adsabs/SciX_UAT_keywords huggingface dataset:
Stochastic Averaging of Radiative Transfer Coefficients for Relativistic Electrons

Synchrotron emissivities, absorptivities, and Faraday rotation and conversion coefficients are needed in modeling a variety of astrophysical sources, including Event Horizon Telescope (EHT) sources. We develop a method for estimating transfer coefficients that exploits their linear dependence on the electron distribution function, decomposing the distribution function into a sum of parts each of whose emissivity can be calculated easily. We refer to this procedure as stochastic averaging and apply it in two contexts. First, we use it to estimate the emissivity of an isotropic κ distribution function with a high-energy cutoff. The resulting coefficients can be evaluated efficiently enough to be used directly in ray-tracing calculations, and we provide an example calculation. Second, we use stochastic averaging to assess the effect of subgrid turbulence on the volume-averaged emissivity and along the way provide a prescription for a turbulent emissivity. We find that for parameters appropriate to EHT sources turbulence reduces the emissivity slightly. In the infrared, turbulence can dramatically increase the emissivity.

Keywords: black holes; magnetohydrodynamics; radiative processes; radiative transfer
Using Tianwen-1 Differential One-way Range Signals to Probe Corotating Interaction Regions: Case Studies

The study reports the first detection of corotating interaction regions (CIRs) in the solar wind by the differential one-way range (DOR) signals transmitted by Tianwen-1. Differential phase delay is calculated for the DOR signals. Variations of the total electron content (TEC) along the signal path between Tianwen-1 and the ground stations are then derived from changes in the differential phase delay. Large TEC variations are found for 2021 February 11, 18, and 19 even after the influence from the Earth's ionosphere has been removed. In situ solar wind measurements and the corresponding simulation results further show that the TEC variations are caused by the CIRs that passed through the signal path on these days. The study demonstrates that CIR structures can be probed using deep-space spacecraft range signals. Such measurements can be used to constrain global solar wind forecast models.

Keywords: solar wind; corotating streams; deep space probes; radio astronomy; remote sensing
Dynamics and Morphology of Cold Gas in Fast, Radiatively Cooling Outflows: Constraining AGN Energetics with Horseshoes

Warm ionized and cold neutral outflows with velocities exceeding 100 km s-1 are commonly observed in galaxies and clusters. However, theoretical studies indicate that ram pressure from a hot wind, driven either by the central active galactic nucleus (AGN) or a starburst, cannot accelerate existing cold gas to such high speeds without destroying it. In this work we explore a different scenario, where cold gas forms in a fast, radiatively cooling outflow with temperature T ≲ 107 K. Using 3D hydrodynamic simulations, we demonstrate that cold gas continuously fragments out of the cooling outflow, forming elongated filamentary structures extending tens of kiloparsecs. For a range of physically relevant temperature and velocity configurations, a ring of cold gas perpendicular to the direction of motion forms in the outflow. This naturally explains the formation of transverse cold gas filaments such as the blue loop and the horseshoe filament in the Perseus cluster. Based on our results, we estimate that the AGN outburst responsible for the formation of these two features drove bipolar outflows with velocity >2000 km s-1 and total kinetic energy >8 × 1057 erg about ~10 Myr ago. We also examine the continuous cooling in the mixing layer between hot and cold gas, and find that radiative cooling only accounts for ~10% of the total mass cooling rate, indicating that observations of soft X-ray and far-ultraviolet emission may significantly underestimate the growth of cold gas in the cooling flow of galaxy clusters.

Keywords: galaxy winds; active galactic nuclei; intracluster medium; filamentary nebulae; cooling flows
Ultradeep ATCA Imaging of 47 Tucanae Reveals a Central Compact Radio Source

We present the results of an ultradeep radio continuum survey, containing ~480 hr of observations, of the Galactic globular cluster 47 Tucanae with the Australia Telescope Compact Array. This comprehensive coverage of the cluster allows us to reach rms noise levels of 1.19 μJy beam-1 at 5.5 GHz, 940 nJy beam-1 at 9 GHz, and 790 nJy beam-1 in a stacked 7.25 GHz image. This is the deepest radio image of a globular cluster and the deepest image ever made with the Australia Telescope Compact Array. We identify ATCA J002405.702-720452.361, a faint (6.3 ± 1.2 μJy at 5.5 GHz, 5.4 ± 0.9 μJy at 9 GHz), flat-spectrum (α = -0.31 ± 0.54) radio source that is positionally coincident with the cluster center and potentially associated with a faint X-ray source. No convincing optical counterpart was identified. We use radio, X-ray, optical, and UV data to show that explanations involving a background active galactic nucleus, a chromospherically active binary, or a binary involving a white dwarf are unlikely. The most plausible explanations are that the source is an undiscovered millisecond pulsar or a weakly accreting black hole. If the X-ray source is associated with the radio source, the fundamental plane of black-hole activity suggests a black hole mass of ~54-6000 M ⊙, indicating an intermediate-mass black hole or a heavy stellar-mass black hole.

