adsabs/SciX_UAT_keywords huggingface dataset:
A Population of Compact Radio Variables and Transients in the Radio-bright Zone at the Galactic Center Observed with the Jansky Very Large Array

Using Jansky Very Large Array data obtained from high-resolution observations at 5.5 GHz at multiple epochs in 2014 and 2019, we have detected a population of radio variables and transients in the radio-bright zone at the Galactic center. With observations covering a sky area of 180 arcmin2 at an angular resolution of 0"4, we report new detections of 110 Galactic center compact radio (GCCR) sources with a size of <1″. The flux densities of GCCR sources exceed 70 μJy, with at least 10σ significance. Among these sources, 82 are variable or transient and 28 are nonvariable. About 10% of them are expected to be extragalactic background sources. We discuss the possible astrophysical nature of the detected sources. As compared to the Galactic disk (GD) population of normal pulsars (NPs) and millisecond pulsars (MSPs), a majority (80%) of the GCCR sources appear to fall within the high flux density tail of the pulsar distribution, as extrapolated from a sample of NPs in the GD. However, MSPs extrapolated from the GD population are too weak to have contributed significantly to the GCCR population that has been detected. We also cross-correlated the GCCR sources with X-ray sources in Chandra X-ray catalogs and found that 42 GCCR sources have candidate X-ray counterparts. Most of the GCCR sources having X-ray counterparts are likely to be associated with unresolved or slightly resolved radio jets launched from X-ray binaries with a compact object, either a black hole or a neutron star.

Keywords: galactic center; black holes; radio pulsars; millisecond pulsars; neutron stars; white dwarf stars; discrete radio sources; radio transient sources; interstellar medium; radio continuum emission; radio interferometry
Hybrid Origins of the Cosmic-Ray Nucleus Spectral Hardening at a Few Hundred GV

Many experiments have confirmed the spectral hardening at a few hundred GV of cosmic-ray (CR) nucleus spectra, and three general different origins have been proposed: the primary source acceleration, the propagation, and the superposition of different kinds of sources. The AMS-02 CR nucleus spectra of He, C, N, O, Ne, Mg, Si, and B (which includes B and its dominating parent species) are collected to study the necessity of employing a break in diffusion coefficient and independent breaks in primary source injection spectra to reproduce the spectral hardening at a few hundred GV. For comparison, three different schemes are introduced to do the global fitting. The fitting results show that both the break in diffusion coefficient and the independent breaks in primary source injection spectra are needed, which correspond to the spatially dependent propagation and the superposition of different kinds of sources, respectively. Consequently, the nucleus spectral hardening in a few hundred GV should have hybrid origins. Moreover, the CR spectral indices of He and Ne show large deviations from other species in the low-rigidity region, which indicates their different CR origins.

Keywords: galactic cosmic rays
Fortuitous Observations of Potential Stellar Relay Probe Positions with GBT

Recent theoretical and observational works have investigated the possibility that extraterrestrial intelligence could use the Sun as a gravitational lens in order to aid communication across interstellar distances. Unlike other targeted SETI searches where the drift rate of any artificial extraterrestrial signals may be unknown up to some large upper limit, the drift rates of any solar system relay probes would be known and set only by the motion of the Earth. One recent work used purpose-designed Green Bank Telescope (GBT) observations to search for signals from a hypothetical communications probe several hundred astronomical unit from the Sun at the antipode of the α Centauri AB system. To further aid in the advancement of relay-probe searches, we present a table of 1764 archival GBT observations which fortuitously fall near the positions of hypothetical probes communicating with stars within 100 pc and compute the drift rates for these probes.

