Sazuna/UAT_keywords huggingface dataset:
Disk Instability Model for Quasi-periodic Eruptions: Investigating Period Dispersion and Peak Temperature. Quasi-periodic eruptions (QPEs) are a class of X-ray repeating burst phenomena discovered in recent years. Although many models have been proposed to study this phenomenon, there remains significant debate regarding the physical origin of QPEs. In our previous work, we developed a disk instability model with a large-scale magnetic field and successfully reproduced the light curves and spectral characteristics of several QPE sources. We further investigate this model in this work, aiming to explain two key observational features: the dispersion in eruption periods and the peak temperatures during eruptions. This model reveals critical thresholds ( Ṁcrit , β1,crit) that separate systems into stable regimes with minimal period variations and unstable regimes where periods are highly sensitive to the accretion rate and magnetic field parameter, while peak temperatures remain nearly constant across the parameter space. This framework successfully explains both the regular eruptions observed in sources such as GSN 069 and the stochastic behavior in sources such eRO-QPE1, while simultaneously accounting for the observed temperature stability during long-term evolution of QPEs.

Keywords: Accretion, High energy astrophysics, Active galactic nuclei, Magnetic fields
How a Klein–Nishina Modified Eddington-limited Accretion Explains Rapid Black Hole Growth in the Early Universe. The discovery of quasars and their supermassive black holes (SMBHs) over 109  M☉  merely hundreds of millions of years after the Big Bang generates tension with the idea of Eddington-limited accretion and pressures the community into exploring the concept of massive black hole seeds and/or super-Eddington accretion. The observation that many black holes have reached supermassive status while obeying the Eddington limit is puzzling, as accretion models are not spherically symmetric. We address this issue by illustrating the physics behind a picture of inner disk accretion involving a geometrically thick, hot quasi-spherical flow, and argue that such an inner region provides the radiation that instantiates the Eddington limit. Given the energetics of the inner disk edge, we show how the characteristic electron cross section drops below its Thomson value, allowing black holes to grow rapidly despite being Eddington-limited. Indeed, after implementing a modified cross section calculated via the Klein–Nishina formula, we find that SMBH formation time drops by up to 47%. In this context, we show how a 109  M☉  black hole can form from a seed 10  M☉  black hole within 500 Myr by way of accretion and mergers. While our picture is oversimplified and contrived in a number of ways that we discuss, we suggest that our scenario is interesting in that it offers a solution to two issues at the intersection of astrophysics and cosmology, namely the reason the Eddington limit is obeyed and how some black holes have grown rapidly despite that limit.

Keywords: Black holes, Astrophysical black holes
Evolution of Crab Pulsar: Magnetic Inclination Angle and Spin. The well-observed Crab pulsar helps one to uncover the underlying knowledge about pulsar evolution. The routine evolution model simultaneously describes the spin-down caused by magnetic dipole radiation and gravitational-wave radiation (GWR), the damping of the free-body precession owing to the bulk viscosity, and the GWR-induced quenching of the magnetic inclination angle χ. We explore the pulsar evolution based on this routine model supplemented with the effects of shear viscosity, r-mode, electromagnetic torque, and accretion, with the stellar thermal evolution as an important input. The impact of shear viscosity on radio pulsar evolution is negligible, as it only slightly increases the magnetic inclination angle and promotes spin-down in magnetars. Under the observational limit for its saturation amplitude, the r-mode also turns out to be completely negligible. Yet, the electromagnetic torque (under certain conditions), along with the accretion based on our three-dimensional fallback disk accretion model, is shown to suppress the growth of the magnetic inclination angle. When applied to the Crab pulsar, the routine model best reproduces the magnetic inclination angle χ, the spin period P, and the spin period derivative  Ṗ  simultaneously, indicating the important role of bulk viscosity. The inclusion of the electromagnetic torque and accretion works even worse, suggesting these two factors perhaps are overestimated for the Crab pulsar. Intriguingly, the calculated magnetic inclination angle derivative  χ̇  is (6  .° 3 × 10‑3–0 .° 3) century–1 with the routine model, also in agreement with the observed tiny  χ̇=0.°62century–1 .

