Sazuna/UAT_keywords huggingface dataset:
Leonessa: An Extremely Metal-poor Galaxy Undergoing Secular Chemical Evolution. Extremely metal-poor (XMP) galaxies are systems with gas-phase oxygen abundances below ∼5% solar metallicity (12+log(O/H) ≤ 7.35). These galaxies populate the metal-poor end of the mass─metallicity and luminosity─metallicity relations (MZR and LZR, respectively). Recent studies have found XMP galaxies in the nearby Universe to be outliers on the LZR, where they show enhanced luminosities relative to other galaxies of similar gas-phase oxygen abundance. Here, we present a study of the recently discovered XMP galaxy Leonessa and characterize the system's properties using new imaging from the Hubble Space Telescope and spectra from the Green Bank Telescope and Hobby─Eberly Telescope. We use these observations to measure a tip of the red giant branch distance (15.86 ± 0.78 Mpc) to Leonessa, the H I gas mass, the gas-phase oxygen abundance, and N/O ratio. We find Leonessa is an isolated, gas-rich (gas fraction μ = 0.69), low-mass (log(M⋆/M⊙) = 6.12 ± 0.08), XMP (12+log(O/H) = 7.32 ± 0.04), star-forming galaxy at a distance of 15.86 ± 0.78 Mpc. Our measurements show that Leonessa agrees with the MZR, but disagrees with the LZR; we conclude the LZR offset is due to recent star formation enhancing the system's luminosity. To investigate possible chemical evolution pathways for nearby XMP galaxies, we also compile a comparison sample of 150 dwarf galaxies (53 XMP systems) taken from the literature with gas-phase metallicity measurements based on the direct method. We find evidence for an anticorrelation between gas-phase oxygen abundance and H I gas-to-stellar mass ratios. We posit Leonessa is undergoing a chemical evolution pathway typical of field dwarf galaxies. *Based on observations made with the NASA/ESA Hubble Space Telescope: Program HST-GO-16048; Green Bank Observatory, Robert C. Byrd Green Bank Telescope: Program GBT20B-468; and McDonald Observatory Hobby─Eberly Telescope.

Keywords: Stellar photometry, Galaxy chemical evolution, Interstellar atomic gas, Distance measure, Interstellar emissions, Metallicity
HAWC, VERITAS, Fermi-LAT, and XMM-Newton Follow-up Observations of the Unidentified Ultra-high-energy Gamma-Ray Source LHAASO J2108+5157. We report observations of the ultra-high-energy gamma-ray source LHAASO J2108+5157, utilizing VERITAS, HAWC, Fermi-LAT, and XMM-Newton. VERITAS has collected ∼40 hr of data that we used to set ULs to the emission above 200 GeV. The HAWC data, collected over ∼2400 days, reveal emission between 3 and 146 TeV, with a significance of 7.5σ, favoring an extended source model. The best-fit spectrum measured by HAWC is characterized by a simple power law with a spectral index of 2.45 ± 0.11stat. Fermi-LAT analysis finds a point source with a very soft spectrum in the LHAASO J2108+5157 region, consistent with the 4FGL-DR3 catalog results. The XMM-Newton analysis yields a null detection of the source in the 2─7 keV band. The broadband spectrum can be interpreted as a pulsar and a pulsar wind nebula system, where the GeV gamma-ray emission originates from an unidentified pulsar, and the X-ray and TeV emissions are attributed to synchrotron radiation and inverse Compton scattering of electrons accelerated within a pulsar wind nebula. In this leptonic scenario, our X-ray upper limit provides a stringent constraint on the magnetic field, which is ≲1.5 μG.

