adsabs/SciX_UAT_keywords huggingface dataset:
A Transition Discovered in the Subcritical Regime of 1A 0535+262

We present NICER observations of the accreting X-ray pulsar 1A 0535+262 during its faint state (≲6 × 1036 erg s‑1), observed in several type I and type II outbursts. We discovered a transition of temporal and spectral properties around the luminosity L t = 3.3 × 1035 erg s‑1, below which spectra are relatively soft and the pulse profiles have only a narrow peak. The spectra are harder and a secondary hump gradually appears in the pulse profiles when L ≳ L t. We discuss possible physical mechanisms for this transition, including different Comptonization seed photons, the disappearance of gas shocks on the neutron star surface, and the combination of plasma and vacuum polarization effects.

Keywords: accretion; high mass x-ray binary stars; pulsars
Li Distribution, Kinematics, and Detailed Abundance Analysis among Very Metal-poor Stars in the Galactic Halo from the HESP-GOMPA Survey

We present a study on the detailed elemental abundances of newly identified, bright, very metal-poor stars with the detection of lithium, initially observed as part of the SDSS/MARVELS pre-survey. These stars were selected for high-resolution spectroscopic follow-up as part of the HESP-GOMPA survey. In this work, we discuss the Li abundances detected for several stars in the survey, which include main-sequence stars, subgiants, and red giants. Different classes of stars are found to exhibit very similar distributions of Li, which points toward a common origin. We derive a scaling relation for the depletion of Li as a function of temperature for giants and main-sequence stars; the majority of the samples from the literature were found to fall within 1σ (0.19 and 0.12 dex K-1 for giants and dwarfs, respectively) of this relationship. We also report the existence of a slope of the Li abundance as a function of distance from the Galactic plane, indicating mixed stellar populations. Most Li-rich stars are found to be in or close to the Galactic plane. Along with Li, we have derived detailed abundances for C, odd-Z, α-, Fe-peak, and neutron-capture elements for each star. We have also used astrometric parameters from Gaia-EDR3 to complement our study, and derived kinematics to differentiate between the motions of the stars-those formed in situ and those accreted. The stellar population of the Spite plateau, including additional stars from the literature, is found to have significant contributions from stars formed in situ and through accretion. The orbits for the program stars have also been derived and studied for a period of 5 Gyr backwards in time.

Keywords: stellar atmospheres; abundance ratios; stellar abundances; lithium stars; metallicity; population ii stars; chemically peculiar stars; high resolution spectroscopy; stellar nucleosynthesis; nucleosynthesis; stellar populations
The Physical Origin of the Periodic Activity for FRB 20180916B

Fast radio bursts (FRBs) are transient radio signals with millisecond-duration, large dispersion measure and extremely high brightness temperature. Among them, FRB 20180916B has been found to have a 16 days periodically modulated activity. However, the physical origin of the periodicity is still a mystery. Here, we utilize the comprehensive observational data to diagnose the periodic models. We find that the ultralong rotation model is the most probable one for the periodic activity. However, this model cannot reproduce the observed rotation measure (RM) variations. We propose a self-consistent model, i.e., a massive star binary containing a slowly rotational neutron star and a massive star with large mass loss, which can naturally accommodate the wealth of observational features for FRB 20180916B. In this model, the RM variation is periodic, which can be tested by future observations.

Keywords: radio transient sources; high mass x-ray binary stars; magnetars
Unveiling an Automated Modeling Method for S0 Galaxies

The objective of this study is to unveil an optimum and precise automated method to model S0 galaxies. Preliminary results of the study confirm that GALFIT-a two-dimensional image decomposition program-with a Sérsic profile can be used to model S0 galaxies. Since the input parameters affect the accuracy of the model, different combinations of parameters were tested to obtain the best-fit model. An automated GALFIT fitting process was developed using Python and Astropy module to increase the efficiency of the process. A sample of S0 galaxy images observed at the Kitt Peak National Observatory was used to test the precision of the implemented method. The accuracy of the created models was tested both visually and statistically. The new method will allow us to model a large sample of S0 galaxies accurately and determine their structure and properties.