Keywords: technosignatures; gravitational lensing; radio astronomy
Atmospheric Parameters and Abundances of Cool Red Giant Stars

Understanding the atmospheric parameters of stars on the top of the RGB is essential to reveal the chemical composition of the Milky Way, as they can be used to probe the farthest parts of our Galaxy. Our goal is to determine the chemical composition of 21 RGB stars with T eff < 4200 K selected from the APOGEE-2 DR17 database using new observations carried out with the spectrograph mounted on the 1 m telescope of the Hungarian Piszkéstető Observatory and the SONG spectrograph (R = 77,000) on the Hertzsprung SONG telescope in the 4500–5800 Å wavelength range. This is the first time the spectrograph (R = 18,000) on the 1 m telescope at Piszkéstető Observatory was used to measure the abundances of stars. We created a new LTE spectral library using MARCS model atmospheres and SYNSPEC by including the line list of 23 molecules to determine atmospheric parameters (T eff, $\mathrm{log}g$ , [Fe/H], [α/Fe]) and abundances of Si, Ca, Ti, V, Cr, Mn, and Ni with FERRE. The resulting parameters were compared to that of APOGEE. We found a good agreement in general, the average difference is ‑11.2 K in T eff, 0.11 dex in $\mathrm{log}g$ , 0.10 dex in [Fe/H], and ‑0.01 dex in [α/Fe]. Our analysis successfully demonstrates the ability of the spectrograph at Piszkéstető Observatory to reliably measure the abundance of bright stars.

Keywords: red giant stars; high resolution spectroscopy; ground-based astronomy; chemical abundances
Globally Optimal and Scalable N-way Matching of Astronomy Catalogs

Building on previous Bayesian approaches, we introduce a novel formulation of probabilistic cross-identification, where detections are directly associated to (hypothesized) astronomical objects in a globally optimal way. We show that this new method scales better for processing multiple catalogs than enumerating all possible candidates, especially in the limit of crowded fields, which is the most challenging observational regime for new-generation astronomy experiments such as the Rubin Observatory Legacy Survey of Space and Time. Here we study simulated catalogs where the ground truth is known and report on the statistical and computational performance of the method. The paper is accompanied by a public software tool to perform globally optimal catalog matching based on directional data.

Keywords: astronomical methods; catalogs; photometry; bayes factor
Emergent Gravity Fails to Explain Color-dependent Galaxy-Galaxy Lensing Signal from SDSS DR7

We test Verlinde's Emergent Gravity (EG) theory using a galaxy-galaxy lensing technique based on Sloan Digital Sky Survey DR7 data. In the EG scenario, we do not expect a color dependence of the galaxy sample in the apparent dark matter predicted by EG, which is exerted only by the baryonic mass. If the baryonic mass is similar, then the predicted lensing profiles from the baryonic mass should be similar according to EG, regardless of the color of the galaxy sample. We use the stellar mass of the galaxy as a proxy of its baryonic mass. We divide our galaxy sample into five stellar mass bins, and further classify them as red and blue subsamples in each stellar mass bin. If we set the halo mass and concentration as free parameters, ΛCDM is favored by our data in terms of the reduced χ2, while EG fails to explain the color dependence of the excess surface density from the galaxy-galaxy lensing measurement.

Keywords: quantum gravity; weak gravitational lensing
A Blueprint for the Milky Way's Stellar Populations. II. Improved Isochrone Calibration in the SDSS and Pan-STARRS Photometric Systems

We improve the identification and isolation of individual stellar populations in the Galactic halo based on an updated set of empirically calibrated stellar isochrones in the Sloan Digital Sky Survey and Pan-STARRS 1 photometric systems. Along the Galactic prime meridian (l = 0° and 180°), where proper motions and parallaxes from Gaia DR2 can be used to compute rotational velocities of stars in the rest frame of the Milky Way, we use the observed double color-magnitude sequences of stars having large transverse motions, which are mostly attributed to groups of stars in the metal-poor halo and the thick disk with halo-like kinematics, respectively. The Gaia sequences directly constrain color-magnitude relations of model colors, and help to improve our previous calibration using Galactic star clusters. Based on these updated sets of stellar isochrones, we confirm earlier results on the presence of distinct groups of stars in the metallicity versus rotational-velocity plane, and find that the distribution of the most metal-poor ([Fe/H] < -2) stars in our sample can be modeled using two separate groups on prograde and retrograde orbits, respectively. At 4-6 kpc from the Galactic plane, we find approximately equal proportions of the Splashed Disk, and the metal-rich (⟨[Fe/H]⟩ ∼ -1.6) and metal-poor (⟨[Fe/H]⟩ ∼ -2.5) halos on prograde orbits. The Gaia-Sausage-Enceladus, the metal-weak thick disk, and the retrograde halo structure(s) (⟨[Fe/H]⟩ ∼ -2.2) constitute approximately 10% of the rest of the stellar populations at these distances.

