Sazuna/UAT_keywords huggingface dataset:
Magnetic Reconnection in the Heliospheric Current Sheet Embedded in an Interplanetary Coronal Mass Ejection Sheath. Solar wind conditions in the heliosphere are altered by solar wind transients such as shocks and interplanetary coronal mass ejections (ICMEs). The interactions of coherent solar wind structures such as the heliospheric current sheet (HCS) with the transients remain incompletely understood, however. Here, we report the first in situ spacecraft observations showing two magnetic reconnection exhausts in the HCS embedded in the sheath region of an ICME. The dropout (presence) of strahl electrons indicates the disconnection (connection) of an exhaust region from (to) Sun. We perform a multispacecraft analysis, which shows that reconnection in the HCS embedded in an ICME sheath can operate in a quasi-steady manner at 1 au, with the observations across the outflow regions showing a good correlation with the Walén relation. The two reconnecting HCS crossings in the ICME sheath are compared to two other HCS crossings, one in the sheath and one in the solar wind upstream of the ICME shock, which show no magnetic reconnection signatures. These results highlight interplanetary transients as space plasma environments that can host dynamics infrequently observed in the solar wind at Earth's orbit.

Keywords: Solar wind, Interplanetary magnetic fields, Solar coronal mass ejections, Solar magnetic reconnection
Separating Flare and Secondary Atmospheric Signals with RADYN Modeling of Near-infrared JWST Transmission Spectroscopy Observations of TRAPPIST-1. Although TRAPPIST-1's temperate planets have the highest transmission signals of any known system, flares contaminate 50%─70% of transits at the 1000 ppm level, far above 100 ppm secondary atmospheric signals. Efforts to mitigate flare contamination and assess impacts on radiation environments are each hampered by a lack of empirical spectral analysis and physics-based modeling. We present spectrotemporal analysis and radiative-hydrodynamic modeling of 5.5 hr of NIRISS and NIRSpec observations of six TRAPPIST-1 flares of 2.2─8.7 × 1030 erg. The flare lines and continua are characterized using grid searches of RADYN beam-heating models spanning 104 times in electron beam parameters. Best-fit models indicate these flares result from moderate-intensity beams with emergent electron fluxes of Fe = 1012 erg s−1 cm−2 and energies ≤37 keV, although all models overpredict the Paschen jump. These models predict X-ray and extreme UV (XUV), far-UV, and near-UV counterparts to the IR peak fluxes of 8.9─28.9 × 1027, 4.3─13.9 × 1026, and 3.4─11.4 × 1027 erg s−1, respectively. Scaling the flare rate into the XUV suggests flaring contributes 1.35 −0.15+2.0×  quiescence yr−1. We bin integrations of similar flare effective temperature to construct fiducial flare spectra from 2000 to 4500 K, in order to develop separate empirical and RADYN-based mitigation pipelines. Both pipelines are applied to all 5.5 hr of R = 10 data, resulting in maximum residuals from 1 to 2.8 μm of 100─140 ppm and typical residuals of 54 ± 14 and 65 ± 17 ppm for the empirical and RADYN-based pipelines, respectively. Injection testing supports a 3σ detection capability for CO2 atmospheres with features of 150─250 ppm, with weak evidence (Bayes factor ≍ 3) still obtained at 130 ppm. Our results motivate multiwavelength observations to improve model fidelity and test high-energy predictions.

Keywords: M dwarf stars, Transmission spectroscopy, Stellar flares, Planet hosting stars, Exoplanet atmospheres
A Possible Periodic Rotation Measure Evolution in the Repeating FRB 20220529. Fast radio bursts (FRBs) are mysterious millisecond-duration radio transients of extragalactic origin. Some of them repeat, while others apparently do not. Investigations of periodic activity in repeating FRB have been conducted to probe their origins. While periodicity in the burst rate has been reported, studies of periodicities in other properties, such as dispersion measure and rotation measure (RM), are sparse. FRB 20220529 was monitored by the Five-hundred-meter Aperture Spherical radio Telescope for nearly 3 yr, providing an opportunity to investigate periodicity in its observed properties. Here we report a possible period of ∼200 days in the RM evolution, with a significance of 4.1σ estimated via the Lomb─Scargle algorithm and 3.1σ with the phase-folding method. Periodicity in the burst rate was also investigated. It may indicate that the FRB progenitor is in a binary system, which is consistent with the significant RM increase and prompt recovery of this FRB on a week timescale. Other scenarios, such as a system with an intermediate-mass black hole, are also explored.

