adsabs/SciX_UAT_keywords huggingface dataset:
Seven Reflares, a Mini Outburst, and an Outburst: High-amplitude Optical Variations in the Black Hole X-Ray Binary Swift J1910.2-0546

We present long-term (2012-2022) optical monitoring of the candidate black hole X-ray binary Swift J1910.2-0546 with the Faulkes Telescopes and Las Cumbres Observatory network. Following its initial bright 2012 outburst, we find that the source displayed a series of at least seven quasi-periodic, high-amplitude (~3 mag) optical reflares in 2013, with a recurrence time increasing from ~42 to ~49 days. In 2014, the source experienced a mini outburst with two peaks in the optical. We also study the recent 2022 outburst of the source at optical wavelengths, and perform a comparative analysis with the earlier rebrightenings. A single X-ray detection and only two radio detections were obtained during the 2013 reflaring period, and only optical detections were acquired in 2014. During the reflaring in both 2013 and 2014, the source showed bluer-when-brighter behavior, having optical colors consistent with blackbody heating and cooling between 4500 and 9500 K, i.e., the temperature range in which hydrogen starts to ionize. Finally, we compare the flaring behavior of the source to rebrightening events in other X-ray binaries. We show that the repeated reflarings of Swift J1910.2-0546 are highly unusual, and propose that they arise from a sequence of repetitive heating and cooling front reflections traveling through the accretion disk.

Keywords: stellar mass black holes; low-mass x-ray binary stars; accretion; jets; optical astronomy
The Star Formation in Radio Survey: 3-33 GHz Imaging of Nearby Galaxy Nuclei and Extranuclear Star-forming Regions

We present 3, 15, and 33 GHz imaging toward galaxy nuclei and extranuclear star-forming regions using the Karl G. Jansky Very Large Array as part of the Star Formation in Radio Survey. With 3-33 GHz radio spectra, we measured the spectral indices and corresponding thermal (free-free) emission fractions for a sample of 335 discrete regions having significant detections in at least two radio bands. After removing 14 likely background galaxies, we find that the median thermal fraction at 33 GHz is 92% ± 0.8% with a median absolute deviation of 11%, when a two-component power-law model is adopted to fit the radio spectrum. Limiting the sample to 238 sources that are confidently identified as star-forming regions and not affected by potential AGN contamination (i.e., having galactocentric radii rG ≥ 250 pc) results in a median thermal fraction of 93% ± 0.8% with a median absolute deviation of 10%. We further measure the thermal fraction at 33 GHz for 163 regions identified at 7″ resolution to be 94% ± 0.8% with a median absolute deviation of 8%. Together, these results confirm that free-free emission dominates the radio spectra of star-forming regions on scales up to ∼500 pc in normal star-forming galaxies. We additionally find a factor of ∼1.6 increase in the scatter of the measured spectral index and thermal fraction distributions as a function of decreasing galactocentric radius. This trend is likely reflective of the continuous star formation activity occurring in the galaxy centers, resulting in a larger contribution of diffuse nonthermal emission relative to star-forming regions in the disk.

Keywords: star formation; h ii regions; radio continuum emission; radio interferometry; radio astronomy; galaxies; galaxy evolution; spiral galaxies; extragalactic radio sources
Opacities of S-type Stars: The Singlet B 1Π-X 1Σ+, B 1Π-A 1△, and C 1Σ+-X 1Σ+ Band Systems of ZrO

The ZrO B 1Π-X 1Σ+, B 1Π-A 1Δ, and C 1Σ+-X 1Σ+ band systems are important opacity sources in the near-infrared and optical spectra of S-type stars. A total of 21 rovibronic bands with v″ ≤ 7 and $v^{\prime} \leqslant 5$ were observed and fit for the B 1Π-X 1Σ+ transition, five bands for the 90ZrO B 1Π-A 1Δ transition and one band for the 90ZrO C 1Σ+-X 1Σ+ transition. All band systems were analyzed using high-temperature, high-resolution emission spectra collected at the National Solar Observatory (Kitt Peak). A modern spectroscopic analysis was performed using the PGOPHER program to provide updated spectroscopic constants. In general, we improve the accuracy of the line positions reported in the literature and slightly extend the vibrational analysis. Equilibrium molecular constants were then derived and combined with new ab initio calculations of transition dipole moment functions to produce line lists with line strengths.

