Sazuna/UAT_keywords huggingface dataset:
Magnitude of Short-wavelength Electric Field Fluctuations in Simulations of Collisionless Plasma Shocks. Large-amplitude electrostatic fluctuations are routinely observed by spacecraft upon traversal of collisionless shocks in the heliosphere. Kinetic simulations of shocks have struggled to reproduce the amplitude of such fluctuations, complicating efforts to understand their influence on energy dissipation and shock structure. In this paper, one-dimensional particle-in-cell simulations with realistic proton-to-electron mass ratio are used to show that in cases with upstream electron temperature Te exceeding the ion temperature Ti, the magnitude of the fluctuations increases with the electron plasma-to-cyclotron frequency ratio ωpe/Ωce, reaching realistic values at ωpe/Ωce ≳ 30. The large-amplitude fluctuations in the simulations are shown to be associated with electrostatic solitary structures, such as ion phase-space holes. In the cases where upstream temperature ratio is reversed, the magnitude of the fluctuations remains small.

Keywords: Shocks, Space plasmas
Characterizing the Nature of Periodic Amplitude Modulation in Pulsars. In recent years, periodic amplitude modulation has emerged as a unique emission feature in the single-pulse sequence of pulsars alongside periodic nulling and subpulse drifting. Despite ample evidence for the uniqueness of this phenomenon, the periodic modulation in several pulsars are often confused with subpulse drifting, primarily due to lack of clear characterization of the emission features from a representative sample of pulsars. In this work, we present a detailed analysis of the single-pulse behavior from 17 pulsars exhibiting periodic amplitude modulation, 6 of them being new detections. The pulsar switches between different intensity states as a result of periodic amplitude modulation, and we propose a novel statistical scheme to identify these emission states. The periodic modulation can be divided into three broad categories: phase stationary modulation, modulations with phase shift, and intermittent periodic modulations. The phase stationary behavior is seen when the emission intensity across a major part of the pulse window changes periodically. The phase shifts are associated with intensity changes at specific locations within the emission window in a periodic manner, while in some pulsars, the periodic modulations become more prominent only at specific intervals resulting in intermittent behavior.

Keywords: Radio pulsars
The HST Legacy Archival Uniform Reduction of Local Group Imaging (LAURELIN). I. Photometry and Star Formation Histories for 36 Ultra-faint Dwarf Galaxies. We present uniformly measured resolved stellar photometry and star formation histories (SFHs) for 36 nearby (≲400 kpc) ultra-faint dwarf galaxies (UFDs; −7.1 ≤ MV ≤ +0.0) from new and archival Hubble Space Telescope (HST) imaging. We measure homogeneous distances to all systems via isochrone fitting and find good agreement (≤2%) for the 18 UFDs that have literature RR Lyrae distances. From the ensemble of SFHs, we find (i) an average quenching time (here defined as the lookback time by which 80% of the stellar mass formed, τ80) of 12.48 ± 0.18 Gyr ago ( z=4.6−0.5+0.6 ), which is compatible with reionization-based quenching scenarios; and (ii) modest evidence of a delay (≲800 Myr) in quenching times of UFDs thought to be satellites of the LMC or on their first infall, relative to long-term Galactic satellites, which is consistent with previous findings. We show that robust SFH measurement via the ancient main-sequence turnoff (MSTO) requires a minimum effective luminosity (i.e., luminosity within the observed field of view) of MV ≤ −2.5, which corresponds to ∼100 stars around the MSTO. We also find that increasing the signal-to-noise ratio above ∼100 at the MSTO does not improve SFH precision, which remains dominated by stochastic effects associated with the number of available stars. A main challenge driving the precision of UFD SFHs is the limitations in the accuracy of foreground dust maps. We make all photometry catalogs public as the first data release of a larger HST archival program targeting all dwarf galaxies within ∼1.3 Mpc.