Keywords: black holes; globular star clusters; intermediate-mass black holes; millisecond pulsars; radio astronomy
Bursts before Burst: A Comparative Study on FRB 200428-associated and FRB-absent X-Ray Bursts from SGR J1935+2154

Accompanied by an X-ray burst, the fast radio burst (FRB) FRB 200428 was recently confirmed as originating from the Galactic magnetar soft gamma repeater (SGR) SGR J1935+2154. Just before and after FRB 200428 was detected, the Five-hundred-meter Aperture Spherical radio Telescope (FAST) had been monitoring SGR J1935+2154 for eight hours. From UTC 2020 April 27 23:55:00 to 2020 April 28 00:50:37, FAST detected no pulsed radio emission from SGR J1935+2154, while Fermi/Gamma-ray Burst Monitor registered 34 bursts in the X/soft γ-ray band, forming a unique sample of X-ray bursts in the absence of FRBs. After a comprehensive analysis on light curves, time-integrated, and time-resolved spectral properties of these FRB-absent X-ray bursts, we compare this sample with the FRB-associated X-ray burst detected by Insight-HXMT, INTEGRAL, and Konus-Wind. The FRB-associated burst distinguishes itself from other X-ray bursts by its nonthermal spectrum and a higher spectral peak energy, but otherwise is not atypical. We also compare the cumulative energy distribution of our X-ray burst sample with that of first repeating FRB source, FRB 121102, with the calibration of FRB 200428-X-ray burst association. We find a similarity between the two, offering indirect support of the magnetar origin of cosmological FRBs. The event rate density of magnetar bursts is about ∼150 times higher than the FRB event rate density at the energy of FRB 200428. This again suggests that, if all FRBs originate from magnetars, only a small fraction of X-ray bursts are associated with FRBs.

Keywords: magnetars; soft gamma-ray repeaters; radio transient sources
Polarization in the GG Tau Ring-Confronting Dust Self-scattering, Dust Mechanical and Magnetic Alignment, Spirals, and Dust Grain Drift

We report Atacama Large Millimeter/submillimeter Array (ALMA) polarization observations at 3 and 0.9 mm toward the GG Tau A system. In the ring, the percentage is relatively homogeneous at 3 mm, being 1.2%, while it exhibits a clear radial variation at 0.9 mm with a mean increasing from 0.6% to 2.8% toward larger radius (r). The polarization orientation at r > 1.″85 appears nearly azimuthal at both wavelengths. At r < 1.″85, the pattern remains azimuthal at 3 mm but becomes radial at 0.9 mm. The dust self-scattering model with ${a}_{\max }$ of 1 mm could reproduce the observed polarization orientation and percentage at 0.9 mm, but the expected polarization percentage at 3 mm would be 0.2%, much smaller than the detected 1.2%. Dust alignment with poloidal magnetic field could qualitatively reproduce the flip in polarization at r < 1.″85 and also the detected polarization percentage. A closer inspection of the nearly azimuthal pattern reveals that polarization orientations are systematically deviating by -9.°0 ± 1.°2 from the tangent of the orbit ellipses. This deviation agrees with the direction of the spiral pattern observed in the near-infrared, but it is unclear how dust grains could be aligned along such spirals. For the scenario where the -9° deviation (-7.°3 after considering the inclination effect) measures the radial component of the dust drift motion, the expected inward drifting velocity would be ~12.8% of the Keplerian speed, a factor of 2.8 larger than the theoretical predictions. Possible additional interpretations of the polarization are discussed, but there is no single mechanism that could explain the detected polarization simultaneously.

