Sazuna/UAT_keywords huggingface dataset:
Late-time Radio Flares in Tidal Disruption Events. Radio monitoring unveiled late (hundreds to a thousand days) radio flares in a significant fraction of tidal disruption events. We propose that these late-time radio flares are a natural outcome if the surrounding density profile flattens outside the Bondi radius. At the Bondi radius, the outflow is optically thin (above a few GHz) to synchrotron self-absorption. As more and more material is swept up, the radio emission rises asymptotically as ∝ t 3 until the outflow begins to decelerate. A detection of such a rise and a late-time maximum constrains the black hole mass and the mass and energy of the radio-emitting outflow. We show that this model can give reasonable fits to some observed light curves, leading to reasonable estimates of the black hole and outflow masses. We also find that the slope of the density profile within the Bondi radius determines whether an early-time (∼102 days) radio peak exists.

Keywords: Tidal disruption, Transient sources, Radio transient sources
The Dynamics of Debris Disk Creation in Neutron Star Mergers. The detection of GW170817/AT2017gfo inaugurated an era of multimessenger astrophysics, in which gravitational-wave and multiwavelength photon observations complement one another to provide unique insight into astrophysical systems. A broad theoretical consensus exists, in which the photon phenomenology of neutron star mergers largely rests upon the evolution of the small amount of matter left on bound orbits around the black hole or massive neutron star remaining after the merger. Because this accretion disk is far from inflow equilibrium, its subsequent evolution depends very strongly on its initial state, yet very little is known about how this state is determined. Using both snapshot and tracer particle data from a numerical relativity/MHD simulation of an equal-mass neutron star merger that collapses to a black hole, we show how gravitational forces arising in a nonaxisymmetric, dynamical spacetime supplement hydrodynamical effects in shaping the initial structure of the bound debris disk. The work done by hydrodynamical forces is ∼10 times greater than that due to time-dependent gravity. Although gravitational torques prior to remnant relaxation are an order of magnitude larger than hydrodynamical torques, their intrinsic sign symmetry leads to strong cancellation; as a result, hydrodynamical and gravitational torques have a comparable effect. We also show that the debris disk's initial specific angular momentum distribution is sharply peaked at roughly the specific angular momentum of the merged neutron star's outer layers, a few r  g  c, and identify the regulating mechanism.

Keywords: Stellar accretion disks, Neutron stars, General relativity, Black holes
The Potential for Organic Synthesis in the Ocean of Enceladus. The Cassini spacecraft detected a soup of organics in the plume of Saturn's moon Enceladus. Those compounds could provide building blocks for the potential emergence or sustenance of microbial life in Enceladus' subsurface ocean. However, the sources and stabilities of organics in Enceladus' ocean are still poorly understood. Here, we perform nonequilibrium thermodynamic calculations to assess the energetics of abiotic synthesis for a broad spectrum of small organic molecules under both cold oceanic and hydrothermal conditions on Enceladus. Most of the organics that we studied are thermodynamically favorable to synthesize at micromolal dissolved concentrations over wide ranges of pH (8.5–11) and redox conditions. This suggests that many of the organic compounds detected by Cassini may be derived from reactions of CO2 and H2. However, some widely assumed precursors of biomolecules, such as formaldehyde, hydrogen cyanide, and acetylene, are found to be unstable and therefore unfavorable to be synthesized. We found that higher temperatures also seem to favor the synthesis of organic species under Enceladus hydrothermal conditions. Detection of thermodynamically unstable species in the plume might reflect artifacts introduced during high-speed spacecraft flybys and/or active synthesis via degradation of primordial chondritic organics, or biological activities.