Keywords: Local Group, Dwarf galaxies, Galaxy stellar content, Star formation, Galaxy evolution, Reionization
Betelgeuse's Buddy: X-Ray Constraints on the Nature of α Ori B. The ∼2100 daylong secondary period of Betelgeuse's optical lightcurve and radial velocity motivated the prediction of a low-mass stellar companion, expected to be at maximal apparent separation from Betelgeuse around 2024 December. We carried out Director's Discretionary Time observations with the Chandra X-ray Observatory to identify any X-ray emission from the companion and constrain its nature as either a compact object or young stellar object (YSO). Past X-ray observations occurred at the wrong phase of the companion's orbit for optimal detection prospects and/or lacked the deep exposure required to constrain the typical X-ray luminosities of YSOs. In our 41.85 ks exposure with Chandra, we do not detect an X-ray source at the position of Betelgeuse. For an estimated hydrogen column density NH = 6 × 1022 cm‑2, we place a limit on the X-ray luminosity LX ≲ 2 × 1030 erg s‑1 (≲4.7 × 10‑4 L⊙) in 0.5–8 keV for a 10 MK plasma temperature spectral model, or LX ≲ 5 × 1029 erg s‑1 (≲1.2 × 10‑4 L⊙) for an absorbed power law with photon index Γ = 2. These limits robustly exclude an accreting compact object (white dwarf or neutron star) as the companion. Solar-mass YSOs with an age similar to Betelgeuse (∼10 Myr) display a range of X-ray luminosities (1028‑32 erg s‑1), and we can place upper bounds within this range for most absorbing columns. Based on these considerations, we conclude that the companion to Betelgeuse is likely a low-mass YSO.

Keywords: X-ray astronomy, X-ray photometry, Binary stars, Red supergiant stars, Close binary stars, Massive stars, X-ray telescopes, Young stellar objects, Pre-main sequence stars, Long period variable stars, Variable stars, Compact objects
Mass Loss and Subsequent Thermal Evolution of Surviving Helium White Dwarfs Shocked by Thermonuclear Supernovae. Following a Type Ia supernova (SN Ia) in a double white dwarf (WD) binary, a surviving WD companion leaves at its orbital velocity ≍ 1000─3000 km s─1. The Gaia mission has discovered seven such hypervelocity WDs with inflated radii indicative of shock heating by SN ejecta. We study the interaction between SN ejecta and Roche lobe filling, 0.08─0.45 M⊙ helium WD companions using 3D hydrodynamical simulations with Athena++. Given the importance of the later thermal evolution, we include an accurate equation of state for the degenerate helium WD donor. We show that a lower-mass, larger-radius WD companion is more strongly impacted by SN ejecta and undergoes substantial mass loss. We find a tight relation between the fractional mass loss and the ratio between the ejecta ram pressure and donor volume-averaged pressure, which can be used for predicting mass loss in other systems. In the most extreme case, the companion becomes a very inflated ≍0.02 M⊙ object. We find helium mass loss ≍ 0.005−0.06 M⊙ with velocities ≍ 1000−4000 km s−1, which may lead to emission lines in the nebular phase. The surviving helium WD receives a kick velocity, but its final velocity is essentially determined by its orbital velocity, ≲1600 km s−1. We model the postexplosion evolution of the shock-heated companions using MESA, and find reasonable agreement with the hypervelocity stars D6-2, J0546+0836, J1332─3541, and SDSS J1637+3631. A surviving ≳0.3 M⊙ helium WD can be ruled out in SN 1972E and SN 2011fe, and any surviving helium WD is likely ruled out in SN remnants 0509-67.5 and SN 1006.

