adsabs/SciX_UAT_keywords huggingface dataset:
Kerr-scalaron Metric and Astronomical Consequences near the Galactic Center Black Hole

Astronomical tests of spacetime metric and gravitation theory near the Galactic center (GC) black hole, Sgr A*, have gained momentum with the observations of compact stellar orbits near the black hole and measurement of the black hole shadow. Deviation from the Kerr metric is a potential signature of modified gravity theory. In this work, we use the Newman–Janis algorithm to construct an axially symmetric and asymptotically flat metric in f(R) scalaron gravity theory. We call it the Kerr-scalaron metric. To study the astronomical consequences of the new metric, we use the compact stellar orbits and the black hole shadow. We use the observed size of the emission ring of the GC black hole shadow for estimating the deviation of the new metric from general relativity. It has been found that scalarons with masses within 10‑17–10‑16 eV are compatible with the observed emission ring size for a black hole spin of χ = 0.9. The Schwarzschild limit of the pericenter shift is estimated for compact stellar orbits near the black hole. General relativistic pericenter shift in wider orbits, including S stars such as S4716 and S2, has been reproduced with these scalarons. The parameter f SP measuring the deviation from Schwarzschild pericenter shift has been found to be f SP = 1.00–1.04 within stellar orbits with semimajor axes of 45–100 au. Scalarons have the capability to dominate Schwarzschild precession for orbits with semimajor axes much smaller than 45 au. Lense–Thirring (LT) precession with the new metric is estimated for the compact orbits. The massive scalarons produce LT precession with magnitude (12.25–24.5) μas yr‑1 in the orbit of S2. The LT precession timescale is within 0.1% of the age of the S stars.

Keywords: classical black holes; gravitation; galactic center
Discovery of Two Polars from a Crossmatch of ZTF and the SRG/eFEDS X-Ray Catalog

Magnetic cataclysmic variables (CVs) are luminous Galactic X-ray sources, which have been difficult to find in purely optical surveys due to their lack of outburst behavior. The eROSITA telescope on board the Spektr-RG mission is conducting an all-sky X-ray survey and recently released the public eROSITA Final Equatorial Depth Survey (eFEDS) catalog. We crossmatched the eFEDS catalog with photometry from the Zwicky Transient Facility and discovered two new magnetic CVs. We obtained high-cadence optical photometry and phase-resolved spectroscopy for each magnetic CV candidate and found them both to be polars. Among the newly discovered magnetic CVs is eFEDS J085037.2+044359/ZTFJ0850+0443, an eclipsing polar with orbital period P orb = 1.72 hr and WD mass M WD = 0.81 ± 0.08M ⊙. We suggest that eFEDS J085037.2+044359/ZTFJ0850+0443 is a low magnetic field strength polar, with B WD ≲ 10 MG. We also discovered a non-eclipsing polar, eFEDS J092614.1+010558/ZTFJ0926+0105, with orbital period P orb = 1.47 hr and magnetic field strength B WD = 36-42 MG.

Keywords: cataclysmic variable stars; binary stars; x-ray astronomy; time domain astronomy
The Quasar Luminosity Function at z 5 via Deep Learning and Bayesian Information Criterion

Understanding the faint end of quasar luminosity function (LF) at a high redshift is important since the number density of faint quasars is a critical element in constraining ultraviolet (UV) photon budgets for ionizing the intergalactic medium (IGM) in the early universe. Here, we present quasar LF reaching M 1450 ~ -22.0 AB mag at z ~ 5, about 1 mag deeper than previous UV LFs. We select quasars at z ~ 5 with a deep learning technique from deep data taken by the Hyper Suprime-Cam Subaru Strategic Program, covering a 15.5 deg2 area. Beyond the traditional color selection method, we improved the quasar selection by training an artificial neural network to distinguish z ~ 5 quasars from nonquasar sources based on their colors and adopting the Bayesian information criterion that can further remove high-redshift galaxies from the quasar sample. When applied to a small sample of spectroscopically identified quasars and galaxies, our method is successful in selecting quasars at ~83% efficiency (5/6) while minimizing the contamination rate of high-redshift galaxies (1/8) by up to three times compared to the selection using color selection alone (3/8). The number of our final quasar candidates with M 1450 < -22.0 mag is 35. Our quasar UV LF down to M 1450 = -22 mag or even fainter (M 1450 = -21 mag) suggests a rather low number density of faint quasars and the faint-end slope of $-{1.6}_{-0.19}^{+0.21}$ , favoring a scenario where quasars play a minor role in ionizing the IGM at high redshift.

