Sazuna/UAT_keywords huggingface dataset:
The Importance of Heat Flux in Low-Mach-number, Quasi-parallel Collisionless Shocks. Collisionless plasma shocks are a common feature of many space and astrophysical systems. They are sources of high-energy particles and nonthermal emission, channeling as much as 20% of the shock's energy into nonthermal particles. The generation and acceleration of these nonthermal particles have been previously studied and shown to affect shock hydrodynamics to the zeroth order. In this work, we use self-consistent hybrid particle-in-cell simulations to examine the effect of self-generated nonthermal ion populations on the nature of collisionless, quasi-parallel shocks. Accelerated nonthermal particles downstream of the shock diffuse into the upstream region, taking energy away from the shock, which increases the compression ratio, slows the shock down, and flattens the nonthermal population's spectral index for lower-Mach-number shocks. We show that this enhances shock compressibility when the heat flux is included in the Rankine─Hugoniot jump conditions, results that are roughly consistent with previous theories of "cosmic-ray-modified shocks." Additionally, the simulation data show that heat flux and enthalpy flux cancels out in the upstream region, yielding a relatively simple, alternative closure for the jump conditions which accurately predict for the shock speed and compression ratio. The results have the potential to explain discrepancies between predictions and observations in a wide range of systems, such as inaccuracies in predictions of the arrival times of coronal mass ejections and the conflicting radio and X-ray observations of intracluster shocks. These effects will likely need to be included in fluid modeling to predict shock evolution accurately.

Keywords: Galaxy clusters, Space plasmas, Interplanetary shocks, Planetary bow shocks, Plasma astrophysics, Supernova remnants, Shocks, Magnetohydrodynamical simulations
EP241217a: A Likely Type II GRB with an Achromatic Bump at z = 4.59. EP241217a is an X-ray transient detected by the Einstein Probe lasting for about 100 s and without accompanying γ-ray detection. The optical spectroscopy reveals the redshift of EP241217a is 4.59. By combining the γ-ray upper limit provided by GECAM-C, there is a considerable possibility that EP241217a is a typical type II gamma-ray burst, but it is fainter than the detection threshold of any available γ-ray monitors (i.e., Eγ,iso ≲ 1053 erg). The X-ray light curve exhibits a plateau lasting for ∼5 × 104 s. However, the joint analysis with optical data suggests the presence of an achromatic bump peaking at ∼3 × 104 s after the trigger, indicating the actual duration of the X-ray plateau may be significantly shorter than it appears. To interpret the achromatic bump, we adopt the scenario of a mildly relativistic jet coasting in a wind-like medium and encountering a rapid density enhancement of the circumburst medium, which is likely induced by the interaction of the progenitor's stellar wind and the interstellar medium. However, this model cannot fully explain observed data, and some issues do exist, e.g., the observed spectrum is harder than the model prediction. Consequently, we conclude that the scenario of a mildly relativistic jet coasting in the wind-like medium cannot explain all observed features of EP241217a. In addition, some alternative models commonly invoked to explain X-ray plateaus are discussed, but there are more or less issues when they were applied to EP241217a. Therefore, further theoretical modeling is encouraged to explore the origin of EP241217a.

Keywords: X-ray bursts
A Hierarchical Shock Model of Ultra-high-energy Cosmic Rays. We propose that a hierarchical shock model—including supernova remnant shocks, galactic wind termination shocks, and accretion shocks around cosmic filaments and galaxy clusters—can naturally explain the cosmic-ray spectrum from ∼1 GeV up to ∼ 200 EeV. While this framework applies to the entire cosmic-ray spectrum, in this work we focus on its implications for ultra-high-energy cosmic rays (UHECRs). We perform a hydrodynamic cosmological simulation to investigate the power processed at shocks around clusters and filaments. The downstream flux from nearby shocks around the local filament accounts for the softer, lower-energy extragalactic component around the ankle, and the upstream escaping flux from nearby clusters accounts for the transition to a hard spectral component at the highest energies. This interpretation is in agreement with UHECR observations. We suggest that a combination of early Universe galactic outflows, cosmic-ray streaming instabilities, and a small-scale turbulent dynamo can increase magnetic fields enough to attain the required rigidities. Our simulation suggests that the available volume-averaged power density of accretion shocks exceeds the required UHECR luminosity density by 3 orders of magnitude. We show that microgauss magnetic fields at these shocks could explain both the origin of UHECRs and potentially contribute to the diffuse radio synchrotron background below 10 GHz. The shock-accelerated electrons produce a hard radio background without overproducing diffuse inverse Compton emission. These results motivate further observational tests with upcoming facilities to help distinguish accretion shocks from other UHECR sources.