Keywords: AM Canum Venaticorum stars, White dwarf stars, Hypervelocity stars, Type Ia supernovae
New Metrics for Identifying Variables and Transients in Large Astronomical Surveys. A key science goal of large sky surveys such as those conducted by the Vera C. Rubin Observatory and precursors to the Square Kilometre Array is the identification of variable and transient objects. One approach is analyzing time series of the changing brightness of sources, namely, light curves. However, finding adequate statistical representations of light curves is challenging because of the sparsity of observations, irregular sampling, and nuisance factors inherent in astronomical data collection. The wide diversity of objects that a large-scale survey will observe also means that making parametric assumptions about the shape of light curves is problematic. We present a Gaussian process (GP) regression approach for characterizing light-curve variability that addresses these challenges. Our approach makes no assumptions about the shape of a light curve and, therefore, is general enough to detect a range of variable and transient source types. In particular, we propose using the joint distribution of GP amplitude hyperparameters to distinguish variable and transient candidates from nominally stable ones and apply this approach to 6394 radio light curves from the ThunderKAT survey. We compare our results with two variability metrics commonly used in radio astronomy, namely ην and Vν, and show that our approach has better discriminatory power and interpretability. Finally, we conduct a rudimentary search for transient sources in the ThunderKAT data set to demonstrate how our approach might be used as an initial screening tool. Computational notebooks in PYTHON and R are available to help deploy this framework to other surveys.

Keywords: Bayesian statistics, Light curve classification , Sky surveys, Transient detection, Gaussian Processes regression, Hierarchical models, Time series analysis, Radio transient sources
Identifying the Secondary Jet in the RadioAstron Image of OJ 287. The 136 yr long optical light curve of OJ 287 is explained by a binary black hole model where the secondary is in a 12 yr orbit around the primary. Impacts of the secondary on the accretion disk of the primary generate a series of optical flares that follow a quasi-Keplerian relativistic mathematical model. The orientation of the binary in space is determined from the behavior of the primary jet. Here, we ask how the jet of the secondary black hole projects onto the sky plane. Assuming that the jet is initially perpendicular to the disk, and that it is ballistic, we follow its evolution after the Lorentz transformation to the observer's frame. Since the orbital speed of the secondary is of the order of one-tenth of the speed of light, the result is a change in the jet direction by more than a radian during an orbital cycle. We match the theoretical jet line with the recent 12 μas resolution RadioAstron map of OJ 287 and determine the only free parameter of the problem, the apparent speed of the jet relative to speed of light. It turns out that the Doppler factor of the jet, δ ∼ 5, is much lower than in the primary jet. Besides following a unique shape of the jet path, the secondary jet is also distinguished by a different spectral shape than in the primary jet. The present result on the spectral shape agrees with the huge optical flare of 2021 November 12, also arising from the secondary jet.

Keywords: Active galactic nuclei
General Relativity Can Prevent a Runaway Greenhouse on Potentially Habitable Planets Orbiting White Dwarfs. Planets orbiting in the habitable zones of white dwarfs have recently been proposed as promising targets for biosignature searches. However, since the white dwarf habitable zone resides at 0.01–0.1 au, planets residing there are subject to tidal heating if they have any orbital eccentricity. Previous work identified nearby planetary companions as potential roadblocks to habitability of planets around white dwarfs, as such companions could induce secular oscillations in eccentricity for the potentially habitable planet, which could in turn heat a surface ocean and induce a runaway greenhouse for even very low values (e ∼ 10‑4) of the eccentricity of the potentially habitable planet. In this work, we examine the potential for general relativistic orbital precession to protect habitable planets orbiting white dwarfs from such a runaway greenhouse and demonstrate that, for some system architectures, general relativity can be protective for planetary habitability.

Keywords: Greenhouse effect, White dwarf stars, Exoplanets, Exoplanet dynamics
Magnetic Island Structures Associated with Kinetic Alfvén Solitary Wave in Two-fluid Plasma. We investigate the formation of magnetic islands and the onset of tearing instability within kinetic Alfvén solitary waves in a two-fluid space plasma. These localized structures, featuring internal magnetic shear, naturally support magnetic reconnection processes analogous to tearing modes in magnetically confined fusion plasmas. Treating the solitary wave as a quasi-static background equilibrium, we analyze the resulting topological modifications by making use of the magnetic flux function, in analogy with tokamak plasmas. We find that the spatial size of the magnetic island scales with the square root of the magnetic flux perturbation. Introducing the tearing stability index  Δ' , we derive the growth rate of the tearing mode using a resistive magnetohydrodynamic framework. Our analysis reveals that tearing modes grow only when the mode's wavenumber exceeds the inhomogeneity scale, becoming marginally stable when the two are comparable. The emergence of magnetic islands has significant implications for cross-field particle transport, turbulence, and acceleration. In particular, the electron diffusion across magnetic fields may be governed by the island size rather than the Larmor radius, potentially accounting for anomalous transport. Furthermore, in certain regimes, unbounded island growth may signal the collapse of the solitary wave structure, enabling a large-scale transfer of magnetic energy to particle energy. These results suggest a unifying mechanism connecting magnetic reconnection in space plasmas with tearing instabilities in laboratory fusion plasmas.

