Sazuna/UAT_keywords huggingface dataset:
Synchrotron Circular Polarization in Gamma-Ray Burst Prompt Optical Emission: Relativistic Thermal Electron Contribution. Synchrotron circular polarization of a nonthermal power-law electron distribution in gamma-ray bursts (GRBs) has been studied. However, some numerical simulations have shown that the resulting distribution of electrons is a combination of a thermal component and a nonthermal power-law component. In this paper, we investigate synchrotron circular polarization using such a hybrid energy distribution of relativistic thermal and nonthermal electrons within a globally toroidal magnetic field in GRB prompt optical emission. Our results show that compared to the solely nonthermal electron model, the synchrotron circular polarization degree (PD) in the hybrid electron model can vary widely in the optical band, depending on different parameters. The lower the electron temperature, the higher the circular PD. The time-averaged circular PD in the hybrid electron model can be higher than ∼1% when the electron temperature is as low as ∼1010 K, while in the solely nonthermal electron model it is usually lower than ∼1%. We further calculate the radiative transfer of the circular and linear polarization in the optical band. Our results show that both of the circular and linear PDs decrease with the increase of optical depth, but the linear PDs decline faster than the circular PDs. To further examine the physical mechanisms of both radiation and particle acceleration, we expect that instruments will be capable of measuring the circular polarization of GRB prompt optical emission in the future.

Keywords: Starlight polarization, Gamma-ray bursts, Interstellar synchrotron emission
Analysis on the Turbulent Properties of Current Sheets in the Solar Wind. The energy dissipation of solar wind turbulence, closely linked to solar wind heating, is a fundamental process in space physics. Previous research indicates that current sheets in the solar wind significantly influence both energy dissipation and particle heating. However, varying definitions of current sheets can lead to inconsistent results, and treating current sheets and the ambient solar wind as uniform entities may obscure the unique turbulent properties within these structures. By analyzing 117 typical current sheets near 1 au in the solar wind over the period from 2005 to 2017, we systematically investigate the turbulent properties of these current sheets. For the first time, we present a direct relationship between the magnetic field decrease rate, thermal energy increase, intermittency, and the magnetic power spectrum inside these structures. We observe a notable increase in proton temperature accompanied by a significant decrease in the magnetic field within the current sheets. Current sheets with larger magnetic field shear angles and more pronounced magnetic field drops tend to exhibit greater inhomogeneity at the sub-ion scale. Most importantly, our analysis reveals that the steeper magnetic field spectrum in the sub-ion range is directly associated with small-scale intermittent structures, a phenomenon observable only within current sheets and gradually diminishing over longer time scales. These results imply that the fine structures within the current sheets might affect the turbulent energy dissipation, and it is necessary to distinguish the current sheet itself from the ambient solar wind when studying turbulent properties and the associated energy dissipation processes.

Keywords: Interplanetary turbulence, Solar wind, Interplanetary physics
Quasithermal Noise in Magnetized Plasma: Theory and Simulations. Quasithermal noise (QTN) spectroscopy has been extensively used as an accurate tool to measure electron density and temperature in unmagnetized space plasmas. The observed spectrum depends on both antenna geometry and plasma kinetic properties. When considering the QTN signal of a magnetized plasma, the description becomes more complicated. We utilize particle-in-cell (PIC) simulations to study the QTN characteristics in magnetized plasmas based on Maxwellian and two-Maxwellian distributions. We compare these results with numerical computations, finding good agreement between the two. However, there are some minor differences. Specifically, when the angle between the wavevector (k) and the background magnetic field was 60°, clear harmonics appeared in the QTN signal from the numerical computations, whereas in the PIC simulations, these harmonics appeared at an angle of 80°. This discrepancy may be related to additional noise in the PIC simulation and our parameter limitations. In addition, Bernstein harmonics become more pronounced as the angle increases between  k  and the background magnetic field. These results demonstrate that PIC simulations can be used to more accurately characterize QTN signals in complex, magnetized systems.