Keywords: lenticular galaxies
A Simple, Entropy-based Dissipation Trigger for SPH

Smoothed particle hydrodynamics (SPH) schemes need to be enhanced by dissipation mechanisms to handle shocks. Most SPH formulations rely on artificial viscosity and, while this works well in pure shocks, attention must be paid to avoid dissipation where it is not wanted. Commonly used approaches include limiters and time-dependent dissipation parameters. The former try to distinguish shocks from other types of flows that do not require dissipation while in the latter approach the dissipation parameters are steered by some source term ("trigger") and, if not triggered, they decay to a predescribed floor value. The commonly used source terms trigger on either compression, $-{\rm{\nabla }}\cdot {\boldsymbol{v}}$ , or its time derivative. Here we explore a novel way to trigger SPH-dissipation: since an ideal fluid conserves entropy exactly, its numerical nonconservation can be used to identify "troubled particles" that need dissipation because they either pass through a shock or become noisy for other reasons. Our new scheme is implemented into the Lagrangian hydrodynamics code MAGMA2 and is scrutinized in a number of shock and fluid instability tests. We find excellent results in shocks and only a very moderate (and desired) switch-on in instability tests. The new scheme is robust, trivial to implement into existing SPH codes, and does not add any computational overhead.

Keywords: hydrodynamical simulations
Triple Spiral Arms of a Triple Protostar System Imaged in Molecular Lines

Most stars form in multiple-star systems. For a better understanding of their formation processes, it is important to resolve the individual protostellar components and the surrounding envelope and disk material at the earliest possible formation epoch, because the formation history can be lost in a few orbital timescales. Here we present Atacama Large Millimeter/submillimeter Array observational results of a young multiple protostellar system, IRAS 04239+2436, where three well-developed large spiral arms were detected in the shocked SO emission. Along the most conspicuous arm, the accretion streamer was also detected in the SO2 emission. The observational results are complemented by numerical magnetohydrodynamic simulations, where those large arms only appear in magnetically weakened clouds. Numerical simulations also suggest that the large triple spiral arms are the result of gravitational interactions between compact triple protostars and the turbulent infalling envelope.

Keywords: star formation
The Factory and the Beehive. IV. A Comprehensive Study of the Rotation X-Ray Activity Relation in Praesepe and the Hyades

X-ray observations of low-mass stars in open clusters are critical to understanding the dependence of magnetic activity on stellar properties and their evolution. Praesepe and the Hyades, two of the nearest, most-studied open clusters, are among the best available laboratories for examining the dependence of magnetic activity on rotation for stars with masses ≲1 M ⊙. We present an updated study of the rotation-X-ray activity relation in the two clusters. We updated membership catalogs that combine pre-Gaia catalogs with new catalogs based on Gaia Data Release 2. The resulting catalogs are the most inclusive ones for both clusters: 1739 Praesepe and 1315 Hyades stars. We collected X-ray detections for cluster members, for which we analyzed, re-analyzed, or collated data from ROSAT, the Chandra X-ray Observatory, the Neil Gehrels Swift Observatory, and XMM-Newton. We have detections for 326 Praesepe and 462 Hyades members, of which 273 and 164, respectively, have rotation periods-an increase of 6× relative to what was previously available. We find that at ≈700 Myr, only M dwarfs remain saturated in X-rays, with only tentative evidence for supersaturation. We also find a tight relation between the Rossby number and fractional X-ray luminosity L X/L bol in unsaturated single members, suggesting a power-law index between -3.2 and -3.9. Lastly, we find no difference in the coronal parameters between binary and single members. These results provide essential insight into the relative efficiency of magnetic heating of the stars' atmospheres, thereby informing the development of robust age-rotation-activity relations.

Keywords: late-type dwarf stars; stellar activity; open star clusters; stellar rotation; x-ray point sources; binary stars
A Fast, Simple, Robust Algorithm for Coronal Temperature Reconstruction

We describe a new algorithm for reconstruction of differential emission measures (DEMs) in the solar corona. Although a number of such algorithms currently exist, they can have difficulty converging for some cases and can be complex, slow, or idiosyncratic in their output (i.e., their inversions can have features that are a result of the inversion code and instrument response, not of the solar source); we will document some of these issues in this paper. The new algorithm described here significantly reduces these drawbacks and is particularly notable for its simplicity; it is reproduced here, in full, on a single page. After we describe the algorithm, we compare its performance and fidelity with some prevalent methods. Although presented here for extreme-ultraviolet data, the algorithm is robust and extensible to any other wavelengths (e.g., X-rays) where the DEM treatment is valid.

