Sazuna/UAT_keywords huggingface dataset:
Numerical Simulations of the Effects of Out-of-plane Velocity Shear on Magnetic Reconnection in the Near-Sun Solar Wind. Motivated by Parker Solar Probe observations of magnetic reconnection exhausts in current sheets near the Sun, we present particle-in-cell simulations of reconnection in the presence of shear flow perpendicular to the plane of reconnection. Increasing the strength of the flow shear decreases the reconnection rate, in agreement with observations and theory, and aligns with similar decreases that have been observed in other geometries. In addition, the shear produces asymmetries in the out-of-plane current bounding the reconnection exhaust, similar to those seen in observations.

Keywords: Space plasmas, Solar magnetic reconnection, Plasma physics
Saturation of Ionospheric Currents under Extreme Environments. Strong solar wind conditions can drive large electric potentials across planetary polar caps, but previous studies have shown that the cross polar cap potential reaches a saturation point during periods of high interplanetary magnetic field (IMF), limiting the efficiency of solar wind─magnetosphere coupling. By analogy, field-aligned currents (FACs) have been expected to show similar saturation, but observational studies to date provide no clear evidence. Here we present global magnetohydrodynamic simulations of Earth and Jupiter that demonstrate saturation of ionospheric currents, including FACs, at high IMF strengths. This current saturation reduces auroral radio power, in contrast to the monotonic scaling predicted by the radio-magnetic scaling law model. Our results suggest that FAC saturation is an important feature of magnetosphere─ionosphere coupling and may impose intrinsic limits on planetary radio emissions, with implications for interpreting radio observations of exoplanets.

Keywords: Star-planet interactions, Magnetohydrodynamics, Magnetospheric radio emissions, Exoplanets
VLA FRAMEx. III. Circumnuclear Radio Emission Mechanisms in Hard X-Ray Selected Active Galactic Nuclei. We present Stokes I continuum analysis for a volume-limited sample (&lt;40 Mpc) of hard X-ray selected active galactic nuclei (AGNs) using 4─12 GHz observations with the Karl G. Jansky Very Large Array (VLA). All 25 sources analyzed here have previously been observed with the Very Long Baseline Array (VLBA) to probe their subparsec projected physical scales, but detected emission has only been measured for 12 of the sources at the C band (4.4 GHz), despite expectations. We determined that coronal emission is unlikely to be a dominant emission mechanism for the sources not detected by the VLBA, and the emission measured with the VLA is likely produced beyond parsec spatial scales. We also explore potential radiation mechanisms for the circumnuclear radio emission that is produced beyond the observable parsec physical scales probed with the VLBA but within the ≤30─110 pc spatial scales observed with the VLA. From an energetics perspective, we find that all targets have extranuclear radio emission that is compatible with AGN winds, assuming a maximum of 10% of the bolometric output can supply the mechanical energy observed. We also find that the excess emission is likely too strong for star formation alone when compared to results from optical spectroscopy, but may contribute in smaller capacities.

Keywords: Radio active galactic nuclei, Radio astronomy, Active galaxies, Active galactic nuclei
Periodic 6.7 GHz CH<SUB>3</SUB>OH Maser Emission in G353.273+0.641: First Candidate for a Pulsating High-mass Protostar. We report on the periodic flux variations in the 6.7 GHz CH3OH maser associated with the high-mass protostar G353.273+0.641, based on 13 yr of monitoring mainly by the Hitachi 32 m telescope. We identified a periodicity of 309 days based on a nearly complete light curve, with 833 epochs every few days. A strong correlation is found between the maser and the mid-infrared fluxes at 3.4 and 4.6 μm observed by NEOWISE during these periods, suggesting that the maser emission responds to variations in the protostellar luminosity. The average profile of the maser light curve is asymmetric and shows a steep drop in intensity just before the brightening, resembling that of some pulsating variable stars. Assuming a protostellar pulsation as the origin of maser periodicity, the observed period implies a cool and highly bloated, red supergiant-like structure. Such a bloated structure is consistent with a theoretical model of protostellar evolution under high accretion rates. The inferred protostellar parameters are broadly consistent with the theoretical model of pulsational instability during the early phase of high-mass star formation. However, a periodic accretion scenario caused by an unresolved compact protobinary cannot be completely ruled out. Several irregular peaks that deviate from the periodicity may result from episodic accretion phenomena or jet-launching events independent of the protostellar pulsation. Extremely high-resolution imaging with next-generation interferometers such as the ngVLA will provide a conclusive test for both the protostellar pulsation and the binary accretion scenarios.