Keywords: interstellar medium; circumstellar disks
A New Timescale–Mass Scaling for the Optical Variation of Active Galactic Nuclei across the Intermediate-mass to Supermassive Scales

The variability of active galactic nuclei (AGNs) has long been servicing as an essential avenue of exploring the accretion physics of a black hole (BH). There are two commonly used methods for analyzing AGN variability. First, the AGN variability, characterized by the structure function (SF) of a single band, can be well described by a damped random walk (DRW) process on timescales longer than ∼weeks, shorter than which departures have been reported. Second, the color variation (CV) between two bands behaves timescale dependent, raising challenges to the widely accepted reprocessing scenario. However, both the departure from the DRW process and the timescale-dependent CV are hitherto limited to AGNs, mainly quasars, at the supermassive scale. Here, utilizing the high-cadence multiwavelength monitoring on NGC 4395 harboring an intermediate-mass BH, we unveil at the intermediate-mass scale for the first time, prominent departures from the DRW process at timescales shorter than ∼hours in all three nights and bands, and plausible timescale-dependent CVs in the two longest nights of observation. Furthermore, comparing SFs of NGC 4395 to four AGNs at the supermassive scale, we suggest a new scaling relation between the timescale (τ; across nearly 3 orders of magnitude) and the BH mass (M BH): where the exponent γ is likely ≃0.6–0.8. This exponent differs from most previous measurements, but confirms a few, and is consistent with a recent theoretical prediction, suggesting a similar accretion process in AGNs across different mass scales.

Keywords: active galactic nuclei; intermediate-mass black holes; supermassive black holes; accretion
Possible Systematic Rotation in the Mature Stellar Population of a z = 9.1 Galaxy

We present new observations with the Atacama Large Millimeter/submillimeter Array for a gravitationally lensed galaxy at z = 9.1, MACS1149-JD1. [O III] 88 μm emission is detected at 10σ with a spatial resolution of ~0.3 kpc in the source plane, enabling the most distant morphokinematic study of a galaxy. The [O III] emission is distributed smoothly without any resolved clumps and shows a clear velocity gradient with ΔV obs/2σ tot = 0.84 ± 0.23, where ΔV obs is the observed maximum velocity difference and σ tot is the velocity dispersion measured in the spatially integrated line profile, suggesting a rotating system. Assuming a geometrically thin self-gravitating rotation disk model, we obtain ${V}_{\mathrm{rot}}/{\sigma }_{V}={0.67}_{-0.26}^{+0.73}$ , where V rot and σ V are the rotation velocity and velocity dispersion, respectively, still consistent with rotation. The resulting disk mass of ${0.65}_{-0.40}^{+1.37}\times {10}^{9}$ M ⊙ is consistent with being associated with the stellar mass identified with a 300 Myr old stellar population independently indicated by a Balmer break in the spectral energy distribution. We conclude that the most of the dynamical mass is associated with the previously identified mature stellar population that formed at z ~ 15.

Keywords: galaxy formation; galaxy dynamics; galaxy evolution; high-redshift galaxies
MPS-ATLAS Library of Stellar Model Atmospheres and Spectra

Stellar spectra contain a large amount of information about the conditions in stellar atmospheres. However, extracting this information is challenging and demands comprehensive numerical modeling. Here, we present stellar spectra synthesized using the recently developed state-of-the-art MPS-ATLAS code on a fine grid of stellar fundamental parameters. These calculations have been extensively validated against solar and stellar observations and can be used for various astrophysical applications. The spectra are available at the Max Planck Digital Library (https://doi.org/10.17617/3.NJ56TR) and have been also incorporated into the ExoTiC-LD python package (https://github.com/Exo-TiC/ExoTiC-LD/ which returns stellar limb darkening coefficients used for the software package Exoplanet Timeseries Characterisation (ExoTic).

Keywords: stellar atmospheres; stellar photospheres
ALCHEMI Finds a "Shocking" Carbon Footprint in the Starburst Galaxy NGC 253