Keywords: Enceladus, Astrobiology, Saturnian satellites
Multiwavelength Observations of Sgr A*. II. 2019 July 21 and 26. We report on the final two days of a multiwavelength campaign of Sgr A* observing in the radio, submillimeter, infrared (IR), and X-ray bands in 2019 July. Sgr A* was remarkably active, showing multiple flaring events across the electromagnetic spectrum. We detect a transient ∼35 minute periodicity feature in Spitzer light curves on 2019 July 21. Time-delayed emission was detected in Atacama Large Millimeter/submillimeter Array light curves, suggesting a hotspot within the accretion flow on a stable orbit. On the same night, we observe a decreased flux in the submillimeter light curve following an X-ray flare detected by Chandra, and we model the feature with an adiabatically expanding synchrotron hotspot occulting the accretion flow. The event is produced by a plasma 0.55 R S in radius with an electron spectrum p = 2.84. It is threaded by a ∼130 Gauss magnetic field and expands at 0.6% the speed of light. Finally, we reveal an unambiguous flare in the IR, submillimeter, and radio, demonstrating that the variable emission is intrinsically linked. We jointly fit the radio and submillimeter light curves using an adiabatically expanding synchrotron hotspot and find it is produced by a plasma with an electron spectrum p = 0.59, 187 Gauss magnetic field, and radius 0.47 R S that expands at 0.029c. In both cases, the uncertainty in the appropriate lower and upper electron energy bounds may inflate the derived equipartition field strengths by a factor of 2 or more. Our results confirm that both synchrotron- and adiabatic-cooling processes are involved in the variable emission's evolution at submillimeter and IR wavelengths.

Keywords: Supermassive black holes, Galactic center, Active galactic nuclei
Direct Evidence of the Hybrid Nature of Extreme-ultraviolet Waves and the Reflection of the Fast-mode Wave. We performed an analysis of the extreme-ultraviolet (EUV) wave event on 2022 March 31. The event originated from active region (AR) 12975 located at N13W52 in the field of view of the Atmospheric imaging Assembly (AIA) and exactly at the west limb viewed by the EUV Imager (EUVI) of the Solar Terrestrial Relations Observatory-Ahead (STEREO-A) satellite. The EUV wave was associated with an M9.6 class flare. The event was also well observed by Mauna Loa Solar Observatory and COR1 coronagraphs. We revealed here evident coexistence of two components of EUV waves in AIA as well as in EUVI images, i.e., a fast-mode wave and a nonwave, which was predicted by the EUV wave hybrid model. The speeds of the fast-mode and nonwave EUV wave components in AIA vary from ∼430 to 658 km s‑1 and ∼157 to 205 km s‑1, respectively. The computed speeds in STEREO-A for the fast-mode wave and nonwave components are ∼520 and ∼152 km s‑1, respectively. Another wave emanated from the source AR and interacted with ambient coronal loops, showing evident reflection in the EUV images above the solar limb. The speed of the reflected wave in the plane of the sky is ∼175 km s‑1. With the precise alignments, we found that the fast-mode EUV wave is just ahead of the coronal mass ejection (CME), and the nonwave component is cospatial with the frontal loop of the accompanied CME. The event also showed stationary fronts.

Keywords: Solar coronal waves, Solar flares, Solar coronal mass ejections
On the Magnetic Braking Law in Black Hole Low-mass X-Ray Binaries. Magnetic braking (MB) plays an important role in the evolution of close low-mass X-ray binaries (LMXBs). It is also essential to the formation of ultracompact X-ray binaries (UCXBs). There have been lively investigations on the MB mechanism(s) in both single stars and close binaries including cataclysmic variables and neutron star (NS) LMXBs but with diverse conclusions. In this paper we explore the effect of MB on the black hole (BH) LMXB evolution. We combine binary population synthesis with detailed binary evolution to obtain the expected properties of the Galactic BH LMXB population. The simulated results are compared with the observational data including the BH mass, companion mass, companion temperature, orbital period, and mean accretion rate. Our results reveal that the MB laws with relatively low efficiency (i.e., RM12 and RVJ83) exhibit better agreement with observations, contrary to what was found for NS LMXBs. This raises the interesting question about whether MB really follows the same unified law in different types of binaries. We also predict that only a very small fraction (≲2.5%) of BH LMXBs can evolve to be UCXBs. This explains why no BH UCXB has been discovered so far.