Keywords: X-ray astronomy, Gamma-ray astronomy
Exploration of the Near-infrared Colors of Cold Y Dwarfs from the Ground and Space. The James Webb Space Telescope (JWST) has provided critical mid-infrared data for cold brown dwarfs. It has also provided low-resolution near-infrared spectra, and for faint sources these are the first spectra at these wavelengths. We use these data and other literature sources to synthesize near-infrared photometry on the Mauna Kea Observatories (MKO) system for 19 T and Y dwarfs, on the Euclid system for 44 T and Y dwarfs, and on the Roman systems for 48 T and Y dwarfs. We also synthesize Euclid IE magnitudes for 15 T and Y dwarfs. Using the S. A. Beiler et al. (2024) observational effective temperatures (Teff), together with ATMO 2020++ model colors, we show that the absolute 4.6 μm magnitude can be used as a proxy for Teff. We present a polynomial fit to the MW2:Teff relationship for cool dwarfs with 250 ≲ Teff K ≲ 1000. We select five Y dwarfs with 275 ≲ Teff K ≲ 400 that have a range in near- to mid-infrared colors. Comparison of the JWST spectral energy distribution to ATMO 2020++ models indicate that Y dwarfs that are bluer in J − W2 are lower gravity or more metal rich than their redder counterparts, with stronger CO and CO2 absorption at 4.2 ≲ λ μm ≲ 4.9. The near-infrared color diagrams show significant scatter, with complex dependencies on Teff, metallicity, and gravity. In order to disentangle these effects, opacity sources for cool model atmospheres need to be more complete at λ &lt; 1 μm.

Keywords: Infrared astronomy, Brown dwarfs, Y dwarfs, Infrared photometry
Magnetosonic Waves as a Potential Driver of Observed Temperature Fluctuation Patterns in AGN Accretion Disks. Recent observations have revealed slow, coherent temperature fluctuations in AGN disks that propagate both inward and outward at velocities of ∼0.01─0.1c, a kind of variability that is distinct from reverberation (mediated by the reprocessing of light) between different regions of the disk. We investigate the origin and nature of these fluctuations using global 3D radiation─magnetohydrodynamic simulations of radiation- and magnetic-pressure-dominated AGN accretion disks. Disks with a significant turbulent Maxwell stress component exhibit wave-like temperature perturbations, most evident close to the midplane, whose propagation speeds exactly match the local fast magnetosonic speed and are consistent with the speeds inferred in observations. These fluctuations have amplitudes of 2%─4% in gas temperature, which are also consistent with observational constraints. Disks that are dominated by mean-field Maxwell stresses do not exhibit such waves. While waves may be present in the body of the disk, we do not find them to be present in the photosphere. Although this may in part be due to low numerical resolution in the photosphere region, we discuss the physical challenges that must be overcome for the waves to manifest there. In particular, the fact that such waves are observed implies that the disk photospheres must be magnetically dominated, since radiative damping from photon diffusion smooths out radiation pressure fluctuations. Furthermore, the gas and radiation fluctuations must be out of local thermodynamic equilibrium.

Keywords: Reverberation mapping, Active galactic nuclei, Radiative transfer, Magnetohydrodynamical simulations
Prospects for kSZ<SUP>2</SUP>─21 cm<SUP>2</SUP> Cross Correlations during Reionization. The 21 cm line and the patchy kinetic Sunyaev─Zel'dovich (kSZ) effect are promising and complementary probes of the Epoch of Reionization. A challenge for cross correlating these two signals is that foreground avoidance or removal algorithms applied to the 21 cm data inevitably sacrifice Fourier modes with long wavelengths along the line of sight (i.e., low-k∥ modes), yet only these same modes contribute to the kSZ signal. Here we show that a suitable kSZ2 × 21 cm2 cross-correlation statistic nevertheless remains nonvanishing, even after filtering out the corrupted low-k∥Fourier modes from the 21 cm data. We simulate the kSZ2 × 21 cm2 cross-correlation signal across reionization-era redshifts and find distinctive redshift evolution. This signal peaks early in the reionization history, when the volume-averaged fraction is around 0.1 ≲ xHII ≲ 0.2, after which it changes sign and reaches a minimum near reionization's midpoint (xHII ∼ 0.5), while the signal gradually vanishes as reionization completes. These trends appear generic across three simulated models which differ in their reionization histories. We forecast the detectability of the kSZ2 × 21 cm2 cross-power spectrum for the HERA and SKA1-Low 21 cm experiments in combination with current and next-generation cosmic microwave background (CMB) surveys including the Simons Observatory, CMB-S4, and CMB-HD. We find that a high-significance detection (signal-to-noise ratio ≳ 5σ) is possible with SKA1-Low and CMB-S4.

