Sazuna/UAT_keywords huggingface dataset:
The Potential of Coordinated Gravitational-wave Follow-up for Improving Kilonova Detection Prospects: Lessons from GW190425. The discovery of a kilonova associated with the GW170817 binary neutron star (BNS) merger had far-reaching implications for our understanding of several open questions in physics and astrophysics. Unfortunately, since then, only one robust BNS merger has been detected through gravitational waves, GW190425, and no electromagnetic (EM) counterpart was identified for it following an uncoordinated search. In order to estimate the potential difference that coordinated follow-up could make for future events, we analyze all reported EM follow-up observations of GW190425. We find that even for a large gravitational-wave localization uncertainty, such as this one, most of the 90% probability region can be covered within hours with a coordinated search, given the observational resources expended in this case by the community. However, more than 5 days after the GW190425 merger, its uncoordinated search covered only 50% of the probability, with some areas observed over 100 times, and some never observed. According to some models, the GW190425 kilonova could have been detected, despite the larger distance and higher component masses compared to GW170817. These results emphasize that coordinated follow-up of gravitational-wave events can, in principle, significantly improve both the chances of finding EM counterparts, and the time it takes to do so, compared to uncoordinated searches.

Keywords: Observational astronomy, Time domain astronomy, Astronomical methods, Gravitational wave astronomy
Reactive Molecular Dynamics Simulations of Iron Reduction on Lunar and Asteroidal Surfaces by Micrometeoroids and Solar Wind. Regolith grains and rock surfaces altered by space weathering on the Moon and S-type asteroids contain nanometer-sized metallic iron particles, known as nanophase iron (npFe0). Several processes have been proposed to explain the reduction of Fe2+ and the formation of npFe0, such as vapor deposition due to micrometeoroid impact or solar wind irradiation, and reduction by implanted hydrogen followed by micrometeoroid impact heating. In addition, recent analyses of Chang'e-5 samples suggested disproportionation reduction (3Fe2+ → Fe0 + 2Fe3+). However, despite numerous observations and experiments, the exact chemical pathway driving npFe0 formation remains controversial. Considering the recent analyses, to investigate the charge evolution of Fe atoms during micrometeoroid impacts and solar wind irradiation, reactive molecular dynamics simulations were conducted. The simulations found that micrometeoroid impacts reduced Fe atoms in fayalite by stripping electrons from O atoms. When the pressure induced by the impact reached approximately 10 GPa, disproportionation reduction, yielding Fe3+ in addition to Fe0 near the base of the microcrater, was also induced. Proton irradiation also reduced Fe atoms by breaking Fe–O bonds and forming O–H bonds. This result shows that neutral iron atoms can be produced even in environments without significant vaporization as the Chang'e-5 samples indicate. However, in the simulations, the reduced Fe atoms did not aggregate into npFe0 particles. The simulations in this work consider the potential derived from the crystal structure of olivine. To simulate the formation of npFe0 particles, a different potential may be needed, which requires further studies.

Keywords: Near-Earth objects, Molecular physics, Asteroid surfaces
A Power Spectral Study of PHANGS Galaxies with JWST MIRI: On the Spatial Scales of Dust and Polycyclic Aromatic Hydrocarbons. The interstellar medium (ISM) consists of a diversity of structures across a range of spatial scales, intimately tied to galactic evolution. In this work, Fourier analysis is used to characterize the spatial structures of dust and polycyclic aromatic hydrocarbons (PAHs) in the ISM of Physics at High Angular Resolution in Nearby GalaxieS (PHANGS) James Webb Space Telescope (JWST) galaxies, observed in the four photometric mid-infrared filters from F770W to F2100W (i.e., 7.7–21 μm). We quantify the abundance of structure on different spatial scales using the power-law slope, α, of the spatial power spectra. The distribution of α across all length scales differs significantly between filters, with steeper slopes for PAH-dominated filters ( αF770W=2.19‑0.15+0.16 ,  αF1130W=1.88‑0.37+0.25 ) and shallower for the dust continuum ( αF1000W=1.48‑0.47+0.33 ,  αF2100W=0.94‑0.28+0.23 ). The distribution of α across galaxies is narrower for PAH-dominated than for thermal dust-dominated bands, highlighting that PAHs trace photodissociation regions dominated by similar physical processes, whereas dust structures are an integrated property over the diverse evolutionary histories of their host galaxies. Unlike dust structures, PAH-sensitive bands display a break in the power spectrum: below a characteristic scale,  ℓ0=160pc‑50pc+110pc , PAH structures are suppressed.

