov. More work is needed to understand the applicability of our knowledge of solar system comet taxonomies onto interstellar objects and we discuss future work that could help to clarify the usefulness of the approach.