Keywords: Cosmology, Cosmic microwave background radiation, Sunyaev-Zeldovich effect, H I line emission, Reionization
Proton Temperature and Density Variations in Linear Magnetic Holes in the Solar Wind. Linear magnetic holes (LMHs) are pressure-balanced structures characterized by reduced magnetic pressure and compensatory thermal pressure enhancements, driven by proton temperature and/or density variations. Here we systematically investigate the relative contributions of the proton temperature and density to the thermal pressure by examining LMH characteristics separately in high-speed and low-speed solar wind streams. Using Solar Orbiter data (2020–2024), we analyze 348 LMHs (∼4.5–5.0 day–1; 256 in low-speed and 92 in high-speed streams) identified via a combined partial variance of increments and minimum variance analysis method. Based on their thermal pressure contributions, the selected LMHs are classified into four types: temperature-dominant (T-LMHs), density-dominant (N-LMHs), temperature-and-density-enhanced (TN-LMHs), and background-like (BG-LMHs). We find that in low-speed streams, BG-LMHs are the most common (39%), followed by T-LMHs (25%). In high-speed streams, TN-LMHs are the most frequent (32%). For the first time, we introduce magnetic variance rate (MVR, the normalized temporal gradient of magnetic strength) to quantify phase steepness and a thermal dominance coefficient (TDC, the difference between normalized temperature and density perturbations) to assess the temperature contribution. The results reveal a statistically significant correlation between MVR and TDC exclusively in T-LMHs within low-speed streams (Spearman's r = 0.4056, P = 0.0009), while other types and high-speed streams show no correlation (r &lt; 0.23, P &gt; 0.14). This selective correlation supports the role of ponderomotive forces—generated by steepening of nonlinear Alfvén waves—in heating T-LMHs, particularly in compressible low-speed wind environments. These results provide the first observational evidence linking phase-steepening dynamics to thermal enhancements in LMHs.

Keywords: Solar wind, Interplanetary magnetic fields
SQ-A: A Collision-triggered Starburst in the Intragroup Medium of Stephan's Quintet. We present new observational evidence supporting the hypothesis that SQ-A, a starburst in the intragroup medium (IGrM) of Stephan's Quintet (SQ), is triggered by a high-speed collision between two gas systems, one associated with the IGrM (v ∼ 6900 km s−1) and another with the intruder galaxy NGC 7318b (v ∼ 6000 km s−1). The new CO(2─1) data set from the Atacama Large Millimeter/submillimeter Array has angular resolutions between  0.″2  and  7.″0  and the new Very Large Array H I data cube has an angular resolution of  6.″6×7.″9 . The CO maps show that the two gas systems are bridged by another system with an intermediate velocity of ∼6600 km s−1, whereas the H I data show that the component of v ∼ 6600 km s−1 fits well into a gap in the more extended v ∼ 6000 km s−1 component, albeit with a displacement of ∼5 kpc. Both the bridge and the complementary distributions between different gas systems are common features of starbursts triggered by cloud─cloud collision. An analysis of clumps (sizes of 100─200 pc) reveals very diversified star formation (SF) activity in clumps belonging to different kinematic systems, with the molecular gas depletion time of the v ∼ 6900 km s−1 clumps being more than 10 times longer than that of the v ∼ 6600 km s−1 clumps. The results are consistent with a scenario in which the enhanced SF activity (and the starburst) in the system of v ∼ 6600 km s−1 is due to gas compression generated in cloud─cloud collisions, whereas the suppression of SF in the v ∼ 6900 km s−1 system is due to vortices (i.e. gas rotation) generated in more complex collisions involving dense clouds and diffuse intercloud gas associated with high-speed shocks.

Keywords: Hickson compact group, Galaxy collisions, Millimeter astronomy, Circumgalactic medium, H I line emission, Star formation, Galaxy groups, Intergalactic medium, Shocks, Interacting galaxies
Quantification of the Age Dependence of Mid-infrared Star Formation Rate Indicators. We combine James Webb Space Telescope images of the nearby galaxy NGC 5194 in the hydrogen recombination line Paα (1.8756 μm) from the Cycle 1 program JWST-FEAST with 21 μm dust continuum images from the Cycle 2 Treasury program JWGT to quantify the difference in the calibration of mid-infrared star formation rates (SFRs) between H II regions and galaxies. We use archival Hubble Space Telescope Hα imaging to correct the Paα emission for the effects of dust attenuation. Our data confirm previous results that the dust-corrected Paα flux is tightly correlated with the 21 μm emission at the scales of H II regions. When combined with published JWST data for the H II regions of the galaxy NGC 628 and Spitzer Space Telescope 24 μm data for whole galaxies and for kiloparsec-size galaxy regions, we show that the L(24)─L(Paα) relation has exponent &gt; 1 across six decades in luminosity. In addition, the hybrid 24 μm + Hα SFR indicator has a scaling constant about 4.4 times higher for H II regions than for whole galaxies, also in agreement with previous results. Models of stellar populations with a range of star formation histories reveal that the observed trends can be entirely ascribed to and quantified with the contribution to the infrared emission by stellar populations older than ∼5─6 Myr. Based on the models' results, we provide (1) a calibration for the infrared SFR across 6 orders of magnitude in L(24), from H II regions to luminous galaxies, and (2) a prescription for the scaling constant of the hybrid infrared SFR indicators as a function of the star formation timescale.