Keywords: milky way dynamics; milky way stellar halo; stellar abundances; stellar atmospheres; stellar evolutionary models
Linking Sky-plane Observations of Moving Objects

The Asteroid Terrestrial-impact Last Alert System (ATLAS) observes the visible sky every night in search of dangerous asteroids. With four (soon five) sites ATLAS is facing new challenges for scheduling observations and linking detections to identify moving asteroids. Flexibility in coping with diverse observation sites and times of detections that can be linked is critical, as is optimization of observing time for coverage versus depth. We present new algorithms to fit orbits rapidly to sky-plane observations, and to test and link sets of detections to find the ones which belong to moving objects. The PUMA algorithm for fitting orbits to angular positions on the sky executes in about a millisecond, orders of magnitude faster than the methods currently in use by the community, without sacrifice in accuracy. The PUMA software should be generally useful to anyone who needs to test many sets of detections for consistency with a real orbit. The PUMALINK algorithm to find linkages among sets of detections has similarities to other approaches, notably HelioLinC, but it functions well at asteroid ranges of a small fraction of an astronomical unit. PUMALINK is fast enough to test 10 million possible tracklets against one another in a half hour of computer time. Candidate linkages are checked by the PUMA library to test that the detections correspond to a real orbit, even at close range, and the false alarm rate is manageable. Sky surveys that produce large numbers of detections from large numbers of exposures may find the PUMALINK software helpful. We present the results of tests of PUMALINK on three data sets which illustrate PUMALINK's effectiveness and economy: 2 weeks of all ATLAS detections over the sky, 2 weeks of special ATLAS opposition observations with long exposure time, and 2 weeks of simulated LSST asteroid observations. Detection probabilities of linkages must be traded against false alarm rate, but a representative choice for PUMALINK might be 90% detection probability for real objects while keeping the false alarm rate below 10% for a 100:1 population of false:real. Although optimization of the tradeoffs between detection probability, execution time, and false alarm rate is application specific and beyond the scope of this paper, we provide guidance on methods to distinguish false alarms from correct linkages of real objects.

Keywords: orbit determination; astronomy data analysis; astronomy software; computational methods; asteroids; near-earth objects
The Nominal Ranges of Rocky Planet Masses, Radii, Surface Gravities, and Bulk Densities

The two primary observable quantities of an exoplanet-its mass and radius-alone are not sufficient to probe a rocky exoplanet's interior composition and mineralogy. To overcome this, host-star abundances of the primary planet-building elements (Mg, Si, Fe) are typically used as a proxy for the planet's bulk composition. The majority of small exoplanet hosts, however, do not have available abundance data. Here we present the open-source ExoPlex mass-radius-composition solver. Unlike previous open-source mass-radius solvers, ExoPlex calculates the core chemistry and equilibrium mantle mineralogy for a bulk composition, including effects of mantle FeO content, core light elements, and surface water/ice. We utilize ExoPlex to calculate the planetary radii, surface gravities, and bulk densities for 106 model planets up to 2 R ⊕ across these geochemistries, adopting the distribution of FGK stellar abundances to estimate of the range of bulk exoplanet compositions. We outline the 99.7% distribution of radii, surface gravities, and bulk densities that define planets as "nominally rocky." Planets outside this range require compositions outside those expected from stellar abundance data, likely making them either Fe-enriched super-Mercuries, or volatile-enriched mini-Neptunes. We apply our classification scheme to a sample of 85 well-resolved exoplanets without available host-star abundances. We estimate only nine planets are within the "nominally rocky planet zone" at >70% confidence, while ~20% and ~30% of this sample can be reasonably classified as super-Mercuries or volatile-rich, respectively. Our results provide observers with a self-consistent way to classify broadly a planet as likely rocky, Mercury-like, or volatile-enriched, using mass and radius measurements alone.