Keywords: quiet sun; active sun; active solar corona; quiet solar corona; solar extreme ultraviolet emission; space plasmas; atomic spectroscopy
A Large Sample of Extremely Metal-poor Galaxies at z < 1 Identified from the DESI Early Data

Extremely metal-poor galaxies (XMPGs) at relatively low redshift are excellent laboratories for studying galaxy formation and evolution in the early universe. Much effort has been spent on identifying them from large-scale spectroscopic surveys or spectroscopic follow-up observations. Previous work has identified a few hundred XMPGs. In this work, we obtain a large sample of 223 XMPGs at z < 1 from the early data of the Dark Energy Spectroscopic Instrument (DESI). The oxygen abundance is determined using the direct T e method based on the detection of the [O III]λ4363 line. The sample includes 95 confirmed XMPGs based on the oxygen abundance uncertainty; the remaining 128 galaxies are regarded as XMPG candidates. These XMPGs are only 0.01% of the total DESI observed galaxies. Their coordinates and other properties are provided in the paper. The most XMPGs have an oxygen abundance of ∼1/34 Z ⊙, a stellar mass of about 1.5 × 107 M ⊙, and a star formation rate of 0.22 M ⊙ yr‑1. The two most XMPGs present distinct morphologies suggesting different formation mechanisms. The local environmental investigation shows that XMPGs preferentially reside in relatively low-density regions. Many of them fall below the stellar mass–metallicity relations (MZRs) of normal star-forming galaxies. From a comparison of the MZR with theoretical simulations, it appears that XMPGs are good analogs to high-redshift star-forming galaxies. The nature of these XMPG populations will be further investigated in detail with larger and more complete samples from the ongoing DESI survey.

Keywords: metallicity; scaling relations; dwarf galaxies; redshift surveys
Identification of Hot Plasma Anomalies in Solar Wind Using Fe Ion Charge Distributions

A presence of high Fe charge states in the ionic charge state distributions of the solar wind (SW) plasma, commonly characterized by the mean charge QFe, provides valuable information on heating processes in the SW sources. We study the relationship between the parameter QFe and the charge state distributions of Fe ions using the Solar TErrestrial RElations Observatory/PLAsma and SupraThermal Ion Composition data on the beginning of the 24th Cycle (2010 January-2011 July). We find that the Fe charge state distributions related to SW with QFe ≈ 8-10 have an uni-modal shape peaked around Fe8+-Fe9+. When the QFe value increases, the distributions change: at first, the profile extends to higher charge states and then transforms into a bi-modal shape with a second maximum around Fe16+ and a minimum around Fe12+. We discuss possible reasons for such bi-modality through the example of the interplanetary coronal mass ejection (ICME) event on 2011 February 24-26, where it was related to the heating of an eruptive prominence. For such an analysis, it is informative to have a special measure of the fraction of highly charged ions for the Fe ion charge distribution in SW. In addition to QFe, we introduce a parameter q12 equal to a fraction of Fe ions with charges Q ≥ 12 and show that this parameter can be applied for identifying both the large-scale hot plasma enhancements associated with ICMEs and small hot fragments of SW plasma, which may be associated with small-scale solar activity in various coronal structures.