Keywords: spectral line lists; molecular spectroscopic constants; rotational spectroscopy
A Floor in the Sun's Photospheric Magnetic Field: Implications for an Independent Small-scale Dynamo

Clette recently showed that F 10.7 systematically approaches a quiet Sun daily value of 67 solar flux units (sfu) at solar minima as the number of spotless days on the Sun increases. Previously, a floor of ∼2.8 nT had been proposed for the solar wind (SW) magnetic field strength (B). F 10.7, which closely tracks the Sun's unsigned photospheric magnetic flux, and SW B exhibit different relationships to their floors at 11 yr solar minima during the last ∼50 yr. While F 10.7 approaches 67 sfu at each minimum, the corresponding SW B is offset above ∼2.8 nT by an amount approximately proportional to the solar polar field strength—which varied by a factor of ∼2.5 during this interval. This difference is substantiated by ∼130 yr of reconstructed F 10.7 (via the range of the diurnal variation of the East-component (rY) of the geomagnetic field) and SW B (based on the interdiurnal variability geomagnetic activity index). For the last ∼60 yr, the contribution of the slow SW to SW B has exhibited a floor-like behavior at ∼2 nT, in contrast to the contributions of coronal mass ejections and high-speed streams that vary with the solar cycle. These observations, as well as recent SW studies based on Parker Solar Probe and Solar Dynamics Observatory data, suggest that (1) the Sun has a small-scale turbulent dynamo that is independent of the 11 yr sunspot cycle; and (2) the small-scale magnetic fields generated by this nonvarying turbulent dynamo maintain a constant open flux carried to the heliosphere by the Sun's floor-like slow SW.

Keywords: solar magnetic fields; quiet sun; solar dynamo; solar cycle; solar radio emission; solar-terrestrial interactions; slow solar wind
Explaining the GWSkyNet-Multi Machine Learning Classifier Predictions for Gravitational-wave Events

GWSkyNet-Multi is a machine learning model developed for the classification of candidate gravitational-wave events detected by the LIGO and Virgo observatories. The model uses limited information released in the low-latency Open Public Alerts to produce prediction scores indicating whether an event is a merger of two black holes (BHs), a merger involving a neutron star (NS), or a non-astrophysical glitch. This facilitates time-sensitive decisions about whether to perform electromagnetic follow-up of candidate events during LIGO-Virgo-KAGRA (LVK) observing runs. However, it is not well understood how the model is leveraging the limited information available to make its predictions. As a deep learning neural network, the inner workings of the model can be difficult to interpret, impacting our trust in its validity and robustness. We tackle this issue by systematically perturbing the model and its inputs to explain what underlying features and correlations it has learned for distinguishing the sources. We show that the localization area of the 2D sky maps and the computed coherence versus incoherence Bayes factors are used as strong predictors for distinguishing between real events and glitches. The estimated distance to the source is further used to discriminate between binary BH mergers and mergers involving NSs. We leverage these findings to show that events misclassified by GWSkyNet-Multi in LVK's third observing run have distinct sky areas, coherence factors, and distance values that influence the predictions and explain these misclassifications. The results help identify the model's limitations and inform potential avenues for further optimization.

Keywords: gravitational wave astronomy; gravitational wave sources; convolutional neural networks
Spicule Jets in the Solar Atmosphere Modeled with Resistive MHD and Thermal Conduction

Using numerical simulations, we study the effects of magnetic resistivity and thermal conductivity in the dynamics and properties of solar jets with characteristics of Type II spicules and cool coronal jets. The dynamic evolution of the jets is governed by the resistive MHD equations with thermal conduction along the magnetic field lines on a 2.5D slice. The magnetic field configuration consists of two symmetric neighboring loops with opposite polarity, used to support reconnection and followed by the plasma jet formation. In total, 10 simulations were carried out with different values of resistivity and thermal conductivity that produce jets with different morphological and thermal properties we quantify. We find that an increase in magnetic resistivity does not produce significant effects on the morphology, velocity, and temperature of the jets. However, thermal conductivity affects both temperature and morphology of the jets. In particular, thermal conductivity causes jets to reach greater heights and increases the temperature of the jet-apex. Also, heat flux maps indicate the jet-apex and corona interchange energy more efficiently than the body of the jet. These results could potentially open a new avenue for plasma diagnostics in the Sun's atmosphere.