Keywords: planetary interior; exoplanet structure; exoplanet astronomy; planet hosting stars
"Winged" Radio Sources from the LOFAR Two-meter Sky Survey First Data Release (LoTSS DR1)

A small number of extragalactic radio sources disclose a pair of low-surface-brightness radio lobes, known as "wings," aligned at a certain angle to the primary jets. Such exotic sources are known as "winged" radio sources. Here we report the new identification of a total of 26 "winged" radio sources from the LOFAR Two-meter Sky Survey First Data Release (LoTSS DR1). Out of the 26 "winged" sources, 14 are identified as X-shaped radio galaxies and the remaining 12 as Z-shaped radio galaxies. The available optical counterpart of each radio galaxy is cataloged along with its estimated redshift. Among the 26 sources, 15 candidates are classified as FR-II radio galaxies, and two are classified as FR-I type. For nine candidates, no conclusions are drawn due to their complex morphology. We also calculate the physical parameters such as spectral index, radio luminosity, and power of the sources. We have made a statistical study of the spectral index by combining our estimated value with the spectral index collected from previous works. A mean value of spectral index of 0.71 is obtained.

Keywords: active galactic nuclei; catalogs; radio continuum emission; surveys
Modeling of Galactic Foreground Polarization with Velocity Gradients

The detection of primordial B-mode polarization is still challenging due to the relatively low amplitude compared to the galactic foregrounds. To remove the contribution from the foreground, a comprehensive picture of the galactic magnetic field is indispensable. The Velocity Gradient Technique (VGT) is promising in tracing magnetic fields based on the modern understanding of the magneto-hydrodynamic turbulence. In this work, we apply VGT to an H I region containing an intermediate velocity cloud and a local velocity cloud, which are distinguishable in position-position-velocity space. We show that VGT gives an excellent agreement with the Planck polarization and stellar polarization. We confirm the advantages of VGT in constructing the 3D galactic magnetic field.

Keywords: interstellar medium; interstellar magnetic fields; interstellar dynamics
Solar Flare Predictive Features Derived from Polarity Inversion Line Masks in Active Regions Using an Unsupervised Machine Learning Algorithm

The properties of the polarity inversion line (PIL) in solar active regions (ARs) are strongly correlated to flare occurrences. The PIL mask, enclosing the PIL areas, has shown significant potential for improving machine-learning-based flare prediction models. In this study, an unsupervised machine-learning algorithm, Kernel Principle Component Analysis (KPCA), is adopted to directly derive features from the PIL mask and difference PIL mask, and use those features to classify ARs into two categories - non-strong flaring ARs and strong-flaring (M-class and above flares) ARs - for time-in-advance from one hour to 72 hr at a 1 hr cadence. The two best features are selected from the KPCA results to develop random-forest classifiers for predicting flares, and the models are then evaluated and compared to similar models based on the R value and difference R value. The results show that the features derived from the PIL masks by KPCA are effective in predicting flare occurrence, with overall better Fisher ranking scores and similar predictive statistics as the R value characteristics.

Keywords: solar flares; space weather; classification; solar active regions
Multiwavelength Observations of 2HWC J1928+177: Dark Accelerator or New TeV Gamma-Ray Binary?

2HWC J1928+177 is a Galactic TeV gamma-ray source detected by the High Altitude Water Cherenkov (HAWC) Observatory up to ∼56 TeV. The HAWC source, later confirmed by the High Energy Stereoscopic System, still remains unidentified as a dark accelerator since there is no apparent supernova remnant or pulsar wind nebula detected in the lower-energy bands. The radio pulsar PSR J1928+1746, coinciding with the HAWC source position, has no X-ray counterpart. Our SED modeling shows that inverse Compton scattering in the putative pulsar wind nebula can account for the TeV emission only if the unseen nebula is extended beyond $r\sim 4^{\prime} $ . Alternatively, TeV gamma-rays may be produced by hadronic interactions between relativistic protons from an undetected supernova remnant associated with the radio pulsar and a nearby molecular cloud G52.9+0.1. NuSTAR and Chandra observations detected a variable X-ray point source within the HAWC error circle, potentially associated with a bright infrared (IR) source. The X-ray spectra can be fitted with an absorbed power-law model with NH = (9 ± 3) × 1022 cm-2 and ΓX = 1.6 ± 0.3 and exhibit long-term X-ray flux variability over the last decade. If the X-ray source, possibly associated with the IR source (likely an O star), is the counterpart of the HAWC source, it may be a new TeV gamma-ray binary powered by collisions between the pulsar wind and stellar wind. Follow-up X-ray observations are warranted to search for diffuse X-ray emission and determine the nature of the HAWC source.