Keywords: quasars; bayesian information criterion; astrostatistics; reionization; neural networks; surveys; luminosity function
Extreme r-process Enhanced Stars at High Metallicity in Fornax

We present and discuss three extremely r-process enhanced stars located in the massive dwarf spheroidal galaxy Fornax. These stars are very unique with an extreme Eu enrichment (1.25 ≤ [Eu/Fe]≤1.45) at high metallicities (-1.3 ≤ [Fe/H]≤-0.8). They have the largest Eu abundances ever observed in a dwarf galaxy opening new opportunities to further understand the origin of heavy elements formed by the r-process. We derive stellar abundances of Co, Zr, La, Ce, Pr, Nd, Er, and Lu using one-dimensional, local thermodynamic equilibrium codes and model atmospheres in conjunction with state-of-the art yield predictions. We derive Zr in the largest sample of stars (105) known to date in a dwarf galaxy. Accurate stellar abundances combined with a careful assessment of the yield predictions have revealed three metal-rich stars in Fornax showing a pure r-process pattern. We define a new class of stars, namely, Eu-stars, as r-II stars (i.e., [Eu/Fe] > 1) at high metallicities (i.e., [Fe/H] ≳ -1.5). The stellar abundance pattern contains Lu, observed for the first time in a dwarf galaxy, and reveals that a late burst of star formation has facilitated extreme r-process enhancement late in the galaxy's history (<4 Gyr ago). Due to the large uncertainties associated with the nuclear physics input in the yield predictions, we cannot yet determine the r-process site leading to the three Eu-stars in Fornax. Our results demonstrate that extremely r-rich stars are not only associated with ultra-faint low-mass dwarf galaxies, but can be born also in massive dwarf galaxies. * Based on data obtained from the ESO reduced archive. Program IDs: 080.B-0784(A), 171.B-0588(A), 71.B-0641(A).

Keywords: chemical abundances; r-process; nucleosynthesis; fornax dwarf spheroidal galaxy; dwarf spheroidal galaxies; chemically peculiar stars
An XMM-Newton Early-type Galaxy Atlas

The distribution of hot interstellar medium in early-type galaxies (ETGs) bears the imprint of the various astrophysical processes it underwent during its evolution. The X-ray observations of these galaxies have identified various structural features related to active galactic nucleus (AGN) and stellar feedback and environmental effects such as merging and sloshing. In our XMM-Newton Galaxy Atlas (NGA) project, we analyze archival observations of 38 ETGs, utilizing the high sensitivity and large field of view of XMM-Newton to construct spatially resolved 2D spectral maps of the hot gas halos. To illustrate our NGA data products in conjunction with the Chandra Galaxy Atlas, we describe two distinct galaxies, NGC 4636 and NGC 1550, in detail. We discuss their evolutionary history with a particular focus on the asymmetric distribution of metal-enriched, low-entropy gas caused by sloshing and AGN-driven uplift. We will release the NGA data products to a dedicated website, from where users can download them to perform further analyses.

Keywords: early-type galaxies
Growth of Massive Molecular Cloud Filament by Accretion Flows. I. Slow-shock Instability versus Ambipolar Diffusion

The Herschel Gould Belt Survey showed that stars form in dense filaments in nearby molecular clouds. Recent studies suggest that massive filaments are bound by the slow shocks caused by accretion flows onto the filaments. The slow shocks are known to be unstable to corrugation deformation of the shock front. Corrugation instability could convert the accretion flow's ram pressure into turbulent pressure that influences the width of the filament, which, according to theory, determines the self-gravitational fragmentation scale and core mass. In spite of its importance, the effect of slow-shock instability on star-forming filaments has not been investigated. In addition, the linear dispersion relation obtained from ideal magnetohydrodynamics (MHD) analysis shows that the most unstable wavelength of shock corrugation is infinitesimally small. In the scale of dense filaments, the effect of ambipolar diffusion can suppress the instability at small scales. This study investigates the influence of ambipolar diffusion on the instability of the slow shock. We perform two-dimensional MHD simulations to examine the linear growth of the slow-shock instability, considering the effect of ambipolar diffusion. The results demonstrate that the most unstable scale of slow-shock instability is approximately 5 times the length scale of ambipolar diffusion ℓ AD calculated using post-shock variables, where ℓ AD corresponds to the scale where the magnetic Reynolds number for ambipolar diffusivity is unity.