Keywords: Interstellar dust, Polycyclic aromatic hydrocarbons, Galaxy structure, Interstellar emissions, Dust continuum emission, Interstellar clouds, Interstellar medium, Interstellar dust processes
Discovery of Small-scale Flows in the Void of a Coronal Mass Ejection with High-cadence Images Acquired by the Metis Coronagraph on Board Solar Orbiter. On 2022 March 26, the ESA Solar Orbiter mission observed the early evolution of a coronal mass ejection (CME). On that day, the spacecraft was at a heliocentric distance of 0.32 au and a longitude separation from Earth of 74 .° 5. The CME source region, observed with the Solar Orbiter Polarimetric and Helioseismic Imager magnetometer, shows no preexisting filament or flux rope. The event was first observed in the inner corona by the Solar Orbiter Extreme Ultraviolet Imager instrument with the Full Sun Imager telescope, showing the initial propagation of a flux rope seen face-on in the EUV, and formed during the eruption. Higher up, the event was observed by Metis with the Visible Light channel with an unprecedented time cadence of 20 s and a spatial resolution of 20" corresponding to about 4600 km bin‑1. The sequence of total brightness images shows the existence of small-scale circular flows inside the expanding flux rope, surrounded by multiple nested arch-shaped features. These motions, never reported so far, occur inside the void of the CME, with projected speed ∼40% higher than the CME propagation speed. The formation of the flux rope during the eruption suggests that these motions can be interpreted as a signature of conversion of magnetic writhe into twist, starting from the shearing of a preexisting arcade.

Keywords: Astronomy data analysis, Coronagraphic imaging, Solar corona, Active solar corona, Polarimetry, Astronomy image processing
Properties of the GSN 069 Accretion Disk from a Joint X-Ray and UV Spectral Analysis: Stress-testing Quasi-periodic Eruption Models. We present an analysis of Hubble Space Telescope (HST) and XMM-Newton data of the tidal disruption event (TDE) candidate and quasi-periodic eruption (QPE) source GSN 069. Using ultraviolet (UV) and optical images at HST resolution, we show that GSN 069's emission consists of a point source superimposed on a diffuse stellar component. The latter accounts for ≤5% of the UV emission in the inner 0 .″ 5 × 0 .″ 5 region, while the luminosity of the former cannot be attributed to stars. Analyzing the 2014/2018 HST UV spectra, we show that to leading order the intrinsic spectral shape is ν Lν ∝ ν4/3, with ∼10% far-UV flux variability between epochs. The contemporaneous X-ray and UV spectra can be modeled self-consistently in a thin disk framework. At observed epochs, the disk had an outer radius (Rout) of  O(103Rg) , showing both cooling and expansion over 4 yr. Incorporating relativistic effects via numerical ray tracing, we constrain the disk inclination angle (i) to be 30∘ ≲ i ≲ 65∘ and identify a narrow region of spin–inclination parameter space that describes the observations. These findings confirm that GSN 069 hosts a compact, viscously expanding accretion disk likely formed after a TDE. The implications for QPE models are (i) that no published disk-instability model can explain the disk's stability in 2014 (no QPEs) and its instability in 2018 (QPEs present), and (ii) while the disk size in 2018 allows for orbiter–disk interactions to produce QPEs, in 2014 the disk was already sufficiently extended, yet no QPEs were present. These findings pose challenges to existing QPE models.