Keywords: Shocks, Cosmic web, Cosmic ray sources, Ultra-high-energy cosmic radiation
The Narrow Emission Lines of Seyfert 1 Galaxies: Comparisons with a Large SDSS Sample. We analyzed a large sample of Sloan Digital Sky Survey spectra of Seyfert galaxies, subdividing type 1 Seyferts (Sy1s) based on their narrow to broad Hα components. Comparing their narrow lines (NLs) to type 2 Seyferts (Sy2s) in line-ratio diagrams, most of the NLs of strong Sy1.0/Sy1.2s (with dominant broad lines) are the same as those of "pure" Sy2s. In contrast, only ∼25%─30% of the Sy1.8 and Sy1.9 nuclei (with weak broad lines) are located in the pure Sy2 region, with the rest falling in the composite/star-forming region. We explain these Seyfert+star formation spectra with a simple model. It shows that 85% of NLs in Sy1.9s are from H II regions, while 88% of the NLs in Sy1.0s arise from the same narrow-line region as in pure Sy2s. About ∼6% of the strong and weak Sy1s have NLs dominated by low-ionization nuclear emission-line region (LINER) emission, while ∼15% of intermediate Seyferts (Sy1.5/Sy1.6) do. To confirm this Seyfert 1 active galactic nucleus (AGN)+star formation combination, we used stellar absorption lines to compare their stellar populations. Their Hδ strengths show that LINERs, pure Sy2s, and also the broad-line-dominated Sy1s have old stellar populations. The weak Sy1s show stronger Hδ absorption, indicating larger proportions of young stars. About one-third of the u-band light in Sy1.0/1.2s is blended Balmer lines and continuum from the broad-line region (BLR). The NL gas reddening increases as the BLR strength decreases, from Sy1.0 (0.13 mag) to Sy1.9 (0.40 mag), to Sy2s and LINERs, both with 0.50 mag. Our data do not support the simplest version of Seyfert 1 and 2 unification, where both AGN classes have identical NLs.

Keywords: Metallicity, Photoionization, Star forming regions, Galaxy stellar content, Emission line galaxies, Galaxy ages, LINER galaxies, Seyfert galaxies, Active galactic nuclei, Galaxy spectroscopy, H II regions
Resolved H II Regions in NGC 253: Ionized Gas Structure and Suggestions of a Universal Density─Surface Brightness Relation. We use the full-disk Very Large Telescope/MUSE mosaic of NGC 253 to identify 2492 H II regions and study their resolved structure. With an average physical resolution of 17 pc, this is one of the largest samples of highly resolved spectrally mapped extragalactic H II regions. Regions of all luminosities exhibit a characteristic emission profile described by a double Gaussian with a marginally resolved or unresolved core with radius &lt; 10 pc surrounded by a more extended halo of emission with radius = 20─30 pc. Approximately 80% of the emission of a region originates from the halo component. As a result of this compact structure, the luminosity─radius relations for core and effective radii of H II regions depend sensitively on the adopted methodology. Only the isophotal radius yields a robust relationship in NGC 253, but this measurement has an ambiguous physical meaning. We invert the measured emission profiles to infer density profiles and find central densities of ne ≍ 10─100 cm−3. In the brightest regions, these agree well with densities inferred from the [S II] λλ6716, 6730 doublet. The central density of H II regions correlates well with the surface brightness within the effective radius. We show that this same scaling relation applies to the recent MUSE + Hubble Space Telescope catalog for 19 nearby galaxies. We also discuss potential limitations, including completeness, impacts of background subtraction and spatial resolution, and the generality of our results when applied to other galaxies.