Keywords: Star forming regions, Galaxy stellar content, Scaling relations, Star formation, H II regions
Searching for Historical Extragalactic Optical Transients Associated with Fast Radio Bursts. We present a systematic search for past supernovae (SNe) and other historical optical transients at the positions of fast radio burst (FRB) sources to test FRB progenitor systems. Our sample comprises 83 FRBs detected by the Canadian Hydrogen Intensity Mapping Experiment (CHIME) and its k'niʔatn k'l⌣stk'masqt Outrigger, along with 93 literature FRBs representing all known well-localized FRBs. We search for optical transients coincident in position and redshift with FRBs and find no significant associations within the 5σ FRB localization uncertainties except for a previously identified potential optical counterpart to FRB 20180916B. By constraining the timescale for SN ejecta to become transparent to FRB emission, we predict that it takes at least 6─10 yr before the FRB emission can escape. From this, we infer that ≍7% of matched optical transients, up to 30% of currently known SNe, and up to 40% of core-collapse SNe could have an observable FRB based on timescales alone. We derive the number of new, well-localized FRBs required to produce one FRB-SN match by chance, and find it will take ∼22,700 FRBs to yield one chance association at the projected CHIME/FRB Outrigger detection rate. Looking forward, we demonstrate redshift overlap between SNe detected by the upcoming Vera C. Rubin Observatory and CHIME/FRB Outrigger FRBs, indicating the prospect of an increase in potential associations at redshift z &lt; 1. Our framework is publicly available, flexible to a wide range of transient timescales and FRB localization sizes, and can be applied to any optical transient populations in future searches.

Keywords: Transient sources, Time domain astronomy, Magnetars, Supernovae, Radio transient sources
Elemental and Isotopic Yields from T Coronae Borealis: Predictions and Uncertainties. T Coronae Borealis (T CrB) is a symbiotic recurrent nova system expected to undergo its next outburst within the next 2 yr. Recent hydrodynamic simulations have predicted the nucleosynthetic yields for both carbon─oxygen (CO) and oxygen─neon (ONe) white-dwarf models, but without accounting for thermonuclear reaction rate uncertainties. We perform detailed Monte Carlo postprocessing nucleosynthesis calculations based on updated thermonuclear reaction rates and uncertainties from the 2025 evaluation. We quantify the resulting abundance uncertainties and identify the key nuclear reactions that dominate them. Our results show that both the CO and ONe nova models robustly produce characteristic CNO isotopes. More pronounced abundance differences emerge for elements with A ≥ 20. Sulfur is the most robust observational discriminator between the CO and ONe nova models, with a model-to-model difference of a factor of ≍30 and minimal sensitivity to reaction rate uncertainties. Neon, silicon, and phosphorus exhibit even larger abundance differences (factors of ≍150─250), providing strong diagnostic potential. While their predicted yields are subject to larger uncertainties, these remain smaller than the model-to-model differences, allowing these elements to serve as useful, though less precise, tracers of white-dwarf composition. Chlorine, argon, and potassium also differ between models, but the 1σ-abundance ranges for the CO and ONe models overlap, reducing their present usefulness as composition tracers. We find that only nine nuclear reactions dominate the abundance uncertainties of the most diagnostically important isotopes, and their influence is largely independent of the underlying white-dwarf composition. These results provide guidance for future experimental efforts and for interpreting ejecta compositions in the next eruption of T CrB.