Keywords: Massive stars, Star formation, Time domain astronomy, Interstellar masers, Protostars, Radio astronomy
Photospheric Horizontal Magnetic Field Decrease Preceding a Major Solar Eruption. Significant photospheric magnetic field changes during major solar eruptions—interpreted as coronal feedback from eruptions to the photosphere—are well-observed. However, analogous short-timescale field changes preceding eruptions are rarely reported. In this study, we present the first detailed analysis of a preflare decrease in the photospheric horizontal magnetic field (Bh) associated with an X1.8-class flare, using high-cadence vector magnetic field data from Helioseismic and Magnetic Imager onboard Solar Dynamics Observatory (SDO). We identify a region of gradual, spatially coherent Bh decrease of about 100 G along the flaring polarity inversion line (PIL) during 30 minutes preceding the flare. This decrease is accompanied by a decrease in the force-free parameter αw, with no significant flux emergence or cancellation observed. After the flare onset, Bh exhibited contrasting behaviors in different subregions: a steplike increase near the PIL and a continued decrease in surrounding regions, suggesting that the preflare Bh decrease may also have a coronal origin, like its postflare counterparts. Coronal imaging from the Atmospheric Imaging Assembly onboard SDO reveals that the associated erupting filament underwent a slow-rise phase before the flare, whose timing and location closely match the occurrence of the preflare Bh decrease. We propose that the slow-rise of the preeruptive filament stretched overlying coronal loops, increasing their verticality and thereby reducing Bh at their photospheric footpoints. The results present the first detailed analysis of a preflare Bh decrease and suggest it as a precursor to solar eruptions, causally linked to early filament activation and its impact on the photosphere.

Keywords: Solar magnetic fields, Solar active regions, Solar flares, Solar coronal mass ejections
Proposal of a Novel Physical Parameter Characterizing Solar Wind Speed in a Wave-driven Model. Empirical solar wind speed models play an important role in enabling space weather forecasting with low computational cost. Among these, one model (called the Wang─Sheeley model) is based on the asymptotic expansion factor. However, it is known that it fails in the case of pseudostreamers. In this study, as a first step toward constructing a solar wind speed empirical model based on physical parameters, we investigated the effect of the radial profile of flux-tube shape on the solar wind speed using one-dimensional numerical simulations. In the simulations, ad hoc Alfvén waves are injected from the photosphere at r = R⊙ as the energy source, and the MHD equations are solved out to the interplanetary space at r = 70R⊙ to reproduce solar wind acceleration. As a result, even when the coronal base magnetic field and the asymptotic expansion factor are fixed, the final solar wind speed varies by approximately 300 km s−1 depending on changes in the expansion height or nonmonotonic expansion. Additionally, across all simulations performed, a better correlation is found with the quantities that reflect the information about the radial profile of flux-tube shape than the asymptotic expansion factor. Our results suggest that, as a physical characteristic parameter of the solar wind speed, an operation that can account for the expansion factor throughout the corona is necessary.

Keywords: Slow solar wind, Space plasmas, Magnetohydrodynamics, Fast solar wind, Space weather, Solar wind, Solar physics, Solar coronal holes
IMPACTX: An X-Ray Spectral Model for Polar Dust and Clumpy Torus. We construct a generic X-ray spectral model for the reflection component from the clumpy torus and dusty gas in the polar region (polar dusty gas) in an active galactic nucleus (AGN), designated as Inclusive Spectral Energy Distribution Model of Polar Dust and Clumpy Torus for X-ray (IMPACTX). To calculate the spectra, we utilize the Monte-Carlo-based, three-dimensional radiative transfer code SKIRT. The adopted geometry is the same as that of the IMPACT model, consisting of a clumpy torus defined by Nenkova et al. and a paraboloid-shell polar dusty gas launched at the inner radius of the torus. We discuss the dependence of the X-ray spectrum on geometrical parameters in comparison with the conventional torus-only model (XCLUMPY). As an example, we have simultaneously applied the IMPACTX and IMPACT models to the X-ray and infrared spectra of the nearby Seyfert 2 galaxy NGC 4388, and have found a solution that can well reproduce both spectra. This demonstrates the importance of using both X-ray and infrared data to constrain the nuclear structure of an AGN.