The centers of starburst galaxies may be characterized by a specific gas and ice chemistry due to their gas dynamics and the presence of various ice desorption mechanisms. This may result in a peculiar observable composition. We analyse the abundances of CO2, a reliable tracer of ice chemistry, from data collected as part of the Atacama Large Millimeter/submillimeter Array large program ALCHEMI, a wide-frequency spectral scan toward the starburst galaxy NGC 253 with an angular resolution of 1.″6. We constrain the CO2 abundances in the gas phase using its protonated form HOCO+. The distribution of HOCO+ is similar to that of methanol, which suggests that HOCO+ is indeed produced from the protonation of CO2 sublimated from ice. The HOCO+ fractional abundances are found to be (1-2) × 10-9 at the outer part of the central molecular zone (CMZ), while they are lower (~10-10) near the kinematic center. This peak fractional abundance at the outer CMZ is comparable to that in the Milky Way CMZ, and orders of magnitude higher than that in Galactic disk, star-forming regions. From the range of HOCO+/CO2 ratios suggested from chemical models, the gas-phase CO2 fractional abundance is estimated to be (1-20) × 10-7 at the outer CMZ, and orders of magnitude lower near the center. We estimate the CO2 ice fractional abundances at the outer CMZ to be (2-5) × 10-6 from the literature. A comparison between the ice and gas CO2 abundances suggests an efficient sublimation mechanism. This sublimation is attributed to large-scale shocks at the orbital intersections of the bar and CMZ.

Keywords: astrochemistry; interstellar medium; starburst galaxies; interstellar molecules; molecular clouds
The APO-K2 Catalog. II. Accurate Stellar Ages for Red Giant Branch Stars across the Milky Way

We present stellar age determinations for 4661 red giant branch stars in the APO-K2 catalog, derived using mass estimates from K2 asteroseismology from the K2 Galactic Archaeology Program and elemental abundances from the Apache Point Galactic Evolution Experiment survey. Our sample includes 17 of the 19 fields observed by K2, making it one of the most comprehensive catalogs of accurate stellar ages across the Galaxy in terms of the wide range of populations spanned by its stars, enabling rigorous tests of Galactic chemical evolution models. Taking into account the selection functions of the K2 sample, the data appear to support the age‑chemistry morphology of stellar populations predicted by both inside-out and late-burst scenarios. We also investigate trends in age versus stellar chemistry and Galactic position, which are consistent with previous findings. Comparisons against APOKASC-3 asteroseismic ages show agreement to within ∼3%. We also discuss offsets between our ages and spectroscopic ages. Finally, we note that ignoring the effects of α-enhancement on stellar opacity (either directly or with the Salaris metallicity correction) results in an ∼10% offset in age estimates for the most α-enhanced stars, which is an important consideration for continued tests of Galactic models with this and other asteroseismic age samples.

Keywords: stellar ages; asteroseismology; stellar abundances; milky way evolution; milky way formation; galaxy stellar content; red giant stars; stellar evolutionary models
A Broadband View on Microquasar MAXI J1820+070 during the 2018 Outburst

The microquasar MAXI J1820+070 went into outburst from 2018 mid-March until mid-July, with several faint rebrightenings afterward. With a peak flux of approximately 4 Crab in the 20-50 keV energy range, the source was monitored across the electromagnetic spectrum with detections from radio to hard X-ray frequencies. Using these multiwavelength observations, we analyzed quasi-simultaneous observations from April 12, near the peak of the outburst (∼March 23). Analysis of the X-ray spectrum found it indicative of an accreting black hole binary in the hard state, consistent with the flat/inverted radio spectrum and the accretion disk winds seen at optical wavelengths. Then, we constructed a spectral energy distribution spanning ∼12 orders of magnitude using modeling in JetSeT. The model is composed of an irradiated disk with a Compton hump and a leptonic jet with an acceleration region and a synchrotron-dominated cooling region. JetSeT finds that the spectrum is dominated by jet emission up to approximately 1014 Hz, after which disk and coronal emission dominates. The acceleration region has a magnetic field of B ∼ 1.6 × 104 G, a cross section of R ∼ 2.8 × 109 cm, and a flat radio spectral shape naturally obtained from the synchroton cooling of the accelerated electrons. The jet luminosity is >8 × 1037 erg s-1 (>0.15LEdd), compared to an accretion luminosity of ∼6 × 1037 erg s-1, assuming a distance of 3 kpc. Because these two values are comparable, it is possible that the jet is powered predominately via accretion with only a small contribution needed from the Blanford-Znajek mechanism from the reportedly slowly spinning black hole.

