Sazuna/UAT_keywords huggingface dataset:
Evolution of Magnetohydrodynamic Turbulence in the Expanding Solar Wind: Residual Energy and Intermittency. We conduct 3D magnetohydrodynamic simulations of decaying turbulence in the context of the solar wind. To account for the spherical expansion of the solar wind, we implement the expanding box model. The initial turbulence comprises uncorrelated counterpropagating Alfvén waves and exhibits an isotropic power spectrum. Our findings reveal the consistent generation of negative residual energy whenever nonlinear interactions are present, independent of the normalized cross helicity σc and compressibility. The spherical expansion facilitates this process. The resulting residual energy is primarily distributed in the perpendicular direction, with S2(b) ‑ S2(u) ∝ l⊥ or equivalently  . Here S2(b) and S2(u) are second-order structure functions of magnetic field and velocity respectively. In most runs, S2(b) develops a scaling relation   ( ). In contrast, S2(u) is consistently shallower than S2(b), which aligns with in situ observations of the solar wind. We observe that the higher-order statistics of the turbulence, which act as a proxy for intermittency, depend on the initial σc and are strongly affected by the expansion effect. Generally, the intermittency is more pronounced when the expansion effect is present. Finally, we find that in our simulations, although the negative residual energy and intermittency grow simultaneously as the turbulence evolves, the causal relation between them seems to be weak, possibly because they are generated on different scales.

Keywords: Interplanetary turbulence, Magnetohydrodynamics, Solar wind, Magnetohydrodynamical simulations
SIRI-2 Detection of the Gamma-Ray Burst 221009A. SIRI-2 is a collection of strontium iodide gamma-ray detectors sensitive at approximately 400 keV to 10 MeV, launched on the Department of Defense's STPSat-6 to geosynchronous orbit. SIRI-2 detected the gamma-ray burst (GRB) 221009A and, unlike most GRB detectors, was not saturated and did not require any pulse pileup corrections. The energetics of this burst as measured by SIRI-2 are consistent with those found by other instruments, and the Band function fits to the spectra are consistent with those from the unsaturated Insight and GECAM instruments and similar to corrected spectra from the Fermi Gamma-ray Burst Monitor, but softer than those found by Konus-Wind when that instrument was saturated. The total fluence measured with SIRI-2 was measured to be 0.140 ± 0.002 erg cm‑2, lower than other instruments, likely due to the increasing background of SIRI-2 forcing the calculation to use a smaller time interval. An extrapolation of the distributions of fluences from GRBs to the fluence of 221009A measured with SIRI-2 indicates bursts brighter than this one should occur about once every 4000 yr.

Keywords: Gamma-ray bursts, Jets, High energy astrophysics, Core-collapse supernovae, Gamma-ray transient sources, Gamma-rays
Numerical Simulation of Electron Magnetohydrodynamics with Landau-quantized Electrons in Magnetar Crusts. In magnetar crusts, magnetic fields are sufficiently strong to confine electrons into a small to moderate number of quantized Landau levels. This can have a dramatic effect on the crust's thermodynamic properties, generating field-dependent de Haas–van Alphen oscillations. We previously argued that the large-amplitude oscillations of the magnetic susceptibility could enhance the ohmic dissipation of the magnetic field by continuously generating small-scale, rapidly dissipating field features. This could be important to magnetar field evolution and contribute to their observed higher temperatures. To study this, we performed quasi-3D numerical simulations of electron MHD in a representative volume of neutron star crust matter, for the first time including the magnetization and magnetic susceptibility resulting from Landau quantization. We find that the potential enhancement in the ohmic dissipation rate due to this effect can be a factor ∼3 for temperatures of the order of 108 K, and ∼4.5 for temperatures of the order of 5 × 107 K, depending on the magnetic field configuration. The nonlinear Hall term is crucial to this amplification: without it, the magnetic field decay is only enhanced by a factor ≲2 even at 5 × 107 K. These effects generate a high wavenumber plateau in the magnetic energy spectrum associated with the small-scale de Haas–van Alphen oscillations. Our results suggest that this mechanism could help explain the magnetar heating problem, though due to the effect's temperature-dependence, full magneto-thermal evolution simulations in a realistic stellar model are needed to judge whether it is viable explanation.