Keywords: Disk galaxies, Warm ionized medium, Interstellar medium, Extragalactic astronomy, H II regions
Dense Molecular Clumps with Large Blue Asymmetries: Evidence for Collapse. An analysis of the Millimetre Astronomy Legacy Team 90 GHz survey has produced a sample of 27 candidate dense molecular clumps with large collapse motions, as revealed by large "blue" asymmetrical line profiles of the optically thick HCO+ (1─0) line. New, more sensitive molecular line observations of this sample, conducted with the Mopra 22 m telescope, confirm the blue asymmetries in the HCO+ (1─0) line profiles, with large, positive values of the asymmetry parameter A ( ĀHCO+=0.69±0.01 ), and positive, but smaller asymmetries in the HCN (1─0) and HNC (1─0) lines ( ĀHCN=0.35±0.01  and  ĀHNC=0.28±0.01 ), as expected for a less optically thick tracer in collapsing clumps. The small, positive mean asymmetry parameters for C2H (N = 1─0; J = 3/2─1/2; F = 2─1) and H13CO+ (1─0),  ĀC2H=0.15±0.02  and  ĀH13CO+=0.18±0.03 , likely indicate slightly optically thick emission for at least some clumps. The hyperfine ratios for N2H+ (1─0) are in their optically thin, LTE, values, but for HCN (1─0) they are not; the F = 1 → 1 hyperfine line shows abnormally weak intensities. A simple two-component model shows that self-absorption of the background F = 1 → 1 hyperfine line by the main F = 2 → 1 hyperfine line of a cold, foreground, redshifted cloud can reproduce the observed HCN (1─0) hyperfine intensities and match the HCN (1─0) and HCO+ (1─0) line profiles. All of these results are consistent with self-absorption of the optically thick lines on the red side of the profile, as expected for collapsing clumps. The simple two-cloud model suggests that this sample represents dense clumps with extreme collapse velocities, Vinf ∼ 2.4 km s−1.

Keywords: Molecular clouds
A Compact Symmetric Object Discovered by the VLA Low-band Ionosphere and Transient Experiment. We present new Very Long Baseline Array (VLBA) imaging of a megahertz peaked-spectrum source that was found using commensal low-frequency data taken with the Karl G. Jansky Very Large Array (VLA). The source, J0330─2730, was identified in multiepoch data from the VLA Low-band Ionosphere and Transient Experiment (VLITE). VLITE continuously collects low-frequency data at 340 MHz during regular VLA observations. Our analysis of the VLITE light curve demonstrates that J0330─2730 has significant 340 MHz flux variability at the ∼20% level over a timescale of approximately 1 yr. Our VLBA images reveal a resolved, double-lobed morphology with a projected linear size of 64 pc. We consider plausible mechanisms that could explain the observed 340 MHz variability and the source properties on milliarcsecond scales. We rule out variable Doppler boosting and conclude that refractive interstellar scintillation or variable free─free absorption are the most likely explanations. We argue that the properties of J0330─2730 are consistent with the class of compact symmetric objects and consider the evolutionary stage of the source. The extent of the resolved lobes revealed by the VLBA is significantly smaller than predictions based on the turnover─size relation for a standard synchrotron self-absorbed jet model. We discuss possible explanations for the departure from the turnover─size relation, including jet formation by a transient phenomenon such as a tidal disruption event or a "frustrated jet" impeded by the presence of dense gas or a high-pressure environment. This study highlights the potential of VLITE for the identification of compact and young radio sources.

Keywords: Sky surveys, Radio jets, Very long baseline interferometry, Active galaxies, Radio continuum emission, Time domain astronomy, Interstellar scintillation, Radio active galactic nuclei, Spectral index
Origin of the Asymmetric Gas Distribution near the Co-orbital Lagrange Points of an Embedded Planet. Hydrodynamic simulations of planet─disk interactions often show material accumulation near the co-orbital Lagrange points L4 and L5—features that may correspond to observed crescents in protoplanetary disks. Intriguingly, these simulations also show an asymmetrical distribution of gas between L4 and L5, whose physical origin is not yet understood and could allow us to further constrain the inner workings of planet─disk interactions. We performed 2D hydrodynamic simulations of a single, nonmigrating planet embedded in a gaseous disk to investigate this effect. We find that the asymmetry is primarily controlled by the sign of the radial temperature gradient, with positive gradients enhancing the accumulation at L4 and negative ones enhancing L5. A symmetric distribution is recovered on globally isothermal disks. Furthermore, we find that the azimuthal locations of L4 and L5 deviate from the classical circular restricted three-body problem, following a monotonic trend with the disk pressure scale height hp:  ϕmax≍±60°[1+0.18(hp3M⋆/mp)2/3] . Our simulations show that the longest-lived and largest-amplitude structures are produced by planets opening gaps with depths Σgap/Σ0 ≲ 0.2. We successfully reproduce the observed asymmetry using a semianalytical model that incorporates the azimuthally asymmetric radial velocity background induced by the planet. Overall, our results suggest that asymmetries in the form of crescents and clumps inside of density gaps opened by planets can constrain the local thermodynamic properties of protoplanetary disks.