Keywords: Recurrent novae
RRAT J2325–0530: A Rotating Radio Transient with an Atypical Waiting-time Distribution. We present 1.25 GHz observations of the rotating radio transient (RRAT) J2325–0530, conducted with the Five-hundred-meter Aperture Spherical radio Telescope. Approximately 60% of detected single pulses occur in clusters of two to five consecutive bursts. Consequently, the waiting-time distribution between successive single pulses exhibits a pronounced excess at one rotation period, deviating from the exponential distribution expected for a Poisson process. After grouping consecutive bursts into single emission events, the recalculated waiting-time distribution is well described by a Weibull distribution with a shape parameter k ≳ 1. Monte Carlo simulations incorporating both intrinsic burst on-windows and rotational modulation successfully reproduce the observed one-rotation excess. These results suggest that RRAT J2325–0530 emits through a quasi-random process with on-windows slightly longer than its spin period, modulated by the rotation of the emission beam. Additionally, the polarization position angle shows complex behavior that cannot be fully described by the standard rotating vector model, and several pulses exhibit quasiperiodic microstructures. Taken together, these features indicate complex magnetospheric dynamics underlying the sporadic emission behavior of RRAT J2325–0530.

Keywords: Neutron stars, Radio pulsars
Energy Conversion and Electron Acceleration and Transport in 3D Simulations of Solar Flares. Recent observations and simulations indicate that solar flares undergo extremely complex 3D evolution, making 3D particle transport models essential for understanding electron acceleration and interpreting flare emissions. In this study, we investigate this problem by solving Parker's transport equation with 3D MHD simulations of solar flares. By examining energy conversion in the 3D system, we evaluate the roles of different acceleration mechanisms, including reconnection current sheet (CS), termination shock (TS), and supra-arcade downflows (SADs). We find that large-amplitude turbulent fluctuations are generated and sustained in the 3D system. The model results demonstrate that a significant number of electrons are accelerated to hundreds of keV and even a few MeV, forming power-law energy spectra. These energetic particles are widely distributed, with concentrations at the TS and in the flare looptop region, consistent with results derived from recent hard X-ray (HXR) and microwave (MW) observations. By selectively turning particle acceleration on or off in specific regions, we find that the CS and SADs effectively accelerate electrons to several hundred keV, while the TS enables further acceleration to MeV. However, no single mechanism can independently account for the significant number of energetic electrons observed. Instead, the mechanisms work synergistically to produce a large population of accelerated electrons. Our model provides spatially and temporally resolved electron distributions in the whole flare region and at the flare footpoints, enabling synthetic HXR and MW emission modeling for comparison with observations. These results offer important insights into electron acceleration and transport in 3D solar flare regions.

Keywords: Space plasmas, Solar flares, Solar magnetic reconnection, Interplanetary particle acceleration, Solar corona
CLOUDY and the High-resolution Microcalorimeter Revolution: Optical, UV, and X-Ray Spectra of One-electron Systems. With the launch of the XRISM microcalorimeter mission, space-based X-ray observations will achieve a record spectral resolving power of R ≡ E/∆E ∼ 1200. With this resolving power, emission features associated with fine-structure energy levels of some species will be resolved, sometimes for the first time. The plasma code, CLOUDY, was not originally designed for high-resolution X-ray spectroscopy and throughout its history did not resolve fine-structure components of Lyman lines. Here, we expand CLOUDY to resolve these fine-structure energy levels and obtain predicted X-ray spectra that match the resolution of new microcalorimeter observations. We show how the Lyman lines can be used as column density indicators in the hot X-ray emitting gas in a cluster of galaxies such as Perseus, and examine their sensitivity to external radiation fields and turbulence.

Keywords: X-ray binary stars, High energy astrophysics, High mass x-ray binary stars, Atomic data, Radiative transfer
The Production Mechanism of the High-energy Emission Line in the Brightest Cosmic Burst. As a characteristic feature of the spectrum, the emission line carries critical information on the underlying physics of the radiation. After extensive efforts in decades, the first high-significant detection of a series of emission lines evolving from 37 MeV to 6 MeV has been detected in the ever-bright gamma-ray burst GRB 221009A. However, the physical mechanism of the entire evolutionary trend of the lines remains elusive. To provide a self-consistent interpretation, we propose a novel scenario in which the photons of the line undergo a radiation transfer process called down-Comptonization after generation by the electron─positron pair annihilation. By incorporating the gamma-ray burst dynamical evolution, we systematically reproduce the observed evolution of the central energy, width, and flux of the emission line and further impose stringent constraints on the production of high-energy emission lines in general. Our study provides a new direction to the research of extreme cosmic bursts.

Keywords: Gamma-ray bursts