Keywords: Space plasmas
Systematic Bias in Ionizing Radiation Escape Fraction Measurements from Foreground Large-scale Structures. We investigate the relationship between the Lyα forest transmission in the intergalactic medium (IGM) and the environmental density of galaxies, focusing on its implications for the measurement of ionizing radiation escape fractions. Using a sample of 268 spectroscopically confirmed background galaxies at 2.7 &lt; z &lt; 3.0 and a galaxy density map at z ≈ 2.5 within the COSMOS field, we measure the Lyα transmission photometrically, leveraging the multiwavelength data available from the COSMOS2020 catalog. Our results reveal a weak but statistically significant positive correlation between Lyα optical depth and galaxy density contrast, suggesting that overdense regions are enriched in neutral gas, which could bias escape fraction measurements. This emphasizes the need to account for the large-scale structure of the IGM in analyses of ionizing radiation escape fractions and highlights the advantages of a photometric approach for increasing the number of sampled lines of sight across large fields. The photometric redshifts provided by upcoming all-sky surveys, such as Euclid, will make it possible to account for this bias, which can also be minimized by using fields separated in the sky by many degrees.

Keywords: Intergalactic medium
Saturn's Recurrent Auroral Flashes in the Equinoctial Campaign. The past decades of Cassini's investigation around Saturn have revealed the frequent occurrence of ∼1 hr quasiperiodic (QP) signatures in the in situ magnetic field and plasma measurements and plasma wave observations. Saturn's aurora, as the response to the planet's magnetospheric dynamics, is also observed to exhibit transient features that can reoccur with a strikingly similar periodicity. Previous investigations have associated these features with energetic plasma injections, standing Alfvén waves, and dayside magnetodisk reconnections, which all require a closed-field configuration. However, both previous and our analyses of the Hubble Space Telescope (HST) images, as Saturn approached the equinox (when both poles are in the field of view), reveal that transient auroral emissions are strictly nonconjugate between hemispheres—they occur in either the north or south alone. This picture is matchable with the open-configured, north─south asymmetric magnetopause reconnection. We also present a detailed case analysis of Cassini's measurements for a prolonged period before and after a sequence of transient auroral brightenings observed in HST's visits G1─G4. We show that the northern lobe featured hourly QP pulsations in the magnetic field, electron flux, and whistler-mode hiss. In an interval outside HST exposures, we observed tens to hundreds of keV electrons in the antiparallel direction associated with upward field-aligned currents, which can in turn produce aurora. This is potentially suggestive of reconnection processes at the magnetopause, but are limited in the spatial scale.

Keywords: Planetary magnetospheres, Saturn, Aurorae
Multimodality in the Search for New Physics in Pulsar Timing Data and the Case of Kination-amplified Gravitational-wave Background from Inflation. We investigate the kination-amplified inflationary gravitational-wave background (GWB) interpretation of the signal recently reported by various pulsar timing array (PTA) experiments. Kination is a post-inflationary phase in the expansion history dominated by the kinetic energy of some scalar field, characterized by a stiff equation of state w = 1. Within the inflationary GWB model, we identify two modes that can fit the current data sets (NANOGrav and EPTA) with equal likelihood: the kination-amplification (KA) mode and the ordinary, no-kination-amplification (no-KA) mode. The multimodality of the likelihood motivates a Bayesian analysis with nested sampling. We analyze the free spectra of current PTA data and mock free spectra constructed with higher signal-to-noise ratios using nested sampling. The analysis of the mock spectrum designed to be consistent with the best fit to the NANOGrav 15 yr (NG15) data successfully reveals the expected bimodal posterior for the first time while excluding the reheating mode that appears in the fit to the current NG15 data, making a case for our correct and comprehensive treatment of potential multimodal posteriors arising from future PTA data sets. The resultant Bayes factor is  B≡Zno−KA/ZKA=2.9±1.9 , indicating comparable statistical significance between the two modes. Given the theoretical model-building challenges of producing highly blue-tilted primordial tensor spectra, the KA mode has the advantage of requiring less blue primordial spectra, compared with the no-KA mode. The synergy between future cosmic microwave background polarization, pulsar timing, and laser interferometer measurements of gravitational waves will help resolve the ambiguity implied by the multimodal posterior in PTA-only searches.