Keywords: Hydrodynamical simulations, Planetary-disk interactions, Protoplanetary disks, Dynamical evolution
JWST Insights into Narrow-line Little Red Dots. JWST has revealed a population of red and compact objects with unique "V-shaped" spectral energy distributions (SEDs) known as "Little Red Dots" (LRDs). Many LRDs exhibit broad Balmer lines and thus likely host active galactic nuclei (AGNs). Here we present a study of five LRDs without broad Hα lines at z ≥ 5. They are selected from 32 LRDs that have NIRSpec high- or medium-resolution grating spectra covering Hα, and their Hα line widths are around 250 km s−1. Compared to normal star-forming galaxies, narrow-line LRDs tend to have relatively higher Hα line widths and luminosities. If they are dominated by galaxies, our SED modeling suggests that they are dusty, compact star-forming galaxies with high stellar masses and star formation rates. Alternatively, if their SEDs are produced by AGNs, the inferred black hole masses (&lt;106 M⊙) place them at the low-mass end of the AGN population. They may represent an early stage of super-Eddington growth, where the black holes have yet to accumulate significant masses. With large uncertainties, these black holes appear slightly overmassive relative to the local MBH─M*relation, but consistent or undermassive with respect to the MBH─σstellar and MBH─Mdyn relations. These narrow-line LRDs offer new insights into the diversity of LRDs. Finally, we find that ∼44% of high-redshift broad-line AGNs exhibit V-shaped SEDs. In addition, ∼20% of the previous LRD candidates are not real LRDs, and their V-shaped SEDs are caused by strong line emission.

Keywords: AGN host galaxies, Active galaxies
LVK S241125n: Massive Binary Black Hole Merger Produces Gamma Ray Burst in Active Galactic Nucleus Disk. Recently, the gravitational-wave (GW) event S241125n, detected by LIGO/Virgo/KAGRA (LVK), has been reported to coincide with a candidate detected by Swift-BAT/GUANO and an X-ray candidate found by the Follow-up X-ray Telescope onboard the Einstein Probe (EP) and confirmed by Swift-XRT. We estimate that the joint false alarm rate for the three candidates is 1/30 yr and that the corresponding false alarm probability (FAP) is FAPtriple = 0.037 (1.8σ). The coincidence between the GW and GRB could be an interesting test of their origin and open attractive opportunities for multimessenger observations, if they are actually associated. Motivated by this, we propose a theoretical model in which a binary black hole merger occurs within an active galactic nucleus (AGN) disk. The typically massive and significantly kicked merger remnant accretes disk material at hyper-Eddington rates, and the resulting jet could lead to the GRB associated with the GW event. As the jet interacts with the gas in the AGN disk, the shock breakout produces a Comptonized spectrum, consistent with an unusually soft photon index of the GRB prompt emission observed by Swift-BAT following LVK S241125n. Meanwhile, strong absorption and dust extinction of the afterglow by the high column density typical of AGN disks could explain the unusually hard spectrum observed in the X-ray band by EP, as well as the nondetection of an optical counterpart. Our model is predictive, and we highlight the importance of further constraining the orbital eccentricity of the merger and conducting deep-field observations of the host galaxy to test our explanation.