Keywords: Space plasmas
Investigation of Interstellar Scintillation of the Millisecond Pulsar B1937+21 with the FAST. Pulsar scintillation serves as a valuable tool for investigating interstellar scattering screens and their properties. In this paper, we report on multiepoch interstellar scintillation from the millisecond pulsar B1937+21 (J1939+2134) using Five-hundred-meter Aperture Spherical radio Telescope (FAST) observations at 1250 MHz with 500 MHz bandwidth. The diffractive scintillation properties are investigated for an observing baseline of 3 yr. The scintillation timescale, exponential index, and decorrelation bandwidth are determined to be 7.67 ± 1.61 minutes, 1.54 ± 0.09, and 0.56 ± 0.25 MHz, respectively, from the autocorrelation functions of the dynamic spectra. These scintillation parameters are found to vary temporally. The frequency dependencies of scintillation parameters exhibit single-power spectral behaviors; however, the derived spectral indices deviate from the theoretical Kolmogorov spectrum. The fringe pattern that forms from interference of scattered waves is revealed in the secondary spectrum as a parabolic arc with a well-determined curvature of 0.95 ± 0.50 s3. The parabolic arc is present contemporaneously over a wide frequency range and scales with frequency as a power law with an index of −0.80 ± 0.18, indicating the broadband nature of the scintillation arc. The arc curvature exhibits annual variation and is well approximated by a one-dimensional scattering screen located approximately 95% of the distance toward the pulsar. These findings will contribute to a deeper understanding of the underlying physics of the ionized interstellar medium.

Keywords: Rotation powered pulsars, Millisecond pulsars, Pulsars, Interstellar scintillation, Radio pulsars
A Time-dependent Solution for GSN 069 Disk Evolution and the Nature of Long-lived Tidal Disruption Events. We present the implementation of a fully time-dependent relativistic disk model—based on the light-curve fitting package FitTeD—into the X-ray spectral fitting environment, pyXspec. This implementation enables simultaneous fitting of multi-epoch and multiwavelength spectral data, where the only free parameters are those describing the black hole and the initial conditions, while the subsequent evolution is governed by the dynamical equations of an evolving accretion flow. We use it to fit seven epochs of X-ray spectra and two epochs of UV spectra of the "long-lived" tidal disruption event (TDE) and quasiperiodic eruption (QPE) source GSN 069, from 2010 through late-2019. Our results show that such "long-lived," X-ray-bright TDEs—of which GSN 069 is a prime, but not unique, example—can naturally be explained within the same framework as events with shorter-lived X-ray emission, like ASASSN-14li and AT2019dsg. Their distinction lies in the "viscous" timescale parameter—tied to the disk's angular momentum transport efficiency—which should be treated as a free parameter when modeling the disk evolution of transient events. We examine the implications for QPE models by tracking the time evolution of disk properties such as mass surface density and accretion rate. We argue that existing QPE models may not be able to reproduce the observed connection between the presence (2018) or absence (2014) of eruptions and the disk properties. In the context of orbiter─disk collision models, the change in mass surface density appears insufficient to explain the needed variation in the eruption's temperature. The absence of eruptions in GSN 069 in 2014 remains a challenge for QPE models.