Keywords: Neutron stars, Radio pulsars, Pulsars
X-Ray Measurements of <SUP>44</SUP>Ti in Four Supernova Remnants. We present the results of a search for scandium Kα X-ray fluorescence resulting from 44Ti electron capture decay in four young supernova remnants (SNRs). We analyze archival XMM-Newton data from Tycho's SNR, Kepler's SNR, Cassiopeia A, and SN 1987A. We fit the X-ray spectra in regions of interest with up to four components, thermal and power-law, as needed to fit the spectrum, and add a Gaussian to test for extra Sc emission. While all require the line with modest confidence, none represent definitive detections on their own, and all are consistent with previous gamma-ray detections or limits on the 44Ti mass.

Keywords: Supernova remnants, X-ray astronomy
Observations of Microwave Emission from Solar Jets and Comparison with Magnetohydrodynamic Simulations. We computed the thermal microwave emission from a 3D magnetohydrodynamic (MHD) simulation and compared it with observations of solar jets. The simulation treats the emergence of magnetic flux into the solar atmosphere and its interaction with a low, preexisting ambient magnetic field. This interaction leads to the formation and development of a jet, driven by an eruption. The computed 17 GHz radio emission is compared with a number of observed jets, with respect to their morphology, their flux, and the rise time of the radio flux. We find that the MHD model reproduces the characteristics of lower-intensity jets reasonably well, whereas there are differences with stronger jets. We suggest possible ways to obtain more realistic jets from MHD simulations, so that they better match the real jets.

Keywords: Solar magnetic flux emergence, Solar ultraviolet emission, Solar radio emission, Solar corona, Solar activity, Magnetohydrodynamical simulations
Enabling Robust Exoplanet Atmospheric Retrievals with Gaussian Processes. Atmospheric retrievals are essential tools for interpreting exoplanet transmission and eclipse spectra, enabling quantitative constraints on the chemical composition, aerosol properties, and thermal structure of planetary atmospheres. The James Webb Space Telescope (JWST) offers unprecedented spectral precision, resolution, and wavelength coverage, unlocking transformative insights into the formation, evolution, climate, and potential habitability of planetary systems. However, this opportunity is accompanied by challenges: modeling assumptions and unaccounted-for noise or signal sources can bias retrieval outcomes and their interpretation. To address these limitations, we introduce a Gaussian process (GP)-aided atmospheric retrieval framework that flexibly accounts for unmodeled features and correlated noise in exoplanet spectra. We validate this method on synthetic JWST observations, and show that GP-aided retrievals reduce bias in inferred abundances and better capture model–data mismatches than traditional approaches. We also introduce the concept of mean squared error to quantify the trade-off between bias and variance, arguing that this metric more accurately reflects retrieval performance than bias alone. We then reanalyze the NIRISS/SOSS JWST transmission spectrum of WASP-96 b, finding that GP-aided retrievals yield broader constraints on CO2 and H2O, possibly alleviating tension between previous retrieval results and equilibrium predictions. Our GP framework provides precise and accurate constraints while highlighting regions where models fail to explain the data. As JWST matures and future facilities come online, a deeper understanding of the limitations of both data and models will be essential, and GP-enabled retrievals like the one presented here offer a principled path forward.

Keywords: Exoplanet atmospheric composition , Astrostatistics tools, Bayesian statistics, Infrared spectroscopy, Exoplanets, Hot Jupiters, Astrostatistics, Exoplanet astronomy, James Webb Space Telescope, Gaussian Processes regression, Atmospheric composition, Exoplanet atmospheres
A NuSTAR Study of Quasiperiodic Oscillations from the Ultraluminous X-Ray Sources in M82. The study of quasiperiodic oscillations (QPOs) in X-ray binaries provides valuable insights into the physics of accretion around compact objects. The M82 galaxy hosts multiple ultraluminous X-ray sources, including two prominent ones—X-1 and X-2—where X-1 is suspected to harbor an intermediate-mass black hole. In this work, we analyze data from 39 NuSTAR observations acquired between 2014 and 2024 to investigate the aperiodic X-ray variability in M82. In particular, we study in detail the evolution of the QPOs from M82 X-1 in the range 20─300 mHz. We do not find additional timing features in the data, besides a frequently present broad noise component at lower frequencies. The QPO behaves similarly to other classes of low-frequency oscillations in accreting compact objects, both black holes and neutron stars.