Keywords: Radio bursts, Radio sources, Transient sources, Radio transient sources
Photon Orbit Signatures in Spectra of Black Hole Accretion Disks. Light orbiting an accreting black hole may impact the disk or jet multiple times before escaping to the observer, at a variety of angles with respect to the local magnetic field. In this Letter, we characterize the imprints of these long path lengths and disparate magnetic field impacts in synchrotron spectra of hot accretion disks, as the strongly lensed "photon ring" exhibits a higher synchrotron turnover frequency in each lensed subimage. We apply tools of varying complexity: first, we develop a minimal, unlensed one-zone model that isolates the first two subimages of the accretion flow. By varying the magnetic field geometry encountered by each subimage, we show that distinctive spectral signatures emerge in both total intensity and fractional linear polarization. Second, we examine a semianalytic radiatively inefficient accretion flow model, in which we find that there is generally a frequency at which the first indirect image outshines the direct image even in total flux density. Lastly, we demonstrate that even general relativistic magnetohydrodynamic simulation snapshots show this spectral character. We find a typical correction to the unresolved spectrum of order 10% near the turnover frequency that grows with increasing viewing inclination, growing to order unity at higher frequencies. We predict sensitive spectral studies of the cores of Messier 87* and Sagittarius A* at frequencies exceeding 300 GHz to constrain the existence of the photon ring even without imaging, with prospects for photon ring detection even in other sources with unresolved shadows.

Keywords: Radio spectroscopy, Photon sphere, Astrophysical black holes
Speed-dependent Threshold for Electron Injection into Diffusive Shock Acceleration. Finding the injection threshold for diffusive shock acceleration (DSA) of electrons in collisionless shocks has been a longstanding unsolved problem. Using first-principles kinetic simulations, we identify the conditions for electron injection into DSA and quantify the evolution of the nonthermal tail in self-generated electromagnetic turbulence. By analyzing electron trajectories and their momentum gain during shock-recrossing cycles, we demonstrate that electrons start participating in DSA when their speed is large enough to overrun the shock. We develop a minimal model showing that speed-dependent injection reproduces nonthermal electron spectra observed in kinetic simulations. Our findings establish a new criterion for electron DSA, which has broad implications for the shock-powered nonthermal emission from space/astrophysical systems.

Keywords: High energy astrophysics, Plasma physics, Plasma astrophysics, Galactic cosmic rays, Shocks, Magnetic fields, Cosmic rays
JWST Discovery of Strong Lensing from a Galaxy Cluster at Cosmic Noon: Giant Arcs and a Highly Concentrated Core of XLSSC 122. Our observations with the James Webb Space Telescope have made the remarkable discovery of strong gravitational lensing arcs from XLSSC 122 (z = 1.98)—setting the record for the most distant galaxy cluster that exhibits strong lensing. The discovery of giant arcs enables a strong-lensing analysis and a measurement of the concentration of the dark matter halo. We perform a strong-lensing analysis of the cluster and measure the radial projected mass density profile. Our measurements reveal an exceptionally high concentration in the core of XLSSC 122. A Navarro─Frenk─White profile fit to the inner 100 kpc estimates the concentration to be 6.3 ± 0.5. The high concentration of XLSSC 122 contributes to the emerging picture that massive structure formation in the early Universe may proceed more rapidly than standard models suggest. We estimate the mass within 100 kpc to be M(R &lt; 100 kpc) = 6.5 ± 0.7 × 1013 M⊙ and find M200c = 2.6 ± 1.1 × 1014 M⊙.