Keywords: Accretion, Supermassive black holes, High energy astrophysics, X-ray transient sources, Time domain astronomy
Modeling of Lightcurves from Reconnection-powered Very High-energy Flares from M87*. The black hole at the center of M87 is observed to flare regularly in the very high-energy (VHE) band, with photon energies ≳100 GeV. The rapid variability, which can be as short as 2 days in the VHE lightcurve, constrains some of the flares to originate close to the black hole. Magnetic reconnection is a promising candidate for explaining the flares, where the VHE emission comes from background soft photons that inverse Compton scatter off of high-energy electron–positron pairs in the reconnecting current sheet. In this work, we ray trace photons from a current sheet near the black hole event horizon during a flux eruption in a magnetically arrested state in a general relativistic magnetohydrodynamics simulation. We incorporate beaming of the Compton up-scattered photons, based on results from radiative kinetic simulations of relativistic reconnection. We then construct VHE lightcurves that account for the dynamics of the current sheet and lensing from general-relativistic effects. We find that most of the flux originates in the inner 5 gravitational radii, and beaming is essential to explain the observed flux from the strongest VHE flares. The ray traced lightcurves show features resulting from the changing volume of the reconnecting current sheet on timescales that can be consistent with observations. Furthermore, we find that the amount of beaming depends strongly on two effects: (i) the current sheet inclination with respect to the observer and (ii) the anisotropy in the direction of motion of the accelerated particles.

Keywords: Gamma-ray observatories, Magnetohydrodynamics, Black holes, High energy astrophysics
L1448 IRS3B: Dust Polarization Aligned with Spiral Features, Tracing Gas Flows. Circumstellar disk dust polarization in the (sub)millimeter is, for the most part, not from dust grain alignment with magnetic fields but rather indicative of a combination of dust self-scattering with a yet unknown alignment mechanism that is consistent with mechanical alignment. While the observational evidence for scattering has been well established, that for mechanical alignment is less so. Circum-multiple dust structures in protostellar systems provide a unique environment to probe different polarization alignment mechanisms. We present ALMA Band 4 and Band 7 polarization observations toward the multiple young system L1448 IRS3B. The polarization in the two bands are consistent with each other, presenting multiple polarization morphologies. On the size scale of the inner envelope surrounding the circum-multiple disk, the polarization is consistent with magnetic field dust grain alignment. On the very small scale of compact circumstellar regions, we see polarization that is consistent with scattering around source a and c, which are likely the most optically thick components. Finally, we see polarization that is consistent with mechanical alignment of dust grains along the spiral dust structures, which would suggest that the dust is tracing the relative gas flow along the spiral arms. If the gas-flow dust grain alignment mechanism is dominant in these cases, disk dust polarization may provide a direct probe of the small-scale kinematics of the gas flow relative to the dust grains.

Keywords: Protostars, Circumstellar disks
Polarimetric Signatures of Bulk Comptonization from within the Plunging Region of Accreting Black Holes. Inverse Compton scattering by the thermal motions of electrons is believed to produce polarized hard X-rays in active galactic nuclei and black hole binaries. Meanwhile, plasma within the plunging region of the black hole free-falls into the event horizon with a bulk relativistic speed, which could also imprint polarization on upscattered photons but has not been discussed in detail. To examine this, we computed polarimetric signatures via general relativistic ray tracing of a toy model consisting of an accreting, geometrically thin plasma with moderate optical depth, falling into the black hole with a bulk relativistic speed within the plunging region. We show that the maximum spatially unresolved linear polarization could be as large as approximately 7%–8% when the black hole is viewed near edge-on, while the corresponding resolved linear polarization could be roughly 50%. The large discrepancy between the two is due to (1) dilution from the radiation outside the plunging region and (2) substantial cancellations of the Stokes Q and U fluxes. The resultant polarization contributed by bulk Comptonization could nevertheless exceed that of thermal electron scattering in a Novikov–Thorne disk. Our results thus suggest a new model for imprinting considerable polarization on the electromagnetic observables of accreting black holes. Measurements of X-ray polarization from black hole binaries and the central black hole of active galactic nuclei could provide direct detection of the plunging region and help constrain plasma properties in the immediate vicinity of the event horizon.