Keywords: Low-mass x-ray binary stars, Stellar mass black holes
Spectra of GRB 221009A at Low Energies Derived from Ground-based Very Low-frequency Measurements. The gamma-ray burst (GRB) event GRB 221009A was the brightest event that has ever been detected to date. Owing to its unexpected brightness, the temporal and/or spectral information of the prompt emission cannot be accurately measured by many satellites (with the only exception of GECAM-C), since they suffered from significant pulse pileup and data saturation effects. Similarly, the X45 solar flare event occurring on 2003 November 4 saturated space-borne X-ray detectors, and it was through ground-based measurements of very low-frequency (VLF) signals that the magnitude of this event was determined, since VLF signals are particularly sensitive to the disturbance on the D-region ionosphere caused by low-energy photons. Therefore, in this study, we first report measurements of VLF signals from the JJI and VTX transmitter as recorded in Shiyan, China, when GRB 221009A occurred. The amplitude change was ∼1.25 and ∼2.31 dB for the JJI and VTX transmitter, respectively. Using a suite of well-validated models, we have further simulated the influence on the D-region ionosphere induced by low-energy photons (&lt;100 keV) of GRB 221009A. Compared with the pre-GRB condition, the electron density was enhanced by 39.75% and 626.61% at 60 and 70 km altitude for the VTX-SYS path and 39.73% and 621.11% at 60 and 70 km altitude for the JJI-SYS path, respectively, with the altitude of notable electron density change being as low as ∼30 km. Moreover, we have compared modeling results of VLF signal change with our measurements during GRB 221009A. The good agreements obtained in terms of amplitude change and overall trend validate the fluxes and spectra of GRB 221009A at low energies (&lt;20 keV) as measured by GECAM-C.

Keywords: Gamma-ray bursts
GRB 221009A/SN 2022xiw: A Supernova Obscured by a Gamma-Ray Burst Afterglow?. We present optical photometry for the afterglow of GRB 221009A, in some respects the most extraordinary gamma-ray burst (GRB) ever observed. Good quality in the R-band light curve is obtained, covering 0.32–19.57 days since the Fermi-Gamma-Ray Burst Monitor trigger. We find that a weak bump emerges from the declining afterglow at t ≈ 11 days; a supernova (SN) may be responsible. We use a smooth broken power-law and 56Ni model to fit the light curve. The best-fitting results reveal that the SN ejected a total mass of M ej = 3.70 M ⊙, a 56Ni mass of M Ni = 0.23 M ⊙, and a kinetic energy of  ESN,K=2.35×1052erg . We also compare GRB 221009A with other GRB–SN events based on a GRB-associated SN sample and find that only SN 2003lw and SN 2011kl can be obviously revealed in the afterglow of GRB 221009A by setting these objects at its distance. This suggests that a supernova (SN 2022xiw) is possibly obscured by the brighter afterglow emission from GRB 221009A.

Keywords: Gamma-ray bursts, Supernovae
An Alternative Channel to Black Hole Low-mass X-Ray Binaries: Dynamical Friction of Dark Matter?. Both the anomalous magnetic braking of Ap/Bp stars and the surrounding circumbinary disk models can account for the formation of black hole (BH) low-mass X-ray binaries (LMXBs), while the simulated effective temperatures of the donor stars are significantly higher than the observed values. Therefore, the formation of BH LMXBs is still not completely understood. In this work, we diagnose whether the dynamical friction between dark matter and the companion stars can drive BH binaries to evolve toward the observed BH LMXBs and alleviate the effective temperature problem. Assuming that there exists a density spike of dark matter around BH, the dynamical friction can produce an efficient angular momentum loss, driving BH binaries with an intermediate-mass companion star to evolve into BH LMXBs for a spike index higher than γ = 1.58. Our detailed stellar evolution models show that the calculated effective temperatures can match the observed value of most BH LMXBs for a spike index range of γ = 1.7–2.1. However, the simulated mass-transfer rates when γ = 2.0 and 2.1 are too high to be consistent with the observed properties showing that BH LMXBs appear as soft X-ray transients. Therefore, the dynamical friction of dark matter can only alleviate the effective temperature problem of those BH LMXBs with a relatively short orbital period.

Keywords: Dark matter distribution, X-ray binary stars, Black holes, Stellar evolution
Diagnostics of Magnetohydrodynamic Modes in the Interstellar Medium through Synchrotron Polarization Statistics. One of the biggest challenges in understanding magnetohydrodynamic (MHD) turbulence is identifying the plasma mode components from observational data. Previous studies on synchrotron polarization from the interstellar medium (ISM) suggest that the dominant MHD modes can be identified via statistics of Stokes parameters, which would be crucial for studying various ISM processes such as the scattering and acceleration of cosmic rays, star formation, and dynamo. In this paper, we present a numerical study of the synchrotron polarization analysis (SPA) method through systematic investigation of the statistical properties of the Stokes parameters. We derive the theoretical basis for our method from the fundamental statistics of MHD turbulence, recognizing that the projection of the MHD modes allows us to identify the modes dominating the energy fraction from synchrotron observations. Based on the discovery, we revise the SPA method using synthetic synchrotron polarization observations obtained from 3D ideal MHD simulations with a wide range of plasma parameters and driving mechanisms, and present a modified recipe for mode identification. We propose a classification criterion based on a new SPA+ fitting procedure, which allows us to distinguish between Alfvén mode and compressible/slow mode dominated turbulence. We further propose a new method to identify fast modes by analyzing the asymmetry of the SPA+ signature and establish a new asymmetry parameter to detect the presence of fast mode turbulence. Additionally, we confirm through numerical tests that the identification of the compressible and fast modes is not affected by Faraday rotation in both the emitting plasma and the foreground.