Keywords: Neutron stars, Magnetars, Magnetic fields, Magnetohydrodynamical simulations
A Parameter Study of the Electromagnetic Signatures of an Analytical Mini-disk Model for Supermassive Black Hole Binary Systems. Supermassive black holes (SMBHs) are thought to be located at the centers of most galactic nuclei. When galaxies merge, they form SMBH binary (SMBHB) systems, and these central SMBHs will also merge at later times, producing gravitational waves. Because galaxy mergers are likely gas-rich environments, SMBHBs are also potential sources of electromagnetic (EM) radiation. The EM signatures depend on gas dynamics, orbital dynamics, and radiation processes. The gas dynamics are governed by general-relativistic magnetohydrodynamics (MHD) in a time-dependent spacetime. Numerically solving the MHD equations for a time-dependent binary spacetime is computationally expensive. Therefore, it is challenging to conduct a full exploration of the parameter space of these systems and the resulting EM signatures. We have developed an analytical accretion-disk model for the mini-disks of an SMBHB system and produced images and light curves using a general-relativistic ray-tracing code and a superimposed harmonic binary BH metric. This analytical model greatly reduces the time and computational resources needed to explore these systems, while incorporating some key information from simulations. We present a parameter-space exploration of the SMBHB system, in which we study the dependence of the EM signatures on the spins of the BHs, the mass ratio, the accretion rate, the viewing angle, and the initial binary separation. Additionally, we study how the commonly used fast-light approximation affects the EM signatures and evaluate its validity in general-relativistic MHD simulations.

Keywords: Accretion, Black holes, Gravitational lensing, General relativity, Radiative transfer
Linear Bending Wave Propagation in Laminar and Turbulent Disks. Bending waves are perhaps the most fundamental and analytically tractable phenomena in warped disk dynamics. In this work, we conduct 3D grid-based, numerical experiments of bending waves in laminar, viscous hydrodynamic and turbulent, weakly magnetized disks, capturing their behavior in unprecedented detail. We clearly elucidate the theory from first principles, wherein the general Fourier–Hermite formalism can be simplified to a reduced framework which extends previous results toward locally isothermal disks. We obtain remarkable agreement with our laminar simulations wherein the tilt evolution is well described by the reduced theory, while higher-order vertical modes should be retained for capturing the detailed disk twisting and internal velocity profiles. We then relax this laminar assumption and instead launch bending waves atop a magnetorotationally turbulent disk. Although the turbulence can be quantified with an effective α parameter, the bending waves behave distinctly from a classical viscous evolution and are readily disrupted when the turbulent velocity is comparable to the induced warping flows. This may have implications for the inclination damping rates induced by planet–disk interactions, the capture rate of black holes in active galactic nucleus disks or the warped shapes assumed by disks in misaligned systems.

Keywords: Magnetohydrodynamical simulations, Astrophysical fluid dynamics, Protoplanetary disks, Circumstellar disks
The 100 pc White Dwarf Sample in the SDSS Footprint. II. A New Look at the Spectral Evolution of White Dwarfs. We increase the spectroscopic completeness of the 100 pc white dwarf sample in the Sloan Digital Sky Survey footprint with 840 additional spectra. Our spectroscopy is 86% complete for white dwarfs hotter than Teff = 5000 K, where Hα remains visible and provides reliable constraints on the atmospheric composition. We identify 2108 DA white dwarfs with pure hydrogen atmospheres, and show that ultramassive DA white dwarfs with M ≥ 1.1 M⊙ are an order of magnitude less common below 10,000 K. This is consistent with a fraction of them getting stuck on the crystallization sequence due to 22Ne distillation. In addition, there are no ultramassive DA white dwarfs with M ≥ 1.1M⊙ and Teff ≤ 6000 K in our sample, likely because Debye cooling makes them rapidly fade away. We detect a significant trend in the fraction of He atmosphere white dwarfs as a function of temperature; the fraction increases from 9% at 20,000 K to 32% at 6000 K. This provides direct evidence of convective mixing in cool DA white dwarfs. Finally, we detect a relatively tight sequence of low-mass DQ white dwarfs in color─magnitude diagrams for the first time. We discuss the implications of this tight DQ sequence, and conclude with a discussion of the future prospects from the upcoming Ultraviolet Transient Astronomy Satellite mission and the large-scale multi-fiber spectroscopic surveys.