Keywords: X-ray astronomy, Accretion, X-ray binary stars, Ultraluminous x-ray sources, Time domain astronomy, Compact objects
Dialog Concerning the Two Shock Codes. In this paper, we summarize the shock physics and the treatment of radiative transfer in two well-established shock codes—the MAPPINGS code and the Cox/Raymond code. We compare the ionization states, temperatures, electron densities, and energy transportations of the shock models with shock velocities of 50, 110, 150, and 300 km s‑1. In summary, both codes adopt the Rankine–Hugoniot flow equation to describe the shock flows, giving the same shock physical properties in the immediate area behind the shock fronts. The different treatments of radiative transfer in these two codes lead to somewhat different computations of the ionization and thermal structures of shocks, as well as the emission-line fluxes. This work highlights the importance of the delicate treatment of photoionization in shock models, providing insight for the future development of shock codes, such as 3D shock codes.

Keywords: Supernova remnants, Shocks, Radiative transfer, Interstellar medium
Photospheric Helium Abundance in Cool Giants: A Comprehensive Study. High-resolution optical spectra of 16 red giants, two early asymptotic giant branch (AGB) stars, and two supergiants, having no/minimal - to super-lithium (Li)-rich abundances, are analyzed to investigate the helium (He)-enhancement. The spectra of eight giants were obtained from the Himalayan Chandra Telescope, and for the rest of the program stars the spectral data were collected from various public archives. Our detailed abundance analyses of the program stars involve the determination of stellar parameters and abundances for about 20 elements among the key abundances of He, Li, C, N, O, and the 12C/13C ratios. The difference in the Mg abundance derived from Mg I lines and the MgH band, and the difference in carbon abundance from C I and the CH band, are used as a clue to the mild hydrogen-deficiency/helium-enhancement. From this analysis, four red giants, an early-AGB star, and a supergiant star were found to be enhanced in helium. All these He-enhanced stars are also found to be super-Li rich except for the supergiant. Since the He-rich red giants are Li rich as well, this implies that He enrichment is accompanied by Li enrichment, but not vice versa. This is the first spectroscopic measurement of photospheric He abundance in normal and Li-rich field giants. The Li enrichment is observed across the giant branch from red giant branch (RGB)-bump (KIC 9821622) to AGB phase, unlike that expected from the RGB-tip to red clump phase. A plausible scenario for the enrichment of He as well as Li in giants is the fresh synthesis of Li in the interiors of giants and dredging-up along with He to the surface from deeper layers. However, there could be multiple scenarios operating in tandem. This analysis of He and Li enrichment along with other key elements provides more insights to decipher the mystery of Li enrichment in giants.

Keywords: Helium-rich stars, Chemical abundances, Red giant stars, Stellar evolution
Star Formation and Magnetic Field Amplification due to Galactic Spirals. We use global MHD galaxy simulations to investigate the effects of spiral arms on the evolution of magnetic fields and star formation within a self-regulated interstellar medium. The same galaxy is simulated twice: once with self-consistent stellar spiral arms and once more with the stellar spirals suppressed via a novel numerical approach, using the RAMSES adaptive mesh refinement code. Spiral arms continually promote star formation, with 2.6 times higher rates in the spiral galaxy. The higher rate is due to high gas columns gathered along the spiral arms, rather than increasing the star formation efficiency at a given gas column. In both cases, the magnetic field is initially amplified via a small-scale dynamo driven by turbulence due to supernova feedback. Only the spiral galaxy exhibits late-time, consistent field growth due to a large-scale dynamo (e-folding time ∼600 Myr). This results in volume-averaged field strengths of ∼1 μG after 1 Gyr of evolution. The mean fields tend to align themselves with the spiral arms and are coherent up to 10 kpc scales. We demonstrate a novel large-scale dynamo mechanism, whereby spiral-driven radial flows enable the mean-field amplification.