Keywords: Magnetohydrodynamics, Interstellar dynamics, Plasma astrophysics, Interstellar magnetic fields, Interstellar synchrotron emission, Interstellar medium, Magnetohydrodynamical simulations
Modeling Transition Region Hot Loops on the Sun: The Necessity of Rapid, Complex Spatiotemporal Heating and Nonequilibrium Ionization. The study examines the heating profile of hot solar transition region loops, particularly focusing on transient brightenings observed in IRIS 1400 Å slit-jaw images. The findings challenge the adequacy of simplistic, singular heating mechanisms, revealing that the heating is temporally impulsive and requires a spatially complex profile with multiple heating scales. A forward-modeling code is utilized to generate synthetic Interface Region Imaging Spectrograph (IRIS) emission spectra of these loops based on HYDRAD model output, confirming that emitting ions are out of equilibrium. The modeling further indicates that density-dependent dielectronic recombination rates must be included to reproduce the observed line ratios. Collectively, this evidence substantiates that the loops are subject to impulsive heating and that the components of the transiently brightened plasma are driven far from thermal equilibrium. Heating events such as these are ubiquitous in the transition region, and the analysis described above provides a robust observational diagnostic tool for characterizing the plasma.

Keywords: Solar atmosphere, Active solar corona, Solar coronal loops, Solar transition region
A Surprising Excess of Radio Emission in Extremely Stable Quasars: A Unique Clue to Jet Launching?. Quasars are generally divided into jetted radio-loud and nonjetted radio-quiet ones, but why only 10% of quasars are radio-loud has been puzzling for decades. Other than jet-induced phenomena, black hole mass, or Eddington ratio, a prominent difference between jetted and nonjetted quasars has scarcely been detected. Here we show a unique distinction between them, and the mystery of jet launching could be disclosed by a prominent excess of radio emission in extremely stable quasars (ESQs; i.e., type 1 quasars with extremely weak variability in UV/optical over 10 yr). Specifically, we find that &gt;25% of the ESQs are detected by the FIRST/VLASS radio survey, while only ∼6%–8% of the control sample, matched in redshift, luminosity, and Eddington ratio, are radio-detected. The excess of radio detection in ESQs has a significance of 4.4σ (99.9995%) and predominantly occurs at intermediate radio loudness with R ∼ 10–60. The radio detection fraction of ESQs also tends to increase in the ESQ samples selected with more stringent thresholds. Our results are in contrast to the common view that radio-loud quasars are likely more variable in UV/optical due to jet contribution. New clues/challenges posed by our findings highlight the importance of extensive follow-up observations to probe the nature of jets in ESQs and theoretical studies on the link between jet launching and ESQs. Moreover, our results make ESQs, an essential population that has never been explored, unique targets in the burgeoning era of time domain astronomy, like their opposite counterparts of quasars exhibiting extreme variability or changing-look features.

Keywords: Radio loud quasars, Jets, Accretion, Quasars
Photometry of Saturated Stars with Neural Networks. We use a multilevel perceptron (MLP) neural network to obtain photometry of saturated stars in the All-Sky Automated Survey for Supernovae (ASAS-SN). The MLP can obtain fairly unbiased photometry for stars from g ≃ 4 to 14 mag, particularly compared to the dispersion (15%–85% 1σ range around the median) of 0.12 mag for saturated (g &lt; 11.5 mag) stars. More importantly, the light curve of a nonvariable saturated star has a median dispersion of only 0.037 mag. The MLP light curves are, in many cases, spectacularly better than those provided by the standard ASAS-SN pipelines. While the network was trained on g-band data from only one of ASAS-SN's 20 cameras, initial experiments suggest that it can be used for any camera and the older ASAS-SN V-band data as well. The dominant problems seem to be associated with correctable issues in the ASAS-SN data reduction pipeline for saturated stars more than the MLP itself. The method is publicly available as a light-curve option on ASAS-SN Sky Patrol v1.0.