Keywords: magnetohydrodynamics; star formation; interstellar medium; interstellar filaments
A JWST Survey of the Supernova Remnant Cassiopeia A

We present initial results from a James Webb Space Telescope (JWST) survey of the youngest Galactic core-collapse supernova remnant, Cassiopeia A (Cas A), made up of NIRCam and MIRI imaging mosaics that map emission from the main shell, interior, and surrounding circumstellar/interstellar material (CSM/ISM). We also present four exploratory positions of MIRI Medium Resolution Spectrograph integral field unit spectroscopy that sample ejecta, CSM, and associated dust from representative shocked and unshocked regions. Surprising discoveries include (1) a weblike network of unshocked ejecta filaments resolved to ∼0.01 pc scales exhibiting an overall morphology consistent with turbulent mixing of cool, low-entropy matter from the progenitor's oxygen layer with hot, high-entropy matter heated by neutrino interactions and radioactivity; (2) a thick sheet of dust-dominated emission from shocked CSM seen in projection toward the remnant's interior pockmarked with small (∼1″) round holes formed by ≲0.″1 knots of high-velocity ejecta that have pierced through the CSM and driven expanding tangential shocks; and (3) dozens of light echoes with angular sizes between ∼0.″1 and 1' reflecting previously unseen fine-scale structure in the ISM. NIRCam observations place new upper limits on infrared emission (≲20 nJy at 3 μm) from the neutron star in Cas A's center and tightly constrain scenarios involving a possible fallback disk. These JWST survey data and initial findings help address unresolved questions about massive star explosions that have broad implications for the formation and evolution of stellar populations, the metal and dust enrichment of galaxies, and the origin of compact remnant objects.

Keywords: core-collapse supernovae; supernova remnants; neutron stars
Contribution of High-energy GRB Emissions to the Spectrum of the Isotropic Diffuse γ-Ray Background

High-precision measurement of the isotropic diffuse γ-ray background (IGRB) has been extrapolated to the TeV energy region using the Fermi-LAT experiment. Various kinds of astrophysical sources are candidates for its origin. However, a consensus on the dominant source has been difficult to reach. Recent observations of the γ-ray emission of γ-ray bursts (GRBs), denoted as the synchrotron self-Compton (SSC) component in the afterglow phase, in the sub-TeV energy region by MAGIC and HESS experiments shed new light on this topic. In this work, we revisit the contribution from the SSC component of GRBs to the IGRB. First, a sample set of GRB events is obtained and examined using observations from Fermi-LAT. Second, the SSC component, described by the energy ratio Rext and spectral index βext, is assigned to every GRB event. We can obtain the total spectrum contribution based on this GRB sample. We find that when Rext and βext reach ∼20% and -1.6, respectively, the contribution from GRB emission dominates in the energy region of hundreds of GeV. We hope that the LHAASO and CTA experiments under construction can observe a large number of GRBs to fix those parameters in coming years. A surviving tail is expected, which can serve to check our calculations based on future satellite experiments such as HERD and GAMMA400.

Keywords: gamma-ray bursts; gamma-ray sources
Self-gravitational Force Calculation of Second-order Accuracy Using the Multigrid Method on Nested Grids

We present a simple and effective multigrid-based Poisson solver of second-order accuracy in both gravitational potential and forces in terms of the one, two, and infinity norms. The method is especially suitable for numerical simulations using nested mesh refinement. The Poisson equation is solved from coarse to fine levels using a one-way interface scheme. We introduce antisymmetrically linear interpolation for evaluating the boundary conditions across the multigrid hierarchy. The spurious forces commonly observed at the interfaces between refinement levels are effectively suppressed. We validate the method using two- and three-dimensional density-force pairs that are sufficiently smooth for probing the order of accuracy.