Keywords: Strong gravitational lensing, Dark matter, James Webb Space Telescope, High-redshift galaxy clusters
The Inner Kernel of the Classical Kuiper Belt. The "kernel" of the classical Kuiper Belt was discovered by Petit et al. as a visual overdensity of objects with low ecliptic inclinations and eccentricities at semimajor axes near ∼44 au. This raises the question—are there other structures present in the classical Kuiper Belt? If there are, clustering algorithms applied to orbits transformed into free elements may yield the best chance of discovery. Here, we derive barycentric free orbital elements for objects in the classical Kuiper Belt, and use the Density-Based Spatial Clustering of Applications with Noise algorithm to identify a new structure, which we dub the inner kernel, located at a ∼ 43 au just inward of the kernel (a ∼ 44 au), which we also recover. It is as yet unclear whether the inner kernel is an extension of the kernel or a distinct structure. Forthcoming observations, including those by the Vera C. Rubin Observatory's Legacy Survey of Space and Time may provide further evidence for the existence of this structure, and perhaps resolve the question of whether there are two distinct structures.

Keywords: Solar system, Kuiper belt, Trans-Neptunian objects
Beyond Hierarchical Mergers: Accretion-driven Origins of Massive, Highly Spinning Black Holes in Dense Star Clusters. GW231123, the most massive binary black hole (BBH) merger detected by LIGO─Virgo─KAGRA, highlights the need to understand the origins of massive, high-spin stellar black holes (BHs). Dense star clusters provide natural environments for forming such systems, beyond the limits of standard massive star evolution to core collapse. While repeated BBH mergers can grow BHs through dynamical interactions (the so-called "hierarchical merger" channel), most star clusters with masses ≲106 M⊙ have escape speeds too low to retain higher-generation BHs, limiting growth into or beyond the mass gap. In contrast, BH─star collisions with subsequent accretion of the collision debris can grow and retain BHs irrespective of the cluster escape speed. Using N-body (Cluster Monte Carlo) simulations, we study BH growth and spin evolution through this process, and we find that accretion can drive BH masses up to at least ∼200 M⊙, with spins set by the details of the growth history. BHs up to about 150 M⊙ can reach dimensionless spins χ ≳ 0.7 via single coherent episodes, while more massive BHs form through multiple stochastic accretion events and eventually spin down to χ ≲ 0.4. These BHs later form binaries through dynamical encounters, producing BBH mergers that contribute up to ∼10% of all detectable events, comparable to predictions for the hierarchical channel. However, the two pathways predict distinct signatures: hierarchical mergers yield more unequal mass ratios, whereas accretion-grown BHs preferentially form near-equal-mass binaries. The accretion-driven channel allows dense clusters with low escape speeds, such as globular clusters, to produce highly spinning BBHs with both components in or above the mass gap, providing a natural formation pathway to GW231123-like systems.

Keywords: Accretion, Gravitational waves
Discovery of a 9.67 s Pulsar in an Ultraluminous X-Ray Source in NGC 4631 with XMM-Newton. Thanks to a recent observation with XMM-Newton, we discovered periodic pulsations at P = 9.6652 ± 0.0002 s in a new ultraluminous X-ray source (ULX) in the galaxy NGC 4631. This source, dubbed as X−8, shows one of the largest spin-up rates ever observed,  Ṗ=(−9.6±0.5)×10−8  s s−1. These findings indicate that the compact object is a neutron star, and X−8 is a new member of the pulsating ULX class. The 0.3─10 keV luminosity of X−8 is ∼3.4 × 1039 erg s−1, and its X-ray spectrum can be described by an absorbed disk blackbody or a cut-off power law, similar to what is observed in other pulsating ULXs. We discuss two possible causes for the large spin-up rate: Doppler shift from orbital motion of the neutron star and intrinsic spin-up due to accretion torque. This new ULX pulsar adds a key source to the small known population, and will enable future studies to better constrain the physical mechanisms responsible for their super-Eddington luminosities.