Keywords: Accretion, Supermassive black holes, High energy astrophysics, X-ray transient sources, Time domain astronomy
Can the Anomalous Magnetic Braking of Ap/Bp Stars Explain the Orbital Decay of Algol-type Binaries?. Several Algol-type binaries were detected to be experiencing rapid orbital decay, which is in contradiction with the conservative mass transfer model. In this work, we investigate whether anomalous magnetic braking (MB) of intermediate-mass Ap/Bp stars, characterized by surface magnetic fields of ∼102–104 G, could drive orbital decay through magnetic wind–field coupling. Using the stellar evolution code MESA, we simulate the evolution of several main-sequence binaries containing Ap/Bp stars, with typical initial parameters of Algol binaries. Our models indicate that the anomalous MB mechanism could induce orbital decay in long timescales (hundreds of Myr to several Gyr), reproducing several basic Algol parameters such as the effective temperatures and luminosities of donor stars. However, the predicted orbital period decay rates are much lower than those observed in several Algol systems. We analyze the limitations of the anomalous MB model and discuss alternative mechanisms that could account for the long- or short-term orbital period variations observed in Algol systems, including a surrounding circumbinary disk, stellar expansion, the Applegate mechanism, and the light-travel-time effect. Long-term observations are still required to distinguish between these mechanisms in the future.

Keywords: Close binary stars, Stellar mass loss, Stellar magnetic fields, Stellar evolution
Irradiated Pulsar Planets and Companions as 511 keV Positron Annihilation Line Sources. Millisecond pulsars (MSPs) are prolific GeV γ-ray emitters, and nearly 80% of Fermi Large Area Telescope MSPs reside in compact binaries. For the first time in the literature, we demonstrate that the companions in these compact MSPs binaries are also 511 keV annihilation line emitters using MEGAlib simulations (a high-energy radiation transport software built with Geant4) to compute the particle showers and resulting backsplash emission from the pulsar irradiation. The 511 keV signal exhibits strong flux modulation and red- and blueshifts associated with a binary orbit, enabling powerful coherent searches. Measuring the 511 keV emission would enable direct γ-ray characterization of unusual pulsar exoplanets and companions, and allow one to identify the unambiguous presence of active pulsars whose beams do not intercept Earth. Intriguingly, the 511 keV flux is brightest for ultracompact systems, against which pulsar surveys are systematically biased. These ultracompact systems are also possibly prime Laser Interferometer Space Antenna (LISA) Galactic sources. This necessitates future joint LISA─MeV γ-ray techniques to characterize MSP binaries. These MSP binaries may also contribute to the puzzling source of the excess 511 keV photons near the Galactic bulge and center.

Keywords: Neutron stars, Galactic center, High energy astrophysics, Millisecond pulsars, Gamma-ray sources, Gamma-ray lines, Dark matter, Gravitational wave sources, Pulsars, Gravitational waves, Gamma-ray astronomy, Binary pulsars
Investigating Little Red Dots with UV Excess: Are They the High-redshift Siblings of Blue Hot Dust-obscured Galaxies?. Little red dots (LRDs), newly identified compact and dusty galaxies with an unexpectedly high number density observed by JWST, have an unusual "V-shaped" rest-frame UV to near-infrared spectral energy distribution (SED). A group of hyperluminous, obscured quasars with excess blue emission, called blue-excess hot dust-obscured galaxies (BHDs), also exhibit qualitatively similar SEDs to those of LRDs. They represent a rare population of galaxies hosting supermassive black holes (SMBHs) accreting near the Eddington limit at redshifts z ∼ 1–4. In this study, we compare their multiwavelength SEDs to investigate whether LRDs, or a subset of them, could be high-redshift analogs of BHDs. Our analysis reveals that despite their similar "V-shape" SEDs, LRDs appear to be a different population than BHDs. The "V-shape" of BHDs appears at longer wavelengths compared to LRDs due to different selection strategies, suggesting LRDs have much less dust attenuation than typical BHDs. The bluer colors in the rest-frame infrared (continuum) emission of LRDs suggest the absence of hot dust heated by active galactic nuclei (AGN) accretion activities. We also argue that the blue excess in LRDs is unlikely from AGN scattered light. The compact morphologies and lower X-ray detection frequencies of LRDs suggest a distinct formation pathway from BHDs, which are thought to be powered by super-Eddington accretion onto central SMBHs following major galaxy mergers.

Keywords: AGN host galaxies, Active galaxies, Active galactic nuclei, Infrared galaxies, High-redshift galaxies