Keywords: Black hole physics, Black holes, High energy astrophysics, General relativity, Radiative transfer, Polarimetry, Astrophysical black holes
A Spectroscopic Survey of Metal-poor OB Stars in Local Dwarf Galaxy NGC 3109. As JWST uncovers increasingly strong evidence that metal-poor, massive stars in early galaxies dominated reionization, observational constraints on the properties of such stars are more relevant than ever before. However, spectra of individual O- and B-type stars are rare at the relevant metallicities (≲0.2 Z⊙), leaving models of stellar evolution and ionizing flux poorly constrained by data in this regime. We present new medium-resolution (R ∼ 4000) optical spectra of 17 OB stars in the local low-metallicity (0.12 Z⊙) dwarf galaxy NGC 3109. We assign spectral types to the stars and present new criteria for selecting O stars using optical and near-UV photometry from Hubble Space Telescope imaging. We fit the spectra and photometry with grids of stellar atmosphere models to measure stellar temperatures, surface gravities, luminosities, radii, and masses. We find evidence of strong mass loss via radiation-driven stellar winds in two O stars, one of which is the hottest, youngest, and most massive star confirmed in the host galaxy to date. Though its spectrum does not meet conventional Wolf─Rayet spectral classification criteria, this metal-poor O5 If star produces strong He II λ4686 emission, and its evolutionary status is ambiguous. This work nearly doubles the number of OB stars with measured parameters in NGC 3109, including 12 stars with no previously reported parameters, five with no published spectroscopy, and four binary candidates. This large sample of OB stellar parameters provides a new observational testbed to constrain the stellar astrophysics that drove cosmic reionization and influenced the evolution of the earliest galaxies.

Keywords: Early-type stars, Massive stars, OB stars, Stellar winds, Stellar atmospheres
Toward a Self-consistent Hydrodynamical Model of the Solar Tachocline. The solar tachocline is a thin internal boundary layer in the Sun located between the differentially rotating convection zone and the uniformly rotating region of the radiative interior beneath. E. A. Spiegel &amp; J. P. Zahn proposed the first hydrodynamical model, which here we call SZ92, arguing that the tachocline is essentially in a steady state of thermal-wind balance, angular-momentum balance, and thermal equilibrium. Angular momentum transport in their model is assumed to be dominated by strongly anisotropic turbulence, which is primarily horizontal, owing to the strong stable stratification of the radiative interior. By contrast, the heat transport is assumed to be dominated by a predominantly vertical diffusive heat flux, owing to the thinness of the tachocline. In this paper, we demonstrate that these assumptions are not consistent with the new model of stratified turbulence recently proposed by G. P. Chini et al. and K. Shah et al., which has been numerically validated by P. Garaud et al. We then propose a simple self-consistent alternative to the SZ92 model—namely, a scenario wherein angular momentum and heat transport are both dominated by horizontal turbulent diffusion. The thickness of the tachocline in the new model scales as Ω⊙/Nm, where Ω⊙ is the mean angular velocity of the Sun and Nm is the characteristic buoyancy frequency in the tachocline region. We discuss other properties of the model and show that it has several desirable features but does not resolve some of the other well-known problems of the SZ92 model.

Keywords: Stellar rotation
Atmospheric Heating Events Associated with Fine-scale Flux Emergence in Ephemeral Regions. Coronal heating has puzzled solar physicists for decades. The question of why the Sun's upper atmosphere is significantly hotter than its lower atmosphere remains a key mystery. It is commonly believed that the source of coronal heating comes from the Sun's magnetic field, and more complex magnetic dynamics are more efficient in heating. In an earlier work, we studied the secondary (or finer-scale) flux emergence identified in five ephemeral regions (ERs), selected during the last solar minimum. Here we further explore the atmospheric response to the secondary flux emergences (SFEs) that were identified in the first paper. We further reveal that approximately 80% of the 172 identified SFEs are associated with atmospheric heating events. The heating is most likely associated with magnetic reconnection involving the SFE. Overall, a solar quiet region is heated by several hundred thousand degrees during the flux emergence of an ER.

Keywords: Solar activity, Solar coronal heating, Solar magnetic flux emergence, Solar magnetic fields
Enhanced Nuclear Binding near the Proton Drip Line Opens Possible Bypass of the <SUP>64</SUP>Ge Rapid Proton Capture Process Waiting Point. We performed astrophysics model calculations with updated nuclear data to identify a possible bypass of the 64Ge waiting point, a defining feature of the rapid proton capture (rp) process that powers type I X-ray bursts on accreting neutron stars. We find that the rp-process flow through the 64Ge bypass could be up to 36% for astrophysically relevant conditions. Our results call for new studies of 65Se, including the nuclear mass, β-delayed proton emission branching, and nuclear structure as it pertains to the 64As(p, γ) reaction rate at X-ray burst temperatures.