Keywords: Cosmology, Early universe, Nested sampling, Gravitational wave sources, Gravitational waves
Modeling Particle Acceleration and Release from Solar Eruptions. Determining the relative contribution of solar flares versus coronal mass ejections in large solar energetic particle (SEP) events is a long-standing problem. Flare-accelerated particles may travel through complex magnetic fields in the eruption region and escape into interplanetary space, thereby contributing to large SEP events. The process by which flare accelerated particles are released into the heliosphere is poorly understood and yet is critical to advancing our understanding of SEPs. In this work, we address the release problem by solving the focused transport equation in the context of a 2.5D ARMS magnetohydrodynamic simulation of a breakout coronal mass ejection (CME)/flare event. We find that particles accelerated by flare reconnection can be released into interplanetary space through interchange reconnection between closed and open field lines. These particles can contribute directly to SEP events and may become an important seed population for further acceleration by CME-driven shocks. Additionally, we find that the energetic particle fluxes in the inner heliosphere remain elevated for an extended period, allowing them to contribute to SEP acceleration by subsequent CMEs. This study represents the first direct particle modeling of how flare-accelerated particles can contribute to major SEP events.

Keywords: Space plasmas, Solar flares, Solar magnetic reconnection, Interplanetary particle acceleration, Solar corona
Analysis of Multi-epoch JWST Images of ∼300 Little Red Dots: Tentative Detection of Variability in a Minority of Sources. The James Webb Space Telescope (JWST) has revealed a population of red and compact sources at z ≳ 5 known as "little red dots" (LRDs) that are likely active galactic nuclei (AGNs). Here, we present a comprehensive study of the variability of 314 LRDs with multi-epoch JWST observations in five deep fields: Ultra Deep Survey, GOODS-S, GOODS-N, A2744, and COSMOS. Our analyses use all publicly available JWST NIRCam imaging data in these fields, together with multi-epoch JWST MIRI images available. We measure the significance (signal-to-noise ratio or SNRvar) of the variabilities for all LRDs and statistically evaluate their variabilities using the SNRvar distributions. We pay particular attention to the systematic offsets of photometric zero-points among different epochs that seem to commonly exist. The derived SNRvar distributions of the LRDs, including those with broad Hα/Hβ emission lines, follow the standard Gaussian distribution, and are generally consistent with those of the comparison samples of objects detected in the same images. This finding suggests that the LRD population, on average, does not show strong variability, which can be explained by super-Eddington accretion of the black holes in AGNs. Alternatively, many of them may be dominated by galaxies. We also find eight strongly variable LRD candidates with variability amplitudes of 0.24–0.82 mag. The rest-frame optical spectral energy distributions of these variable LRDs should have a significant AGN contribution. Future JWST observations will provide more variability information on LRDs.

Keywords: Active galactic nuclei, High-redshift galaxies
Bullet Shooting Cloud─Cloud Collision in MIR Bubble N65. We report that the formation of the twin-bubble system N65 and N65bis may be caused by the cloud─cloud collision (CCC) from the Bullet Nebula. The blueshifted 13CO gas component (N65a [47, 55] km s−1) is associated with the twin-bubble system, while the redshifted 13CO gas component (N65b [55, 62] km s−1) is linked to the Bullet Nebula. The distinct signatures of CCC, such as the bridge feature, the U-shaped cavity, and the complementary distribution with displacement, are found between N65a and N65b. The collision timescale is estimated to be 1.15─2.0 Myr, which is consistent with the dynamical ages of the two H II regions in N65a (0.73 Myr for N65bis and 1.19 Myr for N65, respectively), indicating their CCC-related origin. A total of 354 young stellar objects (YSOs) are found, which are clustered into eight minimum spanning tree groups. The distribution of M1 (at the post-frontal edge) and M2, M3, M4 (at the pre-frontal edge) suggests that the CCC triggers star formation along the collision path of  b=0.°35 , with younger YSOs present at the pre-frontal edge. Therefore, the bipolar morphology of the twin-bubble system can be interpreted by the collision of N65a and N65b along  b=0.°35  about 2 Myr ago.