Keywords: Supermassive black holes, High energy astrophysics, Seyfert galaxies, Active galactic nuclei, X-ray active galactic nuclei, Astrophysical black holes
BREAKFAST: A Framework for General Joint Burst Advocate Duty and Follow-up Guidance of Multiple Gamma-Ray Monitors. With the growing number of gamma-ray monitors in operation, several research teams have adopted a strategy of joint operation and scientific duty to improve efficiency. A successful example is the GECAM─HXMT─SVOM (GHS) constellation collaboration, which sets a precedent for other gamma-ray monitor constellations. However, joint duty also presents challenges to burst advocates (BAs), including an increased number of triggers and, more importantly, frequent switching between various systems due to incompatibilities among different missions, which complicates the situation. To address the current requirements of multiwavelength and multimessenger astronomy, we developed a customized framework for unified trigger processing within the GHS joint duty, named "BAs' Rapid Evaluation and Analysis Kit for Formulating Alerts and Summary Tools" (BREAKFAST). This framework incorporates a series of automated, semiautomated, and manual pipelines designed to rapidly process triggers of prompt emissions in the gamma-ray band from different instruments, while maintaining flexible compatibility for future missions. The pursuit of BREAKFAST goes beyond merely providing trigger processing for BAs. BREAKFAST also aims to identify high-value targets through rapid analysis and to guide follow-up telescopes by providing timely reports, thereby serving as an important bridge between prompt-emission observations and afterglow observations. To this end, a suite of comprehensive analysis modules is included in BREAKFAST, particularly the specially designed module that predicts X-ray afterglow brightness based on prompt-emission properties. The framework's effectiveness has already been demonstrated in recent observational campaigns, and it is expected to play a significant role in the discovery and observation of peculiar transients in the future.

Keywords: Gamma-ray bursts
Detectability of Lunar-origin Asteroids in the LSST Era. While most near-Earth asteroids (NEAs) are thought to originate from the main belt, recent discoveries have suggested the existence of a lunar-derived NEA population, such as the asteroids Kamo'alewa and 2024 PT5. These objects may hold key clues to the dynamical evolution of NEAs and the recent impact history of the Earth─Moon system. However, the population, distribution, and dynamical characteristics of these lunar-origin asteroids (LOAs) remain poorly constrained. By combining the lunar ejecta production with N-body orbital simulations of the ejecta, we investigate their orbital evolution in the past millions of years and the current LOA population, revealing their significant potential for detection by future surveys. Specifically for the Vera C. Rubin Observatory's upcoming Legacy Survey of Space and Time, we predict an average detection rate of about six LOAs (with D &gt; 5 m) per year. Additionally, we find that the LOAs tend to approach from sunward and antisunward directions, with encounter velocities significantly lower than those of typical NEAs. These findings offer valuable insights in guiding targeted ground-based surveys and planetary defense efforts for LOAs in the future.

Keywords: Near-Earth objects, Ejecta, Sky surveys, Close encounters, N-body simulations, Earth-moon system
On the Origin of the Dominant Waves in the Extended Solar Corona. The extended solar corona at 10─30 solar radii is essentially devoid of all waves below 100 kHz other than triggered ion acoustic waves (TIAWs), which consist of a low-frequency (LF) electromagnetic wave at a frequency of a few Hz coupled to one or more electrostatic waves at a few hundred Hz, such that the amplitudes of the high-frequency (HF) waves peak at a fixed phase of each LF wave period. All the waves in a TIAW event travel at the same phase speed, which is found to be 150 km s−1 (the ion acoustic speed was about 100 km s−1). It has not been possible to explain the TIAW as a resonant wave─wave interaction, so a nonresonant interaction has been considered in which the loss of energy by the LF wave is used to both heat the electrons and grow the HF waves. Evidence in support of this explanation is described, and a particle-in-cell simulation that discusses this process is summarized. This simulation demonstrates wave─particle interactions driven by the enhanced LF fluctuations, which subsequently modify the proton velocity distribution function and create conditions favorable for the growth of HF waves. This interplay between a pair of waves, mediated by modifications of plasma parameters and energy conversion, represents a significant nonlinear process in plasma physics, the study of which will deepen the understanding of energy transfer, wave generation, and plasma dynamics in diverse astrophysical environments.

