Sazuna/UAT_keywords huggingface dataset:
Fast Radio Bursts by Stellar Wind Microlensing of a Faint Background Source. By assuming the inverse square law of solar wind plasma density as representative of other stars, it is shown that just outside a star, the outward deflection of a passing radio signal at ν ≈ 1 GHz (which is capable of penetrating the plasma) is about 5 times larger than the gravitational inward deflection by the star, and the ensuing lens equation, which takes both effects into account is a cubic polynomial with three roots and a new strong lensing caustic. The geometric optics approach is valid for a radio source size ≲1 pc. Microlensing magnification of a steady background source occurs typically over a timescale of milliseconds, resulting in ≈80 fast radio bursts (FRBs) per day over the whole sky, which can only perturb the isotropy of FRB distribution at the several percent level. Moreover, repeating FRBs could be triggered by the periodic interception of the line of sight of the background source by members of a binary system. The temporal signatures of such FRBs are consistent with the power spectrum of solar wind density fluctuations on corresponding scales, except that the mean density of the wind is a few times higher than the solar value.

Keywords: Plasma physics, Solar wind, Radio bursts, Stellar winds, Gravitational lensing
A Galactic Self-portrait: Density Structure and Integrated Properties of the Milky Way Disk. The evolutionary history of the Milky Way disk is imprinted in the ages, positions, and chemical compositions of individual stars. In this study, we derive the intrinsic density distribution of different stellar populations using the final data release of the Apache Point Observatory Galactic Evolution Experiment (APOGEE) survey. A total of 203,197 red giant branch stars are used to sort the stellar disk (R ≤ 20 kpc) into subpopulations of metallicity (Δ[M/H] = 0.1 dex), age ( Δlog(ageyr)=0.1 ), and α-element abundances ([α/M]). We fit the present-day structural parameters and density distribution of each stellar subpopulation after correcting for the survey selection function. The low-α disk is characterized by longer scale lengths and shorter scale heights, and is best fit by a broken exponential radial profile for each population. The high-α disk is characterized by shorter scale lengths and larger scale heights, and is generally well-approximated by a single exponential radial profile. These results are applied to produce new estimates of the integrated properties of the Milky Way from early times to the present day. We measure the total stellar mass of the disk to be  5.27‑1.5+0.2×1010 M⊙, and the average mass-weighted scale length is Rd = 2.37 ± 0.2 kpc. The Milky Way's present-day color of (g ‑ r) = 0.72 ± 0.02 is consistent with the classification of a red spiral galaxy, although it has only been in the "green valley" region of the galaxy color–mass diagram for the last ∼3 Gyr.

Keywords: Stellar populations, Galaxy stellar content, Galaxy evolution, Milky Way evolution, Milky Way Galaxy, Milky Way mass
New Insights into the T Tauri Binary Separation Distribution. For three decades, adaptive optic surveys have revealed an excess of T Tauri binaries across a = 10─100 au in nearby star-forming regions compared to the field population of main-sequence (MS) stars. Such an excess requires that most stars are born in dense clusters and subjected to significant dynamical processing that disrupts such binaries across intermediate separations. However, we demonstrate that the apparent excess is due to an observational selection bias. Close binaries within a &lt; 100 au clear out their dusty circumstellar disks on faster timescales compared to wide binaries and single stars. A magnitude-limited sample is therefore biased toward close binaries that have preferentially cleared out their obscuring disks. We reexamine the separation distribution of pre-MS binaries in low-density Taurus, moderately dense Upper Scorpius (Upper Sco), and the extremely dense Orion Nebula Cluster (ONC). By limiting the samples to primary spectral type/mass instead of magnitude, the artificial excess across a = 10─100 au disappears in all three environments. Across wider separations a = 100─4000 au, Taurus exhibits an excess of companions (mostly tertiaries), the ONC displays a deficit, and Upper Sco matches the field MS population. The field derives from an amalgam of all three environments, where Upper Sco corresponds to the average birth environment of solar-type stars. The total binary fraction within a &lt; 10,000 au in Taurus is only 52% ± 7%, substantially lower than the 100% inferred from the biased observations and only slightly higher than the field MS value of 45%. N-body interactions preferentially disrupt outer tertiaries with only marginal dynamical processing of the inner binaries, especially those within a &lt; 100 au.