Keywords: Gravitational wave sources, Gamma-ray bursts, Active galactic nuclei, High energy astrophysics
Magnetic-field Order in the Southwestern Rim of RCW 86 Constrained Using X-Ray Polarimetry. RCW 86 is a supernova remnant whose origin has recently been linked to an off-center explosion within a cavity created by its progenitor star. In the southwestern region, the forward shock is thought to have reached the cavity wall, encountering diverse environmental conditions. We report on the spatially resolved X-ray polarimetric observation of RCW 86 with the Imaging X-ray Polarimetry Explorer (IXPE). In the 2─4.5 keV energy band we find no significant detection of polarization. Employing a dedicated background subtraction procedure and Bayesian spectropolarimetric fitting, we derive 99% upper limits on the polarization degree of the synchrotron component: 15% in higher-statistics regions and 30%─40% in lower-statistics regions. These upper limits on the polarization degree in several regions exclude the possibility of a strongly coherent magnetic field down to the subparsec scale, and that of a moderately coherent one on the scale of the synchrotron features as resolved by IXPE. The results indicate that the shocks in the southwestern rim of RCW 86 propagate more slowly than the unshocked ejecta at their locations, yet exceed the measured proper motion speeds. This behavior is consistent with reflected shocks occurring in tenuous regions of the shocked ejecta, distinct from regions that are radio-bright.

Keywords: X-ray astronomy, Spectropolarimetry, Supernova remnants, Shocks, Polarimetry
Stellar Flares in the TESS Light Curves of Planet-hosting M Dwarfs. M dwarfs are magnetically active stars that frequently produce flares, which have implications for both stellar evolution and exoplanet studies. Flare occurrence rates and activity levels of M dwarfs correlate with stellar characteristics such as age, mass, and rotation period. We search TESS observations of a known active population of M dwarfs as well as a volume-limited sample of M dwarfs within 15 pc. We detect flares in the light curves of these stars, including 276 of 538 M dwarfs within 15 pc, and calculate cumulative flare frequency distributions (FFDs) for each star. Based on flaring behavior, we categorize stars into relatively higher- and lower-activity groups and fit power laws to their FFDs to compare the power-law exponent (α) across activity levels. We find α = 1.99 ± 0.07 for the combined FFD of the lower-activity M dwarfs, compared to averages of α = 1.94 ± 0.58 for highly active stars with 10─100 detected flares, and α = 2.03 ± 0.43 for those with &gt;100 detected flares, suggesting little evolution in the power-law distribution of flares as M dwarfs transition from high- to low-activity states. The uncertainties for the active star groups reflect the standard deviation of α values across individual stars within each subset. Because stellar flares and associated stellar activity complicate exoplanet observations, we also examine the subset of M dwarfs with James Webb Space Telescope (JWST) transmission spectroscopy follow-up observations in Cycles 1─3. The flares we detect for these targets are consistent with the broader 15 pc sample, providing context for interpreting planetary atmosphere retrievals from JWST spectra.

Keywords: M dwarf stars, M stars, Stellar flares, Optical flares, Exoplanet atmospheres
From Nonparametric Distance Reconstruction to Testing the Etherington Relation and Cosmic Curvature Using 2D and 3D BAO Measurements. We present a joint test of cosmic curvature, Ωk0, and the cosmic distance─duality relation (CDDR) using the Etherington relation, which connects the luminosity and angular diameter distances at the same redshift. In this work, we combine the angular diameter distance measurements from recent Baryon acoustic oscillation (BAO) observations with luminosity distances reconstructed from cosmic chronometers data of Hubble parameter H(z) using a nonparametric technique, Gaussian process. A key part of our analysis is the systematic comparison of different BAO measurements—two dimensional (2D) BAO, three-dimensional (3D) BAO, and 3D Dark Energy Spectroscopic Instrument BAO—to determine whether any potential tension between angular and anisotropic BAO data affects constraints on the distance duality parameter η(z) and Ωk0. We adopt four representative parameterizations of η(z) to examine the correlation between η(z) and Ωk0. Our results show no evidence for violation of the CDDR, with η(z) consistent with unity at the 99% confidence level for all BAO datasets and parameterizations. In all scenarios, the best-fit values of Ωk0 mildly favor a nonflat universe, although a spatially flat universe remains compatible at the 95% confidence level. The constraints on η1 and Ωk0 indicate slight variations across different BAO datasets, but the discrepancies between the 2D and 3D BAO measurements do not introduce any significant bias, and no statistically meaningful tension is observed. Our work provides robust constraints on cosmic curvature and the validity of the CDDR based on nonparametric distance reconstruction.

Keywords: Markov chain Monte Carlo, Baryon acoustic oscillations, Cosmological parameters from large-scale structure