Keywords: Accretion, Ultraluminous x-ray sources, Pulsars
Evidence of Titanate Clouds in the Dayside Atmosphere of the Ultrahot Jupiter WASP-121b. The dayside atmospheres of the hottest ultrahot Jupiters (UHJs) have long been subject to speculation about cloud formation, often without direct observational evidence. Here, we present a detailed analysis of the panchromatic dayside emission spectrum of WASP-121b—one of the hottest known UHJs—covering a broad wavelength range of ∼0.6─5.1 μm, based on archival JWST observations from NIRISS and NIRSpec/G396H. We report statistically significant detections of several key molecular species, including H2O (13.4σ), CO (14.7σ), SiO (4.9σ), TiO (5.4σ), and VO (6.6σ), establishing WASP-121b as one of the most thoroughly characterized exoplanetary atmospheres to date. Additionally, we present the robust detection of titanate (CaTiO3) clouds at 6.7σ—the first such detection in any exoplanet atmosphere. Our analysis further reveals strong evidence of TiO depletion, likely due to sequestration into refractory condensates such as titanate clouds. The precisely constrained molecular abundances yield a super-solar C/O ratio of 0.963 ± 0.024, a subsolar Si/O ratio of 0.034 ± 0.024, and a metallicity of 4.7 −1.38+1.99 ×solar. These findings offer a unique window into the atmospheric chemistry of an extreme UHJ, positioning WASP-121b as a key benchmark for next-generation models of atmospheric evolution and dynamics.

Keywords: Exoplanet atmospheric composition , Exoplanets, Atmospheric clouds, Transmission spectroscopy, James Webb Space Telescope, Exoplanet atmospheric structure, Extrasolar gaseous planets, Gaussian Processes regression, Exoplanet atmospheric evolution
Observed Joy's Law of Bipolar Magnetic Region Tilts at Emergence Supports the Thin Flux Tube Model. Bipolar sunspots, or more generally, bipolar magnetic regions (BMRs), are the dynamic magnetic regions that appear on the solar surface and are central to solar activity. One striking feature of these regions is that they are often tilted with respect to the equator, and this tilt increases with the latitude of appearance, popularly known as Joy's law. Although this law has been examined for over a century through various observations, its physical origin is still not established. An attractive theory that has been put forward behind Joy's law is the Coriolis force acting on the rising flux tube in the convection zone, which has been studied using the thin flux tube model. However, observational support for this theory is limited. If the Coriolis force is the cause of the tilt, then we expect BMRs to hold to Joy's law at their initial emergence on the surface. By automatically identifying the BMRs over the last two solar cycles from high-resolution magnetic observations, we robustly capture their initial emergence signatures on the surface. We find that from their appearance, BMRs exhibit tilts consistent with Joy's law. This early tilt signature of BMRs suggests that the tilt is developed underneath the photosphere, driven by the Coriolis force and helical convection, as predicted by the thin flux tube model. Considerable scatter around Joy's law observed during the emergence phase, which reduces in the postemergence phase, reflects the interaction of the vigorous turbulent convection with the rising flux tubes in the near-surface layer.

Keywords: Solar active region magnetic fields, Sunspot groups, Bipolar sunspot groups, Solar physics, Solar magnetic fields, Solar activity
Two-dimensional Radiation-hydrodynamic Simulations of Luminous Red Novae. Luminous red novae (LRNe) are transients associated with mass ejection during stellar mergers and common envelope evolution (CEE). LRNe have the potential to illuminate the poorly understood phases of binary evolution leading up to the CEE, during the mass ejection phase, and in the immediate aftermath. However, the mechanism responsible for powering LRN light curves and the origin of their observed diversity remain open questions. Here, we perform two-dimensional moving-mesh radiation-hydrodynamic simulations of LRNe that take into account hydrogen and helium recombination and relevant opacities. We study a typical high-mass stellar merger, which dynamically ejects 2 M⊙ with a characteristic velocity of 410 km s−1. This ejecta collides with 2.7 M⊙ of equatorially concentrated circumbinary material (CBM) left behind from a prior phase of nonconservative runaway mass transfer. We find that the resulting light curve is composed of a short blue peak followed by a redder, predominantly shock-powered plateau with luminosities reaching up to 1041 erg s−1 and durations up to 200 days. These luminosities are significantly higher, and the durations much longer, than those produced by a simple spherical ejection of the same mass. They also depend in a complex way on the radial distribution of the CBM and the viewing angle. The shock is embedded in the ejecta, and its observational signatures during the optically thick phase are largely hidden. Our results are broadly compatible with observations of the brightest extragalactic LRNe and pave the way for the transformation of LRNe into powerful probes of binary evolution.