Keywords: Active solar corona, Solar coronal waves
Testing Physical and Phenomenological Models to Explain the Origin of the Soft Excess in Type-1 AGN. Active galactic nuclei (AGN) exhibit an excess of X-ray emission below ∼1 keV compared to the 2─10 keV continuum extrapolation. This feature is commonly named soft excess, and its origin is currently debated for type-1 AGN. To investigate the nature of the soft excess, we perform a systematic analysis of XMM-Newton and NuSTAR simultaneous observations for a sample of 22 type-1 AGN. For this, we consider a baseline model composed of the intrinsic continuum, the reflection component, and the soft excess. We test different soft excess scenarios by fitting models of Comptonization, relativistically blurred reflection, ionized partial-covered absorption, and black-body emission. The final best fit requires multiple components for the soft excess, with a 1%─10% contribution of the relativistically blurred reflection at the accretion disk and 10%─80% of the black body component. We find that 88% of the spectra in the sample require the contribution of disk reflection and 92% require black body, where the luminosities of both components correlate. Ionized partial-covered absorption (warm absorber) is also observed in 84% of the objects. We also find that the black-body luminosity correlates with the accretion rate of the source. Using the temperature of the black body and its luminosity, we derive the expected radius at which this black body emission originates, which shows values consistent with originating at the inner part of the accretion disk.

Keywords: X-ray active galactic nuclei
Forecasting the Constraint on the Hu─Sawicki f(R) Modified Gravity in the CSST 3 × 2 pt Photometric Survey. We forecast the constraint on the Hu─Sawicki f(R) model from the photometric survey operated by the Chinese Space Station Survey Telescope (CSST). The simulated 3 × 2 pt data of galaxy clustering, weak lensing, and galaxy─galaxy lensing measurements within 100 deg2 are used in the analysis. The mock observational maps are constructed from a light cone, redshift sampling, and noise. The angular power spectra are measured with pseudo-Cℓ estimators and compared to theory in the same basis using validated weighting functions and an analytic covariance matrix that includes Gaussian, connected non-Gaussian, and supersample terms. We model the theoretical spectra using two methods. The first one uses MGCAMB to compute the linear modified-gravity clustering power spectra, and the second one adopts the FREMU emulator with a baseline of nonlinear ΛCDM prescription. Parameter inference is performed with COBAYA, and the cosmological and modified-gravity parameters are sampled within the emulator training domain, which are jointly fitted with the systematic parameters. We find that the predictions from the two methods are in good agreement at the overlapping large scales, and the emulator method can correctly provide additional high-ℓ information. The 1σ upper bounds of  log10∣fR0∣  are found to be &lt;−5.42 for cosmic shear only case and &lt;−5.29 for the 100 deg2 CSST 3 × 2 pt probe. The full CSST photometric survey with a 17,500 deg2 survey area is expected to further improve the constraint precision by about 1 order of magnitude. Our results demonstrate that the CSST 3 × 2 pt survey can deliver strict tests on f(R) gravity.