Keywords: Stellar wind bubbles, Molecular clouds, Star formation, Interstellar medium, Pre-main sequence
The M31–M33 Interaction: Impact on M31's Center-of-mass Motion and Satellite Orbits. Inspired by recent studies of the interaction between the Milky Way and the Large Magellanic Cloud and its implications for the Milky Way's global dynamical history, we investigate how the massive satellite galaxy M33 influences the position and velocity of Andromeda's (M31) center of mass (COM) as M33 passes through M31's halo. Using recent six-dimensional phase space measurements for both galaxies, we use backward integration to revisit M33's orbital history in a massive M31 potential (3 × 1012 M⊙) for the first time. As previously concluded, we find that a first infall orbit is still the most statistically significant (≳90%) orbital solution for M33, except for a high-mass M31 combined with M31 proper motions from the Hubble Space Telescope (as opposed to Gaia), where there is a greater likelihood (∼65%) of a previous encounter. However, the minimum distance between M33 and M31 during this passage is typically ≥100 kpc, two to three times larger than the distance required to explain M33's warped stellar and gaseous disks. We quantify the magnitude and direction of M31's evolving COM position (RCOM) and velocity (VCOM) owing to M33, finding RCOM ≈ 100–150 kpc at maximum and VCOM ≈ 20–40 km s‑1. Furthermore, we explore the implications of this phenomenon for the M31 satellite system, specifically whether M33's gravitational influence is linked to the lopsided distribution of M31 satellites and whether M33 significantly perturbs the orbits of other M31 satellites. While M33 alone may not explain the lopsided nature of M31's satellite system, its dynamical impact is nonnegligible and must be accounted for in future dynamical studies of the M31 system.

Keywords: Local Group, Galaxy dynamics, Andromeda Galaxy, Dwarf galaxies
North+Lone Star Supernova Host Survey. I. Local Host-galaxy Hα Surface Brightness and the Hubble Residuals of Type Ia Supernovae. We present optical integral field unit spectroscopy acquired with the George and Cynthia Mitchell Spectrograph on the Harlan J. Smith telescope at McDonald Observatory of 94 galaxies (0.01 &lt; z &lt; 0.058) that have hosted Type Ia supernovae (SNe Ia). We selected host galaxies with star-forming morphology, consistent with the criteria used by A. G. Riess et al. We measured the Hα surface brightness (SB) of each host galaxy within 1 kpc of the location of the supernova. Using distances from the Pantheon+ sample, we find a step in Hubble residuals (HRs) compared to local Hα SB of −0.097 ± 0.051 mag at 1.9σ significance in a sample of 73 host galaxies, where SNe in environments with smaller Hα SB are, on average, less luminous after correction for light-curve shape and color. Almost all of the SNe in our sample were discovered by targeted surveys. Using an independent sample primarily from the untargeted Nearby Supernova Factory survey, M. Rigault et al. found a step of 0.045 ± 0.029 mag where SNe in passive environments are instead brighter, which is in 2.4σ tension with our measurement. M. Rigault et al. designated SNe Ia with comparatively small HRs (&lt;−0.1) and faint local Hα SB (&lt;log10(Hα SB/(erg−1 s−1 kpc2)) = 38.32) as the M2 population. SNe that would be classified as M2 are less highly represented in our sample (7% versus 21%). When we include an additional 12 early-type galaxies, the number of M2 SNe is almost doubled, although the tension with the HR step measured by M. Rigault et al. persists at 1.7σ.

Keywords: Cosmology, Type Ia supernovae, Galaxies
Exploring Effects of Modified Machine Learning Pipelines of Astrochemical Inventories. Machine learning pipelines for astrochemical inventories have been introduced as a useful addition to the astrochemist toolbox, having first been used to model and predict column densities in the Taurus Molecular Cloud (TMC-1). Rapid changes in the field of machine learning have provided new tools in optimizing this pipeline, including improved vector representations. Furthermore, the addition of new detections since the original model allows for a retrospective analysis of model performance, in addition to new data for the model. This study revisits TMC-1, investigating both effects of an increased detection inventory on the model and changes to various portions of the pipeline, yielding a significant improvement in column density predictions. Through these comparisons, we attempt to derive insight into the ultimate effectiveness of these models, as well as their current limitations and words of caution in their use and interpretation. Finally, we provide suggestions for future machine learning of interstellar sources.