Keywords: gravitation; computational methods
A NICER View of a Highly Absorbed Flare in GRS 1915+105

After 26 yr in outburst, the black hole X-ray binary GRS 1915+105 dimmed considerably in early 2018; its flux dropped sharply in mid-2019, and it has remained faint ever since. This faint period, the "obscured state," is punctuated by occasional X-ray flares, many of which have been observed by NICER as part of our regular monitoring program. Here we present detailed time-resolved spectroscopy of one bright flare, whose spectrum shows evidence of high column density partial covering absorption and extremely deep absorption lines (equivalent widths over 100 eV in some cases). We study the time-dependent ionization of the obscuring gas with XSTAR, ultimately attributing the absorption to a radially stratified absorber of density ∼1012-1013 cm-3 at ∼few ×1011 cm from the black hole. We argue that a vertically extended outer disk could explain this obscuration. We discuss several scenarios to explain the obscured state, including massive outflows, an increase in the mass accretion rate, and changes in the outer disk that herald the approach of quiescence, but none are entirely satisfactory. Alternative explanations, such as obscuration by the accretion stream impact point, may be testable with current or future data.

Keywords: astrophysical black holes; stellar mass black holes; accretion; x-ray astronomy; black holes; stellar winds
Critically Evaluated Atomic Data for the Au IV Spectrum

A spectral investigation of triply ionized gold (Au IV) has been carried out in the wavelength region of 500-2106 Å. The gold spectra were photographed at the National Institute of Standards and Technology, USA, on a 10.7 m normal incidence vacuum spectrograph (NIVS) using a sliding spark source as well as on a 3 m NIVS at the Antigonish laboratory in Canada with a triggered spark source. Our analysis is theoretically supported by the pseudorelativistic Hartree-Fock (HFR) formalism with a superposition of configuration interactions implemented in Cowan's suite of codes. Radiative transition parameters are also calculated using the HFR+CPOL (core polarization effects) model and the multiconfiguration Dirac-Hartree-Fock approach, and their comparisons are used to evaluate the transition rate data. All the previously reported levels of the 5d 8, 5d 76s, and 5d 76p configurations are confirmed, except one, and three are newly established. The missing 1 S 0 level of 5d 8 is now established at 55,277.8 cm-1. A total of 981 observed lines (E1 type), classified to 1031 transitions, including 133 newly identified, enabled us to optimize 139 energy levels. Several astrophysically important transitions, forbidden (M1- and E2-type) lines of 5d 8 and 5d 76s, are provided with their Ritz wavelengths and radiative parameters. A critically evaluated set of energy levels, observed and Ritz wavelengths along with their uncertainties, transition rates, and uniformly scaled intensities of Au IV lines have been presented. Also, large-scale atomic data to compute the opacity of Au IV in the kilonova ejecta have been supplemented in this work.

Keywords: atomic physics; atomic spectroscopy; line intensities; line positions; transition probabilities; spectral line identification; atomic data; atomic data benchmarking
The TESS-Keck Survey. IV. A Retrograde, Polar Orbit for the Ultra-low-density, Hot Super-Neptune WASP-107b

We measured the Rossiter-McLaughlin effect of WASP-107b during a single transit with Keck/HIRES. We found the sky-projected inclination of WASP-107b's orbit, relative to its host star's rotation axis, to be $| \lambda | ={118}_{-19}^{+38}$ degrees. This confirms the misaligned/polar orbit that was previously suggested from spot-crossing events and adds WASP-107b to the growing population of hot Neptunes in polar orbits around cool stars. WASP-107b is also the fourth such planet to have a known distant planetary companion. We examined several dynamical pathways by which this companion could have induced such an obliquity in WASP-107b. We find that nodal precession and disk dispersal-driven tilting can both explain the current orbital geometry while Kozai-Lidov cycles are suppressed by general relativity. While each hypothesis requires a mutual inclination between the two planets, nodal precession requires a much larger angle, which for WASP-107 is on the threshold of detectability with future Gaia astrometric data. As nodal precession has no stellar type dependence, but disk dispersal-driven tilting does, distinguishing between these two models is best done on the population level. Finding and characterizing more extrasolar systems like WASP-107 will additionally help distinguish whether the distribution of hot-Neptune obliquities is a dichotomy of aligned and polar orbits or if we are uniformly sampling obliquities during nodal precession cycles.