Keywords: Galaxy magnetic fields, Spiral arms, Magnetohydrodynamics, Interstellar medium
A Study of Spectral Variability Between Flaring and Nonflaring State in M74 X-1. We conducted an extensive long-term spectral and timing study of the ultraluminous X-ray (ULX) source M74 X-1, using data taken between 2001 and 2021 by Chandra and XMM-Newton X-ray observatories. Our analysis shows that flares are present in some observations, whereas they are absent in others. The flaring state exhibits two-component spectra at a lower average flux level, whereas the nonflaring state displays single-component spectra at a higher average flux level. The M74 X-1 spectra are best described by the combination of accretion disk and Comptonization components, dual thermal disk blackbody model, and a modified multitemperature disk blackbody model. Using the dual thermal disk blackbody model, we obtain cool and hot temperatures of Tin (cool) =  0.38‑0.06+0.08  keV and Tin (hot) =  1.67‑0.13+0.18  keV, respectively, suggesting two temperature emitting regions and indicating possible presence of outflowing wind along with the accretion disk. We found a Gaussian feature at Eline =  0.96‑0.11+0.05  keV with σ =  0.11‑0.06+0.13  keV in the spectra of the flaring state, which can be interpreted as the unresolved wind feature in the system when compared to similar feature seen in other ULX sources. Plotting the hardness luminosity diagram, we get a trend of increasing hardness with luminosity, suggesting the presence of geometrical beaming in a low-inclination system. Additionally, using the hot disk blackbody component from the dual thermal disk blackbody model, we estimate the mass of the compact object to be M =  7.1‑1.3+1.4 M⊙, classifying it as a stellar-mass black hole and confirming super-Eddington accretion in the system.

Keywords: X-ray astronomy, Ultraluminous x-ray sources, Accretion
Field-level Comparison and Robustness Analysis of Cosmological N-body Simulations. We present the first field-level comparison of cosmological N-body simulations, considering various widely used codes: Abacus, CUBEP3M, Enzo, Gadget, Gizmo, PKDGrav, and Ramses. Unlike previous comparisons focused on summary statistics, we conduct a comprehensive field-level analysis: evaluating statistical similarity, quantifying implications for cosmological parameter inference, and identifying the regimes in which simulations are consistent. We begin with a traditional comparison using the power spectrum, cross-correlation coefficient, and visual inspection of the matter field. We follow this with a statistical out-of-distribution (OOD) analysis to quantify distributional differences between simulations, revealing insights not captured by the traditional metrics. We then perform field-level simulation-based inference using convolutional neural networks (CNNs), training on one simulation and testing on others, including a full hydrodynamic simulation for comparison. We identify several causes of OOD behavior and biased inference, finding that resolution effects, such as those arising from adaptive mesh refinement (AMR), have a significant impact. Models trained on non-AMR simulations fail catastrophically when evaluated on AMR simulations, introducing larger biases than those from hydrodynamic effects. Differences in resolution, even when using the same N-body code, likewise lead to biased inference. We attribute these failures to a CNN's sensitivity to small-scale fluctuations, in particular in voids and filaments, and demonstrate that appropriate smoothing brings the simulations into statistical agreement. Our findings motivate the need for careful data filtering and the use of field-level OOD metrics, such as PQMass, to ensure robust inference.

Keywords: Cosmology, Large-scale structure of the universe, Convolutional neural networks, Astronomy data analysis, N-body simulations, Sampling distribution
Polarization as a Probe of Neutrino Emission from Blazars. The source of extragalactic neutrinos in the TeV─PeV range is a matter of very active research, with blazar jets having been postulated to be the origin of at least some of the detections. The blazar PKS 0735+178 is a prominent example; during its multiband flare in late 2021, a neutrino event was reported by four observatories, with its origin consistent with the direction of that source. While no new jet component was observed to be ejected during that narrow time frame, our analysis shows that a propagating shock front originating from the core region was the likely source of the multiband flare, using very-long-baseline interferometry images of PKS 0735+178 in polarized light. Taken together, our findings are suggestive of a coherent scenario in which the shock may contribute to the acceleration of protons, with the target photons potentially originating either from the ambient medium surrounding the jet or from proton synchrotron radiation. The necessary conditions for neutrino emission via proton─photon interactions are, hence, present in this jet.