Keywords: Algol variable stars, CCD photometry, Time domain astronomy, Computational methods
The Impact of Stellar Radiative Feedback on Formation of Young Massive Clusters via Fast H I Gas Collisions. Young massive clusters (YMCs) are dense aggregates of young stars and are often speculated as potential precursors to globular clusters. However, the formation mechanism of massive and compact gas clumps that precede YMCs remains unknown. In this paper, we study the formation of such massive clumps via fast H I gas collisions (∼100 km s‑1) as suggested by recent observations and their subsequent evolution into YMCs by using three-dimensional magnetohydrodynamics simulations involving self-gravity and detailed thermal/chemical processes. In particular, the impact of ionization feedback from stellar radiation is included in an approximate fashion where the temperature within the H II regions is elevated to 10,000 K, while supernova feedback is not included. We examine whether the resulting massive clumps can survive this ionization feedback and evolve into YMCs. Our simulations reveal the emergence of gas clumps that not only possess substantial mass (∼105 M ⊙) but also sufficient compactness (∼5 pc). Notably, these clumps exhibit significantly higher escape velocities compared to the sound speed of the H II region, indicating effective gravitational retention of gas against feedback-induced evaporation. Consequently, these conditions foster efficient star formation within the massive gas clumps, ultimately leading to their evolution into YMCs. We also perform simulations involving lower-velocity gas collisions, approximately 15 km s‑1, typical shock velocities induced by galactic superbubbles. In contrast to the high-velocity collisions, we find that molecular cloud formation does not occur in the case of 1 cm‑3 gas collision, while YMC formation is observed in the presence of denser gas of 10 cm‑3. However, the formation of YMCs requires compression periods exceeding 10 Myr in these cases, indicating a potential preference for gas collisions driven by intergalactic interactions rather than galactic superbubbles for YMC formation.

Keywords: Young massive clusters, Star clusters
The MOSDEF-KCWI Survey: Spectral Properties of Lyα Halos around z ∼ 2 Star-forming Galaxies. We present Keck Cosmic Web Imager integral field observations of extended Lyα emission in the circumgalactic medium of 27 typical star-forming galaxies at z ∼ 2, drawn from the Multi-Object Spectrometer for Infra-Red Exploration (MOSFIRE) Deep Evolution Field (MOSDEF) survey. Using composite spectra in two bins of star formation rate (SFR), star formation rate surface density (ΣSFR), and other galactic properties, we measure spatial variations in the Lyα profile across three regions in the Lyα halo. We find single-peaked, redshifted profiles are ubiquitous within a central 7 kpc radius region. Further out in the halo (7–14 and 14–21 kpc), the Lyα profile of the resonantly scattered emission exhibits more diversity, either transitioning to a double-peaked profile or remaining single peaked across the halo. We find a shorter scale length of the Lyα halo surface brightness profile for composite halos with faster winds. The composites have a similar average inclination, suggesting those with faster winds clear channels in the interstellar medium (ISM), reducing the fraction of Lyα photons resonantly scattered to large radii. A uniform expanding shell radiative transfer model reproduces the shape but not the normalization of the observed double-peaked Lyα profiles. Models that adopt a more realistic, clumpy ISM are likely needed to reproduce both the shape and normalization of the Lyα profiles.

Keywords: Galaxy spectroscopy, Galaxy evolution, Circumgalactic medium
Unified Rapid Mass Transfer. We present a method to obtain rapid mass-loss rates in binary systems, specifically at the onset of mass transfer (MT) episodes. The method unifies atmospheric (underflow) and L 1 stream (overflow) mass rates in a single continuous procedure. The method uses averaged 3D properties of the binaries, such as effective binary potential and effective binary acceleration, to both evolve the donor and obtain properties of the matter at the L 1 plane. In the case of underflow, we obtain atmospheric stratification. Our method can be used for binaries with an extensive range of mass ratios, 0.01 ≤ q ≤ 100, and can also be applied to hot donors. The considered examples show that the MT rates obtained with this revised formalism always differ from the optically thin and optically thick MT rates widely used during the computations of binary evolution.

Keywords: Roche lobe overflow, Binary stars, Multiple star evolution