Keywords: exoplanet dynamics; orbit determination; radial velocity
Solar Magneto-seismology of a Magnetic Slab in an Asymmetric Magnetic Environment

Diagnosing the solar atmospheric plasma remains one of the major challenges in solar physics. In recent years, new methods have been developed to apply the powerful concept of solar magneto-seismology (SMS) to obtain information about plasma parameters in solar structures guiding magnetohydrodynamic (MHD) waves that would otherwise be difficult to measure. This paper uses the Cartesian model of a magnetic slab placed in an asymmetric magnetic environment to generalize recently discovered SMS techniques. Utilizing the fact that the asymmetric environment changes the character of the classical kink and sausage eigenmodes, we describe two spatial seismology methods built upon this mixed character of quasi-sausage and quasi-kink modes. First, we present the amplitude ratio technique, which compares the oscillation amplitudes measured at the two boundaries of the slab, and we provide expressions to estimate the internal Alfvén speed in the thin slab and in the incompressible plasma approximations. The second main technique relies on the changed distribution of wave power throughout the slab under the effect of waveguide asymmetry. This minimum perturbation shift technique is then also utilized to provide Alfvén speed estimates that depend on the plasma and magnetic parameters of the environment, as well as the measured slab width and oscillation frequency. Finally, we perform a brief investigation of how the amplitude ratio and the minimum perturbation shift depend on the different sources of waveguide asymmetry, and illustrate our findings with numerical results.

Keywords: solar physics; the sun; magnetohydrodynamics; solar atmosphere; solar coronal seismology; solar coronal waves
Kinetic Equilibrium of Two-dimensional Force-free Current Sheets

Force-free current sheets are local plasma structures with field-aligned electric currents and approximately uniform plasma pressures. Such structures, widely found throughout the heliosphere, are sites for plasma instabilities and magnetic reconnection, the growth rate of which is controlled by the structure's current-sheet configuration. Despite the fact that many kinetic equilibrium models have been developed for one-dimensional force-free current sheets, their two-dimensional (2D) counterparts, which have a magnetic field component normal to the current sheets, have not received sufficient attention to date. Here, using particle-in-cell simulations, we search for such 2D force-free current sheets through relaxation from an initial, magnetohydrodynamic equilibrium. Kinetic equilibria are established toward the end of our simulations, thus demonstrating the existence of kinetic force-free current sheets. Although the system currents in the late equilibrium state remain field aligned as in the initial configuration, the velocity distribution functions of both ions and electrons systematically evolve from their initial drifting Maxwellians to their final time-stationary Vlasov state. The existence of 2D force-free current sheets at kinetic equilibrium necessitates future work in discovering additional integrals of motion of the system, constructing the kinetic distribution functions, and eventually investigating their stability properties.

Keywords: space plasmas; heliosphere; magnetic fields; interplanetary discontinuities
The Molecular Exoskeleton of the Ring-like Planetary Nebula NGC 3132

We present Submillimeter Array (SMA) mapping of 12CO J = 2 → 1, 13CO J = 2 → 1, and CN N = 2 → 1 emission from the ring-like planetary nebula NGC 3132, one of the subjects of JWST Early Release Observation near-infrared imaging. The ∼5″ resolution SMA data demonstrate that the Southern Ring's main, bright, molecule-rich ring is indeed an expanding ring, as opposed to a limb-brightened shell, in terms of its intrinsic (physical) structure. This suggests that NGC 3132 is a bipolar nebula viewed more or less pole-on (inclination ∼15°–30°). The SMA data furthermore reveal that the nebula harbors a second expanding molecular ring that is aligned almost orthogonally to the main, bright molecular ring. We propose that this two-ring structure is the remnant of an ellipsoidal molecular envelope of ejecta that terminated the progenitor star's asymptotic giant branch evolution and was subsequently disrupted by a series of misaligned fast, collimated outflows or jets resulting from interactions between the progenitor and one or more companions.

Keywords: planetary nebulae; stellar winds; molecular gas; radio interferometry