Keywords: Binary stars, T Tauri stars, Star formation, Circumstellar disks, Visual binary stars, Dynamical evolution
Spectral Biases, Starspot Morphology, and Dynamo Transitions on the Pre-main Sequence: Insights from the X-Shooter WTTS Library. Starspots are ubiquitous in young, low-mass stars, yet their impact on the spectral classification and fundamental parameter inference of pre-main-sequence stars (PMS) has been largely overlooked. In this study, we demonstrate that cool starspots systematically distort spectral morphology and bias the effective temperatures, surface gravities, and luminosities derived for nonaccreting weak-lined T Tauri stars (WTTSs). Using a sample of 56 WTTSs with high-resolution, broadband X-Shooter spectra, we perform two-temperature spectral fits that explicitly account for spot coverages and temperature contrasts. These composite models consistently outperform traditional single-temperature fits, particularly in the 3350─4000 K regime, where spot contributions dominate the red-optical and near-IR flux. Moreover, we propose that surface gravity discrepancies between optical and IR measurements are a natural consequence of spot-dominated emission in PMS stars. We find that single-temperature models can overestimate effective temperatures by up to 700 K and underestimate  logg  by 1─2 dex. Using spot-corrected effective temperatures, we derive masses and ages from traditional, magnetic, and spotted evolutionary models, finding that spot corrections systematically raise inferred masses by up to 80% and stellar ages by up to 0.5 dex. These discrepancies are strongest for stars in the 0.3─0.8 M⊙ range. Using starspots as a proxy for magnetic topology, we find evidence that a shift from largely axisymmetric to nonaxisymmetric magnetic fields dominated by small-scale structures coincides with the formation of a radiative core during PMS evolution, effectively distinguishing between the convective and interface dynamo regimes.

Keywords: Starspots, Pre-main sequence stars, Star formation
The Most Distant γ-Ray Flare to Date: A Multiwavelength Campaign on the z = 4.715 Blazar GB6 B1428+4217. In 2023 November, the Fermi Large Area Telescope detected a γ-ray flare from the high-redshift blazar GB6 B1428+4217 (z = 4.715). We initiated a multiwavelength follow-up campaign involving Swift, NuSTAR, the Sierra Nevada and Perkins Observatories, and the Effelsberg 100 m radio telescope. This source, also known as 5BZQ J1430+4204, has shown an anomalous soft X-ray spectrum in previous observations, including possible ionized absorption features or signatures of bulk Comptonization of thermal electrons, which are also detected during the flaring episode. Simultaneous optical data revealed a polarization fraction of ∼8% in the R band, confirming that synchrotron emission dominated over thermal emission from the accretion disk. The hard X-ray flux was enhanced during the flare. Modeling of the broadband spectral energy distribution suggests that the high-energy component is dominated by Compton scattering by external seed photons from the accretion disk. The origin of the flare is consistent with the injection of a hard-spectrum electron population in the emission region. With a γ-ray luminosity among the top 5% of flaring events, GB6 B1428+4217 exemplifies a prototypical MeV blazar. Its Compton-dominated spectral energy distribution and extreme luminosity are in line with expectations from the blazar sequence. High-redshift flares like this are critical for understanding jet physics in the early Universe and may improve detection prospects with future missions such as the Compton Spectrometer and Imager.

Keywords: High energy astrophysics, Blazars, Relativistic jets, Flat-spectrum radio quasars, Radiative processes, Gamma-ray astronomy, High-redshift galaxies
Revisiting the Perseus Cluster. I. Resolving the Si/S/Ar/Ca Ratios by Stellar Convection. Chemical abundance measurements from stars in the Milky Way to the intragalactic medium in the Perseus Cluster have challenged the spherical explosion models. Models in the literature cannot closely match the observed element ratios, where Si, S are overproduced and Ar, Ca are underproduced. In this article, we explore the impact of the model parameters during the evolution of massive stars on the final explosive nucleosynthesis. We investigate the effects of a parameterized model of the convective process, including the mixing length parameter and the semiconvection parameter, on the production of Si-group elements. We search for the value pair that can reduce the discrepancy in the models. We conclude that a mixing length parameter of 2.2 and a semiconvection parameter of 0.03 are required to fit these criteria. Using this updated value pair, we compute a sequence of massive star models from MZAMS = 15–40 M⊙. The high-resolution data from future observations, such as XRISM, will provide further details on less constrained processes in stellar evolution and supernova explosions. Future comparison with supernova models of various progenitor metallicity will further shed light on the supernova population and their relative rates on cosmological scales.