Keywords: x-ray transient sources; stellar mass black holes; spectral energy distribution
Measuring Periods in Aperiodic Light Curves-Applying the GPS Method to Infer the Rotation Periods of Solar-like Stars

Light curves of solar-like stars are known to show highly irregular variability. As a consequence, standard frequency analysis methods often fail to detect the correct rotation period. Recently, Shapiro et al. showed that the periods of such stars could still be measured by considering the Gradient of the Power Spectrum (GPS) instead of the power spectrum itself. In this study, the GPS method is applied to model light curves of solar-like stars covering all possible inclination angles and a large range of metallicities and observational noise levels. The model parameters are chosen such that they resemble those of many stars in the Kepler field. We show that the GPS method is able to detect the correct rotation period in ≈40% of all considered cases, which is more than 10 times higher than the detection rate of standard techniques. Thus, we conclude that the GPS method is ideally suited to measure periods of those Kepler stars lacking such a measurement so far. We also show that the GPS method is significantly superior to autocorrelation methods when starspot lifetimes are shorter than a few rotation periods. GPS begins to yield rotation periods that are too short when dominant spot lifetimes are shorter than one rotation period. We conclude that new methods are generally needed to reliably detect rotation periods from sufficiently aperiodic time series-these periods will otherwise remain undetected.

Keywords: stellar rotation
Analysis of SEP Events and Their Possible Precursors Based on the GSEP Catalog

Solar energetic particle (SEP) events are one of the most crucial aspects of space weather. Their prediction depends on various factors including the source solar eruptions such as flares and coronal mass ejections (CMEs). The Geostationary Solar Energetic Particle (GSEP) events catalog was developed as an extensive data set toward this effort for solar cycles 22, 23, and 24. In the present work, we review and extend the GSEP data set by (1) adding "weak" SEP events that have proton enhancements from 0.5 to 10 pfu in the E >10 MeV channel and (2) improving the associated solar source eruptions information. We analyze and discuss spatiotemporal properties such as flare magnitudes, locations, rise times, and speeds and widths of CMEs. We check for the correlation of these parameters with peak proton fluxes and event fluences. Our study also focuses on understanding feature importance toward the optimal performance of machine-learning (ML) models for SEP event forecasting. We implement random forest, extreme gradient boosting, logistic regression, and support vector machine classifiers in a binary classification schema. Based on the evaluation of our best models, we find both the flare and CME parameters are requisites to predict the occurrence of an SEP event. This work is a foundation for our further efforts on SEP event forecasting using robust ML methods.

Keywords: solar energetic particles
High-energy Studies of the 3HWC J1954+286 Region: Likely Gamma-Ray Detection of the Supernova Remnant G65.1+0.6

We carry out high-energy studies of the region of the Galactic TeV source 3HWC J1954+286, whose location coincides with those of PSR J1954+2836 and supernova remnant (SNR) G65.1+0.6. Analyzing the GeV γ-ray data obtained with the Large Area Telescope on board the Fermi Gamma-ray Space Telescope, we are able to separate the pulsar's emission from that of the region. Excess power-law-like emission of a ~6σ significance level at the region is found, which we explain as arising from the SNR G65.1+0.6. Given the low-significance detection, either a hadronic or a leptonic model can provide a fit to the power-law spectrum. Considering the properties of the pulsar and the SNR, we discuss the possible origin of the TeV source, and suggest that it is likely to be the TeV halo associated with the pulsar.

Keywords: gamma-rays; pulsars; supernova remnants
The Gliese 86 Binary System: A Warm Jupiter Formed in a Disk Truncated at ≈2 au

Gliese 86 is a nearby K dwarf hosting a giant planet on a ≈16 day orbit and an outer white dwarf companion on a ≈century-long orbit. In this study we combine radial velocity data (including new measurements spanning more than a decade) with high angular resolution imaging and absolute astrometry from Hipparcos and Gaia to measure the current orbits and masses of both companions. We then simulate the evolution of the Gl 86 system to constrain its primordial orbit when both stars were on the main sequence; the closest approach between the two stars was then about 9 au. Such a close separation limited the size of the protoplanetary disk of Gl 86 A and dynamically hindered the formation of the giant planet around it. Our measurements of Gl 86 B and Gl 86 Ab's orbits reveal Gl 86 as a system in which giant planet formation took place in a disk truncated at ≈2 au. Such a disk would be just big enough to harbor the dust mass and total mass needed to assemble Gl 86 Ab's core and envelope, assuming a high disk accretion rate and a low viscosity. Inefficient accretion of the disk onto Gl 86 Ab, however, would require a disk massive enough to approach the Toomre stability limit at its outer truncation radius. The orbital architecture of the Gl 86 system shows that giant planets can form even in severely truncated disks and provides an important benchmark for planet formation theory.

Keywords: protoplanetary disks; planetary system formation; planet formation; stellar evolution; close binary stars