Keywords: Jets, High energy astrophysics, Blazars, Neutrino astronomy, Very long baseline interferometry, Cosmological neutrinos, Radio astronomy
The AURORA Survey: The Nebular Attenuation Curve of a Galaxy at z = 4.41 from Ultraviolet to Near-infrared Wavelengths. We use JWST/NIRSpec observations from the Assembly of Ultradeep Rest-optical Observations Revealing Astrophysics survey to constrain the shape of the nebular attenuation curve of a star-forming galaxy at z = 4.41, GOODSN-17940. We utilize 12 H I recombination lines to derive the attenuation curve spanning optical to near-infrared wavelengths (3751─9550 Å). We then leverage a high signal-to-noise ratio spectroscopic detection of the rest-frame ultraviolet continuum in combination with rest-UV photometric measurements to constrain the shape of the curve at ultraviolet wavelengths. While this UV constraint is predominantly based on stellar emission, the large measured equivalent widths of Hα and Hβ indicate that GOODSN-17940 is dominated by an extremely young stellar population &lt;10 Myr in age such that the UV stellar continuum experiences similar attenuation to that of the nebular emission. The resulting combined nebular attenuation curve spans 1400─9550 Å and has a shape that deviates significantly from commonly assumed dust curves in high-redshift studies. Relative to the Milky Way, SMC, and Calzetti curves, the new curve has a steeper slope at long wavelengths (λ &gt; 5000 Å) while displaying a similar slope across blue-optical wavelengths (λ = 3750─5000 Å). In the ultraviolet, the new curve is shallower than the SMC and Calzetti curves and displays no significant 2175 Å bump. This work demonstrates that the most commonly assumed dust curves are not appropriate for all high-redshift galaxies. These results highlight the ability to derive nebular attenuation curves for individual high-redshift sources with deep JWST/NIRSpec spectroscopy, thereby improving the accuracy of physical properties inferred from nebular emission lines.

Keywords: Interstellar dust, Emission line galaxies, James Webb Space Telescope, High-redshift galaxies, Galaxies
Modeling Atmospheric Alteration on Titan: Hydrodynamics and Shock-induced Chemistry of Meteoroid Entry. Meteoroid entry into planetary atmospheres generates bow shocks, resulting in high-temperature gas conditions that drive chemical reactions. In this paper, we perform three-dimensional hydrodynamic simulations of meteoroid entry using the Athena++ code, coupled with chemistry calculations via Cantera to model the nonequilibrium chemistry triggered by atmospheric entry. Our aerodynamical simulations reveal the formation of complex shock structures, including secondary shock waves, which influence the thermodynamic evolution of the gas medium. By tracking thermodynamic parameters along streamlines, we analyze the effects of shock heating and subsequent expansion cooling on chemical reaction pathways. Our results demonstrate that chemical quenching occurs when the cooling timescale surpasses reaction rates, leading to the formation of distinct chemical products that deviate from equilibrium predictions. We show that the efficiency of molecular synthesis depends on the object's size and velocity, influencing the composition of the post-entry gas mixture. Applying our model to Titan, we demonstrate that organic matter can be synthesized in the present environment of Titan. Also, we find that nitrogen, the dominant atmospheric component, remains stable, while water vapor is efficiently removed, a result inconsistent with equilibrium chemistry assumptions. Moreover, we compare our simulation results with laser experiments and find good agreement in chemical yields. Finally, we also evaluate the impact on Titan's atmosphere as a whole, showing that meteoroid entry events could have played a significant role in supplying molecules such as HCN during early Titan's history.

Keywords: Hydrodynamical simulations, Meteoroids, Titan, Planetary atmospheres, Chemical kinetics