Keywords: solar spicules; solar chromosphere; solar physics; solar atmosphere
Energetic Particle Perpendicular Diffusion: Simulations and Theory in Noisy Reduced Magnetohydrodynamic Turbulence

The transport of energetic charged particles (e.g., cosmic rays) in turbulent magnetic fields is usually characterized in terms of the diffusion parallel and perpendicular to a large-scale (or mean) magnetic field. The nonlinear guiding center theory has been a prominent perpendicular diffusion theory. A recent version of this theory, based on the random ballistic spreading of magnetic field lines and a backtracking correction (RBD/BC), has shown good agreement with test particle simulations for a two-component magnetic turbulence model. The aim of the present study is to test the generality of the improved theory by applying it to the noisy reduced magnetohydrodynamic (NRMHD) turbulence model, determining perpendicular diffusion coefficients that are compared with those from the field line random walk (FLRW) and unified nonlinear (UNLT) theories and our test particle simulations. The synthetic NRMHD turbulence model creates special conditions for energetic particle transport, with no magnetic fluctuations at higher parallel wavenumbers so there is no resonant parallel scattering if the particle Larmor radius R L is even slightly smaller than the minimum resonant scale. This leads to nonmonotonic variation in the parallel mean free path λ ∥ with R L. Among the theories considered, only RBD/BC matches simulations within a factor of 2 over the range of parameters considered. This accuracy is obtained even though the theory depends on λ ∥ and has no explicit dependence on R L. In addition, the UNLT theory often provides accurate results, and even the FLRW limit provides a very simple and reasonable approximation in many cases.

Keywords: cosmic rays; magnetic fields; particle astrophysics
ARMADA. I. Triple Companions Detected in B-type Binaries α Del and ν Gem

Ground-based optical long-baseline interferometry has the power to measure the orbits of close binary systems at ∼10 μas precision. This precision makes it possible to detect "wobbles" in the binary motion due to the gravitational pull from additional short-period companions. We started the ARrangement for Micro-Arcsecond Differential Astrometry (ARMADA) survey with the Michigan Infra-Red Combiner (MIRC)/MIRC-X instrument at the Center for High Angular Resoloution Astronomy (CHARA) array for the purpose of detecting giant planets and stellar companions orbiting individual stars in binary systems. We describe our observations for the survey, and introduce the wavelength calibration scheme that delivers precision at the tens of microarcseconds level for <0"2 binaries. We test our instrument performance on a known triple system, κ Peg, and show that our survey is delivering a factor of 10 better precision than previous similar surveys. We present astrometric detections of tertiary components to two B-type binaries: a 30 day companion to α Del, and a 50 day companion to ν Gem. We also collected radial velocity data for α Del with the Tennessee State University Automated Spectroscopic Telescope at Fairborn Observatory. We are able to measure the orbits and masses of all three components in these systems. We find that the previously published radial velocity orbit for the inner pair of ν Gem is not consistent with our visual orbit. The precision achieved for these orbits suggests that our ARMADA survey will be successful at discovering new compact triple systems to A/B-type binary systems, leading to better statistics of hierarchical system architectures and formation history.

Keywords: astrometry; close binary stars; optical interferometry
Search for Coincident Gravitational-wave and Fast Radio Burst Events from 4-OGC and the First CHIME/FRB Catalog

Advanced LIGO and Virgo have reported 90 confident gravitational-wave (GW) observations from compact-binary coalescences from their three observation runs. In addition, numerous subthreshold GW candidates have been identified. Binary neutron star (BNS) mergers can produce GWs and short-gamma-ray bursts, as confirmed by GW170817/GRB 170817A. There may be electromagnetic counterparts recorded in archival observations associated with subthreshold GW candidates. The CHIME/FRB Collaboration has reported the first large sample of fast radio bursts (FRBs), millisecond radio transients detected up to cosmological distances; a fraction of these may be associated with BNS mergers. This work searches for coincident GWs and FRBs from BNS mergers using candidates from the fourth Open Gravitational-wave Catalog and the first CHIME/FRB catalog. We use a ranking statistic for GW/FRB association that combines the GW detection statistic with the odds of temporal and spatial association. We analyze GW candidates and nonrepeating FRBs from 2019 April 1 to 2019 July 1, when both the Advanced LIGO/Virgo GW detectors and the CHIME radio telescope were observing. The most significant coincident candidate has a false alarm rate of 0.29 per observation time, which is consistent with a null observation. The null results imply, at most, ${ \mathcal O }(0.01) \% $ - ${ \mathcal O }(1) \% $ of FRBs are produced promptly from the BNS mergers.