Keywords: Galaxy clusters, Hydrodynamical simulations, Explosive nucleosynthesis, Chemical abundances, Supernovae, Perseus Cluster
Extremely Tenuous, Dusty Terrestrial Atmospheres Remain Stable against Global Collapse: An Argument for Arid Planet Habitability in the Trappist-1 System. The potential habitability of tidally locked planets orbiting M stars remains uncertain. However, recent observations with JWST and numerous theoretical studies investigating atmospheric evolution over time suggest that thick atmospheres (&gt;1 bar) with abundant surface water are increasingly unlikely. A significant challenge to habitability for tidally locked systems is the prevention of atmospheric collapse. For tenuous atmospheres, this challenge is even more extreme, since low-pressure atmospheres cannot transport heat effectively from the day- to nightside. Previous studies find that collapse is likely for CO2 atmospheres with surface pressures less than 100 mbar for planets in the habitable zone of Trappist-1, which would significantly reduce the likelihood for long-term habitability in that system. In this study, we find that modest levels of atmospheric mineral dust mitigate this collapse for a tenuous 7.05 mbar atmosphere on a small Mars-sized planet. Dust heating modifies the vertical and zonal thermal gradients and wind sheers. Due to the rapid planetary rotation (1 yr = 6 sols), this change is reflected in the magnitude of rotational components of the global circulation, including the equatorial jet and midlatitude eddies. We find that heat transport by these components is increased in dusty simulations, and atmospheric collapse on the permanent nightside is prevented. This dust-dominated climate is consistent with a planetary atmosphere evolving towards low pressure and high aridity with time. Our findings therefore provide an alternative target to Earth-like climates around M stars and reopens the possibility of habitability in M star systems.

Keywords: Exoplanet atmospheric dynamics, Planetary climates, Habitable zone, Habitable planets, Exoplanet atmospheres
The Resolved Structure of a Low-metallicity Photodissociation Region. Photodissociation regions (PDRs) are key to understanding the feedback processes that shape interstellar matter in galaxies. One important type of PDR is the interface between H II regions and molecular clouds, where far-ultraviolet radiation from massive stars heats gas and dissociates molecules. Photochemical models predict that as metallicity decreases, the C/CO transition occurs at greater depths in the PDR compared to the H/H2 transition, increasing the extent of CO-dark H2 gas in low-metallicity environments. This prediction has been difficult to test outside the Milky Way due to the lack of high-spatial-resolution observations tracing H2 and CO. This study examines a low-metallicity PDR in the N13 region of the Small Magellanic Cloud (SMC), where we spatially resolve the ionization front, the H2 dissociation front, and the C/CO transition using 12CO J = 2−1, 3−2, and [C I] 1─0 observations from the Atacama Large Millimeter/submillimeter Array and near-infrared spectroscopy of the H2 2.12 1─0 S(1) vibrational line, and H recombination lines from the James Webb Space Telescope. Our analysis shows that the separation between the H/H2 and C/CO boundaries is approximately 0.043 ± 0.013(stat.) ± 0.0036(syst.) pc (equivalent to  0.″146±0.″042(stat.)±0.″012(syst.)  at the SMC's distance of 62 kpc), defining the spatial extent of the CO-dark H2 region. Compared to our plane-parallel PDR models, we find that a constant-pressure model matches the observed structure better than a constant-density one. Overall, we find that the PDR model does well at predicting the extent of the CO-dark H2 layer in N13. This study represents the first resolved benchmark for low-metallicity PDRs.