Keywords: solar corona; solar wind; solar physics; solar coronal mass ejections; atomic physics
Gamma Rays from Kilonova: A Potential Probe of r-process Nucleosynthesis

The mergers of compact binaries with at least one neutron star component are the potential leading sites of the production and ejection of r-process elements. Discoveries of galactic binary pulsars, short gamma-ray bursts, and gravitational-wave detections have all been constraining the rate of these events, while the gravitational wave plus broadband electromagnetic coverage of binary neutron star merger (GW170817) has also placed constraints on the properties (mass and composition) of the merger ejecta. But uncertainties and ambiguities in modeling the optical and infrared emission make it difficult to definitively measure the distribution of heavy isotopes in these mergers. In contrast, gamma rays emitted in the decay of these neutron-rich ejecta may provide a more direct measurement of the yields. We calculate the gamma production in remnants of neutron star mergers, considering two epochs: a kilonova epoch, lasting about two weeks, and a much later epoch of tens and hundreds of thousands of years after the merger. For the kilonova epoch, when the expanding ejecta is still only partially transparent to gamma radiation, we use 3D radiative transport simulations to produce the spectra. We show that the gamma-ray spectra associated with beta- and alpha-decay provide a fingerprint of the ejecta properties and, for a sufficiently nearby remnant, may be detectable, even for old remnants. We compare our gamma spectra with the potential detection limits of next generation detectors, including the Lunar Occultation Explorer (LOX), the All-sky Medium Energy Gamma-ray Observatory (AMEGO), and the Compton Spectrometer and Imager (COSI). We show that fission models can be discriminated via the presence of short-lived fission fragments in the remnant spectra.

Keywords: neutron stars; explosive nucleosynthesis; r-process; gamma-ray sources; gamma-ray lines
Measuring the Hubble Constant with Dark Neutron Star-Black Hole Mergers

Detection of gravitational waves (GWs) from neutron star-black hole (NSBH) standard sirens provides local measurements of the Hubble constant (H 0), regardless of the detection of an electromagnetic (EM) counterpart, given that matter effects can be exploited to break the redshift degeneracy of the GW waveforms. The distinctive merger morphology and the high-redshift detectability of tidally disrupted NSBH make them promising candidates for this method. Also, the detection prospects of an EM counterpart for these systems will be limited to z < 0.8 in the optical, in the era of future GW detectors. Using recent constraints on the equation of state of NSs from multi-messenger observations of NICER and LIGO/Virgo/KAGRA, we show the prospects of measuring H 0 solely from GW observation of NSBH systems, achievable by the Einstein telescope (ET) and Cosmic Explorer (CE) detectors. We first analyze individual events to quantify the effect of high-frequency (≥500 Hz) tidal distortions on the inference of NS tidal deformability parameter (Λ) and hence on H 0. We find that disruptive mergers can constrain Λ up to ${ \mathcal O }(60 \% )$ more precisely than nondisruptive ones. However, this precision is not sufficient to place stringent constraints on the H 0 from individual events. By performing Bayesian analysis on simulated NSBH data (up to N = 100 events, corresponding to a day of observation) in the ET+CE detectors, we find that NSBH systems enable unbiased 4%-13% precision on the estimate of H 0 (68% credible interval). This is a similar measurement precision found in studies analyzing NSBH mergers with EM counterparts in the LVKC O5 era.

Keywords: cosmology; gravitational waves; general relativity; gravitational wave sources; gravitational wave astronomy
Synchrotron Self-Compton Scattering in Sgr A* Derived from NIR and X-Ray Flare Statistics

The flaring activity of Sagittarius A* (Sgr A*) can be analyzed by statistical means to test emission models for its accretion flow. A particular modeling question is whether the observed X-ray flares are the high-energy end of a synchrotron spectrum or if they arise from self-Comptonized photons of a lower-energy synchrotron process. We use already published Chandra X-ray Visionary Project data to statistically investigate the X-ray count-rate distribution of Sgr A*. Two previous workgroups have already undertaken such an analysis on that data. They modeled the flaring part of the emission with a bounded power law, i.e., a power-law distribution with a hard cutoff at the highest measured count rate. With this model, both teams obtain a power-law index ${\alpha }_{X}\sim 2$ . We show that the flare count-rate distribution can also be well described by a truncated, i.e., an exponentially decaying power law. We argue that an exponential truncation is a more natural model than a hard cutoff. With this alternate model, our fit yields a power-law index ${\alpha }_{X}\sim 1.66$ . We find that this slope can be canonically explained by a synchrotron self-Compton (SSC) process. Therefore, we argue that SSC models are the best ones suitable to explain the observed X-ray count-rate distribution.