Keywords: gravitational wave astronomy; radio bursts; high energy astrophysics
CEERS Spectroscopic Confirmation of NIRCam-selected z ≳ 8 Galaxy Candidates with JWST/NIRSpec: Initial Characterization of Their Properties

We present JWST NIRSpec spectroscopy for 11 galaxy candidates with photometric redshifts of z ≃ 9 - 13 and M UV ∈ [ -21, -18] newly identified in NIRCam images in the Cosmic Evolution Early Release Science Survey. We confirm emission line redshifts for 7 galaxies at z = 7.762-8.998 using spectra at ~1-5 μm either with the NIRSpec prism or its three medium-resolution (R ~ 1000) gratings. For z ≃ 9 photometric candidates, we achieve a high confirmation rate of ≃90%, which validates the classical dropout selection from NIRCam photometry. No robust emission lines are identified in three galaxy candidates at z > 10, where the strong [O III] and Hβ lines would be redshifted beyond the wavelength range observed by NIRSpec, and the Lyα continuum break is not detected with the sensitivity of the current data. Compared with Hubble Space Telescope-selected bright galaxies (M UV ≃ -22) that are similarly spectroscopically confirmed at z ≃ 8 - 9, these NIRCam-selected galaxies are characterized by lower star formation rates (SFRs; SFR ≃ 4 M ⊙ yr-1) and lower stellar masses (≃108 M ⊙), but with higher specific SFR (≃40 Gyr-1), higher [O III]+Hβ equivalent widths (≃1100 Å), and elevated production efficiency of ionizing photons ( $\mathrm{log}({\xi }_{\mathrm{ion}}/\mathrm{Hz}\,{\mathrm{erg}}^{-1})\simeq 25.8$ ) induced by young stellar populations (<10 Myr) accounting for ≃20% of the galaxy mass, highlighting the key contribution of faint galaxies to cosmic reionization. Taking advantage of the homogeneous selection and sensitivity, we also investigate metallicity and ISM conditions with empirical calibrations using the [O III]5008/Hβ ratio. We find that galaxies at z ≃ 8 - 9 have higher SFRs and lower metallicities than galaxies at similar stellar masses at z ≃ 2 - 6, which is generally consistent with the current galaxy formation and evolution models.

Keywords: early universe; galaxy formation; galaxy evolution; high-redshift galaxies
Unveiling the Interplay between the GASP Jellyfish Galaxy JO194 and Its Environment with Chandra

X-ray studies of jellyfish galaxies opened a window into the physics of the interplay between the intracluster medium (ICM) and interstellar medium (ISM). In this paper, we present the study of an archival Chandra observation of the GASP jellyfish galaxy JO194. We observe X-ray emission extending from the stellar disk to the unwinding spiral arms with an average temperature of kT = 0.79 ± 0.03 keV. To investigate the origin of the X-ray emission, we compare the observed X-ray luminosities with those expected from the star formation rates (SFRs) obtained from Hα emission. We estimate an X-ray luminosity excess of a factor ~2-4 with respect to the SF; therefore, we conclude that SF is not the main event responsible for the extended X-ray emission of JO194. The metallicity in the spiral arms ( $Z={0.24}_{-0.12}^{+0.19}\,{Z}_{\odot }$ ) is consistent with that of the ICM around JO194 (Z = 0.35 ± 0.07); thus, we suggest that ICM radiative cooling dominates the X-ray emission of the arms. We speculate that the X-ray plasma results from the ISM−ICM interplay, although the nature of this interplay is still mostly unknown. Finally, we observe that the X-ray properties of JO194 are consistent with those of two other GASP galaxies with different stellar mass, phase-space conditions in their hosting clusters, and local ICM conditions. We suggest that the conditions required to induce extended X-ray emission in jellyfish galaxies are established at the beginning of the stripping, and they can persist on long timescales so that galaxies in different clusters and evolutionary stages can present a similar extended X-ray emission.