Keywords: Interstellar medium, Dwarf galaxies, Photodissociation regions
Scattering of Stellar Mass Black Holes and Gravitational-wave Bremsstrahlung Radiation in Active Galactic Nucleus Disks. The dynamics of stellar mass black holes (sBHs) embedded in active galactic nuclei (AGNs) could produce highly eccentric orbits near the central supermassive black hole, leading to repeated close encounters that emit gravitational waves in the Laser Interferometer Gravitational-Wave Observatory (LIGO) frequency band. Many works have focused on the mergers of sBHs in the disk that produce gravitational waves; however, sBHs in hyperbolic orbits also emit gravitational-wave bremsstrahlung that can be detected by ground-based interferometers like LIGO. In this work, we analyze the scattering of sBHs in an AGN disk as they migrate inside the disk, focusing on gravitational-wave bremsstrahlung emission. We determine how the gravitational-wave emission depends on the different parameters of the scattering experiments, such as the mass of the supermassive black hole and the sBH migration rate and mass ratio. We find that scattering with detectable gravitational-wave bremsstrahlung is more frequent around lower-mass supermassive black holes (∼105−6 M⊙). We then conduct a suite of Monte Carlo simulations and estimate the rate for ground-based gravitational-wave detections to be in the range of 0.08─1194 Gpc−3 yr−1, depending on migration forces and detection thresholds, with large uncertainties accounting for variations in possible AGN environments. The expected rate for our Fiducial parameters is 3.2 Gpc−3 yr−1. Finally, we provide first-principle gravitational-wave templates produced by the encounters.

Keywords: High-luminosity active galactic nuclei, Elliptical orbits, Orbital resonances, Gravitational wave sources, Gravitational wave astronomy, Low-luminosity active galactic nuclei, Active galactic nuclei, Hyperbolic orbit, Compact objects, Parabolic orbit, Dynamical friction, Astrophysical black holes
Gravitational-wave Phase Shifts in Eccentric Black Hole Mergers as a Probe of Dynamical Formation Environments. We here quantify the gravitational-wave (GW) phase shift appearing in the waveform of eccentric binary black hole (BBH) mergers formed dynamically in 3-body systems. For this, we have developed a novel numerical method where we construct a reference binary, by evolving the post-Newtonian ( PN ) evolution equations backwards from a point near merger without the inclusion of the third object, that can be compared to the perturbed binary that evolves under the influence from the third BH. From this, we quantify how the interplay between dynamical tides,  PN -effects, and the time-dependent Doppler shift of the eccentric GW source results in unique observable GW phase shifts that can be used to probe the dynamical assembly mechanism of individual GW sources. We further find an analytical expression for the GW phase shift, which has a universal functional form that only depends on the time-evolving BBH eccentricity. The normalization scales with the BH masses and initial separation, which can be linked to the underlying astrophysical environment. GW phase shifts from a chaotic 3-body BH scattering taking place in a cluster, and from a BBH inspiraling in a disk migration trap near a supermassive BH, are also shown for illustration.

Keywords: Astrodynamics, Black holes, Stellar mass black holes, Gravitational wave sources, Gravitational wave astronomy, Gravitational waves, N-body simulations, Astrophysical black holes
Understanding the Transport Properties of Methane–Hydrogen Mixtures under the Interior Conditions of Ice Giants. Inaccuracy in our knowledge of the transport properties of relevant mixtures under planetary interior conditions is a roadblock in predicting the observable properties of planets in our solar system and beyond. In this work, we investigate methane–hydrogen mixtures using data sets obtained from density functional theory calculations for the electronic structure, combined with molecular dynamics simulations for the ions for a wide range of pressure and temperature. Hydrogen concentration significantly affects the equation of state (EOS) but has little influence on transport properties. We provide an analytical expression to model thermal EOS and transport properties as a function of hydrogen content, with the maximum deviation observed at low P–T conditions. These insights are particularly relevant to improve the planetary models and enhance our ability to predict the properties of "ice" giants and beyond.