Keywords: galactic center
Super-resolution of SDO/HMI Magnetograms Using Novel Deep Learning Methods

Image super-resolution is a technique of enhancing the resolution of an image where a high-resolution (HR) image is reconstructed from a low-resolution (LR) image. In this Letter, we apply two novel deep learning models (residual attention model and progressive GAN model) for enhancing Solar Dynamics Observatory (SDO)/Helioseismic and Magnetic Imager (HMI) magnetograms. For this, we consider line-of-sight (LOS) magnetograms taken by SDO/HMI as output and their degraded ones with 4 × 4 binning as input. Deep learning networks try to find internal relationships between LR and HR images from the given input and the corresponding output image. We consider SDO/HMI magnetograms from 2014 May to August for training, from 2014 October to December for validation, and 2015 January to March for test. We find that the deep learning models generate higher-quality results than the bicubic interpolation in terms of visual aspects and metrics. We apply this model to a full-resolution SDO/HMI magnetogram and then compare the generated magnetogram with the corresponding Hinode/The Solar Optical Telescope Narrowband Filtergrams (NFI) magnetogram. This comparison shows that the generated magnetogram is consistent with the Hinode one with a high correlation (CC: 0.94) and a high similarity (SSIM: 0.93), which are better than the bicubic method.

Keywords: the sun; astronomy data analysis; solar atmosphere; solar magnetic fields
Photometric Variability of T Tauri S as a Probe for Circumstellar Material

The irregular photometric variability of the triple system T Tauri has been puzzling since its discovery. A circumbinary disk around T Tau Sa and Sb can explain the recent brightness fluctuations of T Tau Sb, due to changes in extinction depending on the position of T Tau Sb in its orbit. The photometric variations and orbital motion of T Tau Sb indicate a position angle of the disk of ∼30°. Combined with information from the literature, we estimate its inclination to be ∼60° or more.

Keywords: trinary stars; young stellar objects; classical t tauri stars; circumstellar matter
K2 Targets Observed in TESS Cycles 1-3

Kepler, K2, and Transiting Exoplanet Survey Satellite (TESS) have demonstrated the power of high-precision uninterrupted photometry. Combining data sets from these two observatories can extend the baseline of high precision photometry on a target. In addition, for some investigations, the differing passbands of the two telescopes may provide additional information about the causes of the observed variability. In order to facilitate studies combining these unique data sets, we present a list of all the 55,641 targets observed by K2 which fell within the TESS field of view during its first three years of observations.

Keywords: catalogs; exoplanet astronomy; stellar astronomy
Infall Profiles for Supercluster-scale Filaments

We present theoretical expectations for infall toward supercluster-scale cosmological filaments, motivated by the Arecibo Pisces-Perseus Supercluster Survey (APPSS) to map the velocity field around the Pisces-Perseus Supercluster (PPS) filament. We use a minimum spanning tree applied to dark matter halos the size of galaxy clusters to identify 236 large filaments within the Millennium simulation. Stacking the filaments along their principal axes, we determine a well-defined, sharp-peaked velocity profile function that can be expressed in terms of the maximum infall rate V max and the distance ρ max between the location of maximum infall and the principal axis of the filament. This simple, two-parameter functional form is surprisingly universal across a wide range of linear mass densities. V max is positively correlated with the halo mass per length along the filament, and ρ max is negatively correlated with the degree to which the halos are concentrated along the principal axis. We also assess an alternative, single-parameter method using V 25, the infall rate at a distance of 25 Mpc from the axis of the filament. Filaments similar to the PPS have ${V}_{\max }=612\ \pm $ 116 km s-1, ${\rho }_{\max }=8.9\pm 2.1$ Mpc, and V 25 = 329 ± 68 km s-1. We create mock observations to model uncertainties associated with viewing angle, lack of three-dimensional velocity information, limited sample size, and distance uncertainties. Our results suggest that it would be especially useful to measure infall for a larger sample of filaments to test our predictions for the shape of the infall profile and the relationships among infall rates and filament properties.

Keywords: large-scale structure of the universe; dark matter distribution; redshift surveys; galaxies