Keywords: galaxy evolution; x-ray astronomy; galaxy clusters
Intrusion of MeV-TeV Cosmic Rays into Molecular Clouds Studied by Ionization, the Neutral Iron Line, and Gamma Rays

Low-energy (∼ MeV) cosmic rays (CRs) ionize molecular clouds and create the neutral iron line (Fe I Kα) at 6.4 keV. On the other hand, high-energy (≳GeV) CRs interact with the dense cloud gas and produce gamma rays. Based on a one-dimensional model, we study the spatial correlation among ionization rates of gas, 6.4 keV line fluxes, and gamma-ray emissions from a molecular cloud illuminated by CRs accelerated at an adjacent supernova remnant. We find that the spatial distributions of these three observables depend on how CRs intrude the cloud and on the internal structure of the cloud. If the intrusion is represented by slow diffusion, the 6.4 keV line should be detected around the cloud edge where ionization rates are high. On the other hand, if CRs freely stream in the cloud, the 6.4 keV line should be observed where gamma rays are emitted. In the former, the cooling time of the CRs responsible for the 6.4 keV line is shorter than their cloud crossing time, and it is opposite in the latter. Although we compare the results with observations, we cannot conclude whether the diffusion or the free-streaming is dominantly realized. Our predictions can be checked in more detail with future X-ray missions such as XRISM and Athena and by observations of ionization rates that cover wider fields.

Keywords: galactic cosmic rays; molecular clouds; x-ray sources; gamma-ray sources
KMT-2022-BLG-2397: Brown Dwarf at the Upper Shore of the Einstein Desert

We measure the Einstein radius of the single-lens microlensing event KMT-2022-BLG-2397 to be θ E = 24.8 ± 3.6 μas, placing it at the upper shore of the Einstein Desert, 9 ≲ θ E/μas ≲ 25, between free-floating planets (FFPs) and bulge brown dwarfs (BDs). In contrast to the six BD (25 ≲ θ E ≲ 50) events presented by Gould et al. (2022), which all had giant-star source stars, KMT-2022-BLG-2397 has a dwarf-star source, with angular radius θ ast ~ 0.9 μas. This prompts us to study the relative utility of dwarf and giant sources for characterizing FFPs and BDs from finite-source point-lens (FSPL) microlensing events. We find "dwarfs" (including main-sequence stars and subgiants) are likely to yield twice as many θ E measurements for BDs and a comparable (but more difficult to quantify) improvement for FFPs. We show that neither current nor planned experiments will yield complete mass measurements of isolated bulge BDs, nor will any other planned experiment yield as many θ E measurements for these objects as the Korea Microlensing Telescope (KMT). Thus, the currently anticipated 10 yr KMT survey will remain the best way to study bulge BDs for several decades to come.

Keywords: gravitational microlensing; free floating planets; gravitational microlensing exoplanet detection; brown dwarfs
Rotation and Lithium Confirmation of a 500 pc Halo for the Open Cluster NGC 2516

Recent analyses of the Gaia data have identified diffuse stellar populations surrounding nearby open clusters. It is important to verify that these "halos," "tails," and "strings" are of similar ages and compositions as stars in the denser part of the cluster. We present an analysis of NGC 2516 (≈150 Myr), which has a classical tidal radius of 10 pc and an apparent halo of stars spanning 500 pc (20° on-sky). Combining photometry from Gaia, rotation periods from TESS, and lithium measurements from Gaia-ESO and GALAH, we find that the halo of NGC 2516 is the same age as the cluster's core. Two-thirds of kinematically selected halo members out to 250 pc from the cluster center have rotation periods consistent with a gyrochronological age of 150 Myr. A comparison sample of field stars shows no such trend. The lithium abundances of stars in the halo are higher than in the field and correlated with the stellar rotation rate and binarity fraction, as has been noted in other young open clusters. Broadly speaking, this work supports a new paradigm wherein the halos of open clusters are often more populous than their cores. We highlight implications for spectroscopic survey targeting, open cluster dispersal, and planet searches around young stars. * This is Paper III of the Cluster Difference Imaging Photometric Survey.

Keywords: stellar associations; open star clusters; stellar rotation; stellar ages; stellar kinematics