Keywords: Nuclear astrophysics, Nuclear physics, X-ray bursters
Inference of Multichannel r-process Element Enrichment in the Milky Way Using Binary Neutron Star Merger Observations. Observations of GW170817 strongly suggest that binary neutron star (BNS) mergers produce rapid neutron-capture nucleosynthesis (r-process) elements. However, it remains an open question whether these mergers can account for all the r-process element enrichment in the Milky Way's history. Here, we constrain the contributions of the BNS channel using astrophysical neutron star observations. The rate and mass distributions are constrained by LIGO/Virgo/Kagra through the latest catalog GWTC-3, the neutron star equation of state by gravitational-wave, radio, and X-ray observations, and the delay time distribution by short gamma-ray burst (GRB) host galaxy associations. We present a Bayesian framework to consistently combine these observations with abundance information to quantify the contribution and uncertainties of single and multiple astrophysical enrichment sources, and obtain a distribution of per-event BNS r-process element yields consistent with geophysical and astrophysical abundance constraints. We then adopt a Galactic chemical evolution model assuming an instantaneous and fixed amount of Fe enrichment from core-collapse supernovae, and show that BNS-only enrichment scenarios remain inconsistent with the observed r-process abundance trend of disk stars in the Galaxy even with the uncertainties in BNS merger observations. Using stellar abundance observations instead of the short GRB constraints, we can infer a shorter BNS delay time distribution with power-law index α ≤ −2.0 and minimum delay time  tmin≤40  Myr at 90% confidence, consistent with detailed Galactic chemical evolution models. Such delay times are in tension with those predicted by standard BNS formation models. Alternatively, we confirm that a two-channel scenario, in which the second channel tracks the star formation history without significant delay, can account for both Galactic stellar and short GRB observations. We estimate that 45%─90% of the r-process abundance in the Milky Way today would have been produced by this star formation-tracking channel, rather than BNS mergers with significant delay times.

Keywords: Gravitational wave astronomy, R-process, Nuclear astrophysics
Simulation Study of Optical Band Radiation during the Afterglow Phase of Gamma-Ray Bursts Based on the HADAR Experiment. Studying the afterglow of gamma-ray bursts (GRBs) is essential for refining theoretical models, uncovering their underlying physical processes, and exploring potential new physics. Multiwavelength investigations have emerged as a fundamental approach for deciphering the complex nature of GRBs, harnessing the complementary observational capabilities of advanced spaceborne observatories and sophisticated ground-based facilities. The High Altitude Detection of Astronomical Radiation (HADAR) experiment utilizes a novel wide-angle atmospheric Cherenkov detection technology, offering the advantages of a wide field of view and a low energy threshold. It is capable of not only observing the very-high-energy (VHE) gamma radiation from GRBs but also conducting observational studies on the optical afterglow of GRBs. In this work, we simulated the expected observations of HADAR for the optical and VHE gamma radiation afterglow of GRB 190114C. The results indicate that HADAR can effectively observe both bands of this GRB, particularly demonstrating excellent sensitivity in the optical band and providing good imaging of its radiation process. This demonstrates that HADAR is capable of conducting effective observations and research on the optical radiation of similar GRBs. Its contributions can refine relevant theoretical research and may enable attempts to observe gravitational-wave counterparts.

Keywords: Gamma-ray bursts
Survivor Bias in Longitudinal Asymmetry of Impulsive Solar Energetic Particle Events. We follow H.-Q. He and W. Wan in studying the distribution of impulsive solar energetic particle (SEP) events from the updated observations. We confirm their findings of the longitudinal asymmetry of the distribution of SEPs with respect to the east side and west side of the magnetic footpoint, which is indicated as the "east–west azimuthal asymmetry." With a qualitative analysis, H.-Q. He and W. Wan suggested that this asymmetry is caused by perpendicular diffusion and heliospheric magnetic field structure. We use a simple probability-based statistical method to provide the distribution of SEP events as an analytical function of relative longitude. We find that the impulsive SEP events not correlated to observed flares were indeed flare-induced but went unobserved due to the flares occurring on the far side of the Sun. Furthermore, the observed east–west azimuthal asymmetry of SEPs is caused by the survivor bias.

Keywords: Solar energetic particles, Solar flares