Keywords: Common envelope evolution, Transient sources
Dust Budget Crisis in Little Red Dots. Little red dots (LRDs), a population of active galactic nuclei (AGNs) recently identified by JWST, are characterized by their compact morphology and red optical continuum emission, which is often interpreted as evidence for significant dust extinction of AV ≳ 3 mag. However, the dust-reddened AGN scenario is increasingly challenged by their faint near-to-far-infrared emission and a potential "dust budget crisis" in cases when the host galaxy is either undetectably low-mass or absent. In this study, we reevaluate the dust extinction level in LRDs by modeling the UV-to-infrared spectra for various extinction laws and a broad range of dusty distribution parameters. Comparing the predicted infrared fluxes with observational data from JWST MIRI, Herschel, and the Atacama Large Millimeter/submillimeter Array, our analysis finds that the visual extinction is tightly and consistently constrained to AV ≲ 1.0─1.5 mag for A2744─45924, RUBIES-BLAGN-1, and stacked spectral energy distributions from a large sample of LRDs under the SMC extinction law, with slightly weaker constraints for those with gray extinction in the UV range. The revised AV values yield radiative efficiencies of ∼10% for the LRD population, easing the tension with the Sołtan argument for the bulk AGN population at lower redshifts. Moreover, this moderate extinction (or dust-free) scenario, with reprocessed emission spectra testable by future far-infrared observatories, provides a paradigm shift in understanding the natures, environments, and evolutionary pathways of massive black holes in the early Universe.

Keywords: Supermassive black holes, Galaxy formation, Quasars, Astrophysical dust processes, Interstellar medium, High-redshift galaxies
The Pre-outburst Properties of the FU Ori Object HBC 722. FU Ori outbursts are thought to play an important role in stellar assembly and the evolution of protoplanetary disks. However, the progenitor young stellar objects are largely uncharacterized. We obtained a low-resolution optical spectrum of HBC 722 before its FU Ori outburst as part of a survey of young stellar objects in the North America Nebula. The spectrum yields a spectral type of M3.3±0.4, which when combined with archival photometry allows us to measure the stellar and accretion properties of a young star prior to its FU Ori outburst. The pre-outburst accretion rate of 7 × 10−9 M⊙ yr−1 is high for a protoplanetary disk around an M3-M3.5 star, though about 15,000 times weaker than the accretion rate during the outburst. The pre-outburst variability, inferred from archival B-band photometry, is about a factor of 5 with a standard deviation of 0.16 dex and is consistent with variable accretion onto young low-mass stars. The stellar radius is larger than the radius of accreting young stars of similar spectral type by a factor of 2. The extinction to HBC 722 is ∼1.45 ± 0.3 mag, lower than the 2.5─3.7 mag extinction values measured during the outburst. The u-band photometry plays an especially important role in constraining the veiling at longer wavelengths and therefore also the extinction and photospheric luminosity.

Keywords: FU Orionis stars, Classical T Tauri stars, Pre-main sequence stars, Spectral energy distribution, Stellar accretion disks
Dust Recycling and Icy Volatile Enhancement (DRIVE): A Novel Method of Volatile Enrichment in Cold Giant Planets. Giant planet atmospheres are thought to reflect the gas phase composition of the disk when and where they formed. However, these atmospheres may also be polluted via solid accretion or ice sublimation in the disk. Here, we propose a novel mechanism for enriching the atmospheres of these giant planets with volatiles via pebble drift, fragmentation, and ice sublimation. We use a combination of 3D hydrodynamic simulations, radiative transfer, and particle tracking to follow the trajectories and resulting temperatures of solids in a disk containing an embedded planet forming outside the CO snowline. We show that small dust can become entrained in the meridional flows created by the giant planet and advected above the disk midplane where temperatures are well above the sublimation temperature of CO. This transport of small grains occurs over 10 kyr timescales, with individual micron-sized grains cycling between the midplane and surface of the disk multiple times throughout the planetary accretion stage. We find that this stirring of dust results in sublimation of CO gas above the snow surface in the dust trap created exterior to the giant planet, leading to supersolar CO abundances in the pressure bump. This mechanism of dust recycling and icy volatile enrichment in cold giant planets, which we call the DRIVE effect, may explain the enhanced metallicities of both wide-separation exoplanets and Jupiter in our own solar system.

Keywords: Hydrodynamical simulations, Exoplanets, Protoplanetary disks, Planet formation, Solar system astronomy