Keywords: gamma-ray astronomy; x-ray astronomy; gamma-ray telescopes; interstellar medium; gamma-ray sources; non-thermal radiation sources; radio pulsars; high mass x-ray binary stars; x-ray sources; supernova remnants
The Great Dimming of the Hypergiant Star RW Cephei: CHARA Array Images and Spectral Analysis

The cool hypergiant star RW Cephei is currently in a deep photometric minimum that began several years ago. This event bears a strong similarity to the Great Dimming of the red supergiant Betelgeuse that occurred in 2019-2020. We present the first resolved images of RW Cephei that we obtained with the CHARA Array interferometer. The angular diameter and Gaia distance estimates indicate a stellar radius of 900-1760 R ⊙, which makes RW Cephei one of the largest stars known in the Milky Way. The reconstructed, near-infrared images show a striking asymmetry in the disk illumination with a bright patch offset from the center and a darker zone to the west. The imaging results depend on assumptions made about the extended flux, and we present two cases with and without allowing extended emission. We also present a recent near-infrared spectrum of RW Cep that demonstrates that the fading is much larger at visual wavelengths compared to that at near-infrared wavelengths as expected for extinction by dust. We suggest that the star's dimming is the result of a recent surface mass ejection event that created a dust cloud that now partially blocks the stellar photosphere.

Keywords: late-type supergiant stars; stellar mass loss; stellar radii; variable stars
1000 Days of the Lowest-frequency Emission from the Low-luminosity GRB 171205A

We report the lowest-frequency measurements of gamma-ray burst (GRB) 171205A with the upgraded Giant Metrewave Radio Telescope (uGMRT) covering a frequency range of 250-1450 MHz and a period of 4-937 days. It is the first GRB afterglow detected in the 250-500 MHz frequency range and the second brightest GRB detected with the uGMRT. Even though the GRB was observed for nearly 1000 days, there is no evidence of a transition to a nonrelativistic regime. We also analyzed the archival Chandra X-ray data on day ∼70 and day ∼200. We also found no evidence of a jet break from the analysis of combined data. We fit synchrotron afterglow emission arising from a relativistic, isotropic, self-similar deceleration as well as from a shock breakout of a wide-angle cocoon. Our data also allowed us to discern the nature and the density of the circumburst medium. We found that the density profile deviates from a standard constant density medium and suggests that the GRB exploded in a stratified wind-like medium. Our analysis shows that the lowest-frequency measurements covering the absorbed part of the light curves are critical to unraveling the GRB environment. Our data combined with other published measurements indicate that the radio afterglow has a contribution from two components: a weak, possibly slightly off-axis jet and a surrounding wider cocoon, consistent with the results of Izzo et al. The cocoon emission likely dominates at early epochs, whereas the jet starts to dominate at later epochs, resulting in flatter radio light curves.

Keywords: gamma-ray bursts; gamma-ray transient sources; extragalactic radio sources
A Study of Primordial Very Massive Star Evolution. II. Stellar Rotation and Gamma-Ray Burst Progenitors

We calculate new evolutionary models of rotating primordial very massive stars, with initial mass from 100 M ⊙ to 200 M ⊙, for two values of the initial metallicity Z = 0 and Z = 0.0002. For the first time in this mass range, we consider stellar rotation and pulsation-driven mass loss, along with radiative winds. The models evolve from the zero-age main sequence until the onset of pair-instability. We discuss the main properties of the models during their evolution and then focus on the final fate and the possible progenitors of jet-driven events. All tracks that undergo pulsational-pair instability produce successful gamma-ray bursts (GRB) in the collapsar framework, while those that collapse directly to black holes (BH) produce jet-driven supernova events. In these latter cases, the expected black hole mass changes due to the jet propagation inside the progenitor, resulting in different models that should produce BH within the pair-instability black hole mass gap. Successful GRBs predicted here from zero metallicity, and very metal-poor progenitors, may be bright enough to be detected even up to redshift ~20 using current telescopes such as the Swift-BAT X-ray detector and the JWST.

Keywords: stellar evolution; stellar remnants; stellar rotation; population iii stars; stellar winds; gamma-ray bursts