Keywords: Neptune, Methane, Uranus
GeV Gamma-Rays from Molecular Clouds Illuminated by Particles Diffusing from the Adjacent Supernova Remnant G335.2+0.1 Confined in an Expanding Bubble. We report the detection of GeV gamma-ray emission likely associated with supernova remnant (SNR) G335.2+0.1 and the finding of a molecular cloud (∼20'─ 30'  in angular size) that is very likely in physical contact with the SNR and responsible for the gamma-ray emission. Using the 16.8 yr Fermi-LAT data, an extended emission, with a significance of 13.5σ and a radius 0 .° 24 in 0.2─500 GeV in the uniform-disk model, was found to the adjacent east of the SNR. With archival Mopra CO-line data, a large molecular clump at local-standard-of-rest velocity ∼−48 to −43 km s−1 was revealed, appearing coincident with the gamma-ray source. The SNR was found located in a cavity encircled by a "C"-shaped ring-like molecular shell at −45 to −43 km s−1. This morphological agreement, together with the position-velocity diagrams made along lines cutting across the cavity, suggests that the SNR was evolving in the expanding molecular bubble created by the stellar wind of the progenitor with a mass ≳15 M⊙. The giant molecular cloud, visible at around −46 km s−1, and the associated SNR are thus estimated to lie at a kinematic distance of 3.1 kpc, with the H I absorption taken into account. We suggest that the SNR has entered the radiative phase after the blastwave recently struck the cavity wall. With the evolutionary scenario of the SNR, we demonstrate that the gamma-ray emission reported here can be naturally interpreted by the hadronic interaction between the accelerated protons escaped from the SNR shock and the eastern large molecular clump.

Keywords: Supernova remnants, Cosmic rays, Gamma-rays, Molecular clouds
Multiband Optical Variability of the Blazar OP 313 in the Outburst State during 2024─2025. We present the analysis results of flux and spectral variability of the blazar OP 313 across intra-night to short-term timescales using BVRI photometric data, gathered over 25 nights from 2024 November to 2025 May, using two optical telescopes in Aryabhatta Research Institute of Observational Sciences, India. The source was in an outburst state during this period. We searched for intraday variability (IDV), using two powerful statistical tests: the power-enhanced F-test and the nested ANOVA test. The source displayed IDV in the R band for five of the 10 nights, yielding a duty cycle of 34%. During the entire monitoring of the source, it showed variations of over two mag in all B, V, R, and I data bands. We obtained a variability timescale for a variable light curve, giving us an upper limit for the size of the emission region. We generated optical spectral energy distributions of the blazar for these 25 nights, fitted a power law of form (F ν∝ν−αo ) and found the weighted mean spectral index to be 1.471 ± 0.004. An analysis of the color─magnitude diagram shows that, contrary to the redder-when-brighter trend typically observed in FSRQs, this source exhibits a bluer-when-brighter trend on short-term variability timescales—a behavior more commonly associated with BL Lac objects. We explore potential physical mechanisms responsible for the observed spectral variability.

Keywords: Optical observation, BL Lacertae objects, Blazars, Flat-spectrum radio quasars, Active galactic nuclei
Spectropolarimetry of GRB 180427A: Evidence for Distinct Emission Sites with Varying Polarization. The dynamics of the origin of gamma-ray emissions in gamma-ray bursts (GRBs) remains an enigma. Through a joint analysis of GRB 180427A, observed by the Fermi Gamma-ray Space Telescope and AstroSat's Cadmium Zinc Telluride Imager, we identify emissions from two distinct regions with varying polarization properties. Time-resolved polarization analysis reveals a synchronous evolution of the polarization angle (PA) and fraction with two emission pulses, peaking with a delay of 5.09 ± 0.29 s. Spectral analysis indicates that the first pulse is characterized by a stronger blackbody component, while the second pulse exhibits a more prominent nonthermal spectrum (power law with an exponential cutoff). Using a bottom-to-top approach through simulations, we decouple the polarization properties of the individual spectral components, revealing polarization fractions (PFs) of 25%─40% for the blackbody spectrum and 30%─60% for the nonthermal spectrum. At a redshift of z ∼ 0.22, the blackbody emission originates from the jet photosphere at ∼a few 1011 cm, whereas the nonthermal emission arises from an optically thin region at a few 1013 cm. The changing dominance of these emissions explains the observed PA shift of 60° ± 22°. The spectral cutoff at 1 MeV suggests pair opacity due to the jet's relatively lower bulk Lorentz factor (Γ∼ a few tens). The high PF and hard low-energy spectral slopes (α &gt; −0.5) imply a top-hat jet structure observed off-axis, near the jet's edge. This off-axis viewing introduces anisotropy in the observed radiation within the viewing cone (1/Γ), accounting for the observed polarization.

Keywords: Non-thermal radiation sources, Gamma-ray bursts, Spectropolarimetry, Relativistic jets