Keywords: Interstellar molecules, Interdisciplinary astronomy, Astrochemistry
The Nature of Optical Afterglows without Gamma-Ray Bursts: Identification of AT2023lcr and Multiwavelength Modeling. In the past few years, the improved sensitivity and cadence of wide-field optical surveys have enabled the discovery of several afterglows without associated detected gamma-ray bursts (GRBs). We present the identification, observations, and multiwavelength modeling of a recent such afterglow (AT 2023lcr), and model three literature events (AT 2020blt, AT 2021any, and AT 2021lfa) in a consistent fashion. For each event, we consider the following possibilities as to why a GRB was not observed: (1) the jet was off-axis; (2) the jet had a low initial Lorentz factor; and (3) the afterglow was the result of an on-axis classical GRB (on-axis jet with physical parameters typical of the GRB population), but the emission was undetected by gamma-ray satellites. We estimate all physical parameters using afterglowpy and Markov Chain Monte Carlo methods from emcee. We find that AT 2023lcr, AT 2020blt, and AT 2021any are consistent with on-axis classical GRBs, and AT 2021lfa is consistent with both on-axis low Lorentz factor (Γ0 ≈ 5–13) and off-axis (θobs = 2θjet) high Lorentz factor (Γ0 ≈ 100) jets.

Keywords: Gamma-ray bursts, Sky surveys, Surveys, Transient sources, X-ray transient sources, Radio transient sources
UNCOVER: Significant Reddening in Cosmic Noon Quiescent Galaxies. We explore the physical properties of five massive quiescent galaxies at z ∼ 2.5, revealing the presence of nonnegligible dust reservoirs. JWST NIRSpec observations were obtained for each target, finding no significant line emission; multiple star formation tracers independently place upper limits between 0.1 and 10 M⊙ yr‑1. Spectral energy distribution modeling with Prospector infers stellar masses of  log10[M/M⊙]∼10‑11  and stellar-mass-weighted ages between 1 and 2 Gyr. The inferred mass-weighted effective radii (reff ∼ 0.4–1.4 kpc) and inner 1 kpc stellar surface densities ( log10[Σ&lt;1kpc/M⊙kpc2]≳9 ) are typical of quiescent galaxies at z ≳ 2. The galaxies predominately display negative color gradients (redder core and bluer outskirts); for one galaxy, this effect results from a dusty core. Unlike local quiescent galaxies, we identify significant reddening in these typical cosmic noon passive galaxies; all but one require AV ≳ 0.4. This finding is in qualitative agreement with previous studies, but our deep 20-band NIRCam imaging is able to significantly suppress the dust–age degeneracy and confidently determine that these galaxies are reddened. We speculate about the physical effects that may drive the decline in dust content in quiescent galaxies over cosmic time.

Keywords: Interstellar dust, Quenched galaxies, Galaxy evolution, Galaxy quenching, Near infrared astronomy
Temporarily Quiescent Galaxies at Cosmic Dawn: Probing Bursty Star Formation. The bursty, time-variable nature of star formation in the first billion years, as revealed by JWST, drives phases of temporary quiescence in low-mass galaxies that quench after starbursts. These galaxies provide unique probes of the burstiness of early star formation and its underlying physical processes. Using the SERRA cosmological zoom-in simulations, we analyze over 200 galaxies with M⋆ &lt; 109.5M⊙at z ∼ 6–8, finding that most experience quiescent phases driven by stellar feedback, with minimal influence from environmental effects. The fraction of temporarily quiescent galaxies increases with decreasing mass and luminosity, representing the dominant population at M⋆ &lt; 108M⊙ and MUV &gt; ‑17. By forward modeling their spectral energy distributions, we show that they are faint (&lt;MUV&gt; = ‑15.6 for M⋆ = 108M⊙), have strong Balmer breaks (&gt;0.5), and no emission lines. Comparing our predicted fractions with JWST results, we find similar luminosity-dependent trends; however, the observed fractions of temporarily quiescent galaxies at MUV ∼ ‑20 to ‑19 are higher, suggesting that stronger feedback or additional mechanisms beyond supernovae may be at play. We propose searching for F200W dropouts and satellites in the proximity (&lt;5″) of massive (&gt;1010M⊙) galaxies as effective strategies to uncover the hidden majority of faint (MUV &gt; ‑17), temporarily quiescent systems, crucial for constraining early feedback processes in low-mass galaxies.

Keywords: Cosmology, Dwarf galaxies, High-redshift galaxies, Galaxy evolution