Keywords: Large-scale structure of the universe, Cosmological parameters, Scalar-tensor-vector gravity
A Possible Indication of Metallic Iron in White Dwarf Dusty Disks from Their "Dirtiness". Polluted white dwarfs provide unique constraints on the elemental compositions of planetary bodies. The tidal disruption of accreting bodies is thought to form circumstellar dusty disks, whose emission spectra could offer additional insights into the mineral phases of the accreted solid material. Silicates are detected in the mid-infrared spectra of several disks, but cannot fully account for the near-infrared excess in the disks' spectra. Conductive materials, such as metallic iron, are potential sources of near-infrared emissivity. We investigate the role of metallic iron within silicate dust in the observed spectra of the white dwarfs G29-38 and GD56. Using calculations of thermal emission spectra, we analyze the abundance of metallic iron in the dust and the disk structure parameters that best fit the observed spectra. We find that metallic-iron-bearing dust enhances the near-infrared opacity, thereby providing a better fit to the G29-38 spectrum for various silicate compositions than metallic-iron-free dust. The best-fit metal-to-silicate mixing ratio is approximately unity, and for Mg-rich pyroxenes, this value is also consistent with G29-38's stellar atmospheric composition within 1σ observational uncertainties. Based on the spectral fitting and compositional consistency, Fe-rich silicates without metallic iron cannot be ruled out. The observed GD56 spectrum also favors iron-bearing dust. However, the large observational uncertainties of GD56's stellar elemental abundances hinder a precise comparison between the stellar and dust iron abundances. Upcoming high-precision JWST observations will provide a larger sample, enabling a statistical analysis of the correlation between the iron abundances in the atmospheres and circumstellar dust of polluted white dwarfs.

Keywords: Interstellar dust, Dust composition, Infrared spectroscopy, White dwarf stars, Exoplanet astronomy, Debris disks
Study of the Orientation of Erupting Magnetic Flux Ropes in Coronal Mass Ejections from the Solar Corona to 1 au. The southward component of the interplanetary magnetic field, often originating from solar coronal mass ejections (CMEs), plays a crucial role in driving geomagnetic storms. Accurate prediction of the flux rope orientation of CMEs as they arrive at Earth requires a clear understanding of how the orientation of magnetic flux ropes evolves from the solar corona to 1 au. In this study, we investigated six geoeffective CMEs, initiated either from active regions (ARs; three events) or quiet-Sun (QS) filament eruptions (three events). The orientation prior to the eruption is determined by the eruptive filament or the magnetic flux rope near the solar surface. During the CME propagation away from the Sun, the graduated cylindrical shell model and the Grad─Shafranov technique are used to estimate the orientation of the CME magnetic field structure. Our results show that the orientation of flux ropes associated with QS eruptions did not change from the Sun to 1 au. For three rotating events initiated from ARs, the direction of rotation remains consistent during the propagation from the Sun to 1 au. The trend indicates that the heliospheric current sheet has a relatively limited influence on these events. For rotating events, the direction of rotation basically follows the prediction of Lynch's model.

Keywords: Solar-terrestrial interactions, Interplanetary magnetic fields, Solar flares, Solar coronal mass ejections
Polarization Dynamics of X-Ray Synchrotron Emission from a Multi-zone Blazar Jet. The polarization of X-ray synchrotron emission in blazars directly probes the magnetic field geometry and particle acceleration processes in relativistic jets. We use particle-in-cell simulations of magnetic reconnection and magnetized turbulence, coupled to polarization-sensitive radiative transfer code, to interpret Imaging X-ray Polarimetry Explorer (IXPE) observations of Mrk 421 during a high flux state recorded in December of 2023. To evaluate the fitness of the two theoretical scenarios, we rely on a quantitative comparison of the statistical properties of simulated and observed X-ray flux and polarization light curves using five evaluation metrics, rather than attempting to fit individual data points. We propose a turbulence-driven multi-zone model where jet emission is represented as the sum of the radiative output of N independent cells, each described by a particle-in-cell simulation. Comparison of ensembles of simulated Stokes-parameter light curves with IXPE data shows that magnetic-reconnection-dominated models provide the best match to the observed X-ray flux and polarization dynamics. The optimal configuration corresponds to N = 15 emitting cells, which reproduces the observed amplitudes and timescales of the X-ray flux and polarization variations. Magnetized turbulence models underpredict both the flux and polarization variability. Our results indicate that a multi-zone, reconnection-powered emission scenario can describe the X-ray polarization behavior of Mrk 421 and establish a quantitative framework for testing theoretical models against IXPE observations of other high-synchrotron-peaked blazars.

Keywords: Jets, Blazars, Relativistic jets, Polarimetry, Astronomical simulations