Keywords: DB stars, Stellar evolutionary types, Stellar masses, White dwarf stars, DZ stars, Stellar classification, DA stars, Compact objects, DC stars, DQ stars, Stellar evolution
Potential Interior Structures and Habitability of Super-Earth Exoplanets LHS 1140 b, K2-18 b, TOI-1452 b, and TOI-1468 c. We analyze four super-Earth exoplanets, LHS 1140 b, K2-18 b, TOI-1452 b, and TOI-1468 c, which orbit M dwarf stars in the habitable zone. Their relative proximity, within 40 pc, makes them prime candidates for follow-up observations and atmospheric and habitability studies. This paper aims to assess their internal structure and habitability, considering their tidal heating, atmospheric heating, and global transport. We model the interior structure of the planets by applying Bayesian inference to an exoplanet's interior model. A model with constant quality factor is used to calculate the range of tidal heating, and a one-dimensional analytical model of tidally locked planets is used to assess their surface temperature distribution and habitability. Assuming no or only thin atmospheres, K2-18 b and TOI-1468 c are likely to be water worlds. However, TOI-1452 b and LHS 1140 b may have rocky surfaces. We find that tidal heating is not enough to raise the global mean surface temperature, but greenhouse heating can effectively do so. If the considered planets have retained thick atmospheres, K2-18 b, TOI-1468 c, and TOI-1452 b may, for significant atmospheric heating and heat transport factors, be too hot to sustain liquid water on their surfaces. However, the lower instellation of LHS 1140 b and the nonzero probability of it having a rocky surface give more space for habitable conditions on the planet.

Keywords: Exoplanets, Exoplanet tides, Exoplanet structure, Habitable zone, Habitable planets, Exoplanet atmospheres
The Puzzling Long GRB 191019A: Evidence for Kilonova Light. GRB 191019A was a long gamma-ray burst (GRB) lasting ~65 s and, as such, originally thought to be linked to a core-collapse supernova. However, even though follow-up observations identified the optical counterpart close to the bright nucleus of a nearby ancient galaxy (z = 0.248), no associated supernova was found. This led to the suggestion that the burst was caused by the merger of two compact stellar objects, likely in a dense circumnuclear environment. By using a recently developed diagnostic tool based on prompt emission temporal properties, we noticed that GRB 191019A falls among those long GRBs which are associated with compact mergers and with evidence of kilonova light. We thus reanalyzed unpublished GROND multicolor ( ) data obtained between 0.4 and 15 days posttrigger. Image subtraction confirmed the optical counterpart in all four optical bands, with GROND tracking its fading until 1.5 days postburst. Incorporating publicly available Swift-XRT data, a joint fit of an afterglow plus a kilonova model revealed a better match than an afterglow-only scenario. The resulting kilonova properties resemble those of AT2017gfo associated with the binary neutron star merger GW170817, with a total ejected mass of ~0.06 M⊙. Contrary to previous findings inferring a high-density circumburst environment (n0 ~ 107−8 cm−3), our analysis finds standard conditions (n0 ~ 1 cm−3), suggesting the long duration of GRB 191019A was intrinsic rather than due to jet interaction with a dense external medium.

Keywords: Gamma-ray bursts, Compact objects
Morphology of 137 Fast Radio Bursts Down to Microsecond Timescales from the First CHIME/FRB Baseband Catalog. We present a spectro-temporal analysis of 137 fast radio bursts (FRBs) from the first CHIME/FRB baseband catalog, including 125 one-off bursts and 12 repeat bursts, down to microsecond resolution using the least-squares optimization fitting routine fitburst. We measure subburst components within burst envelopes as narrow as ~23 μs (FWHM), with 20% of the sample displaying substructures narrower than 100 μs, offering constraints on emission mechanisms. Scattering timescales in the sample range from 30 μs to 13 ms at 600 MHz. Our measured values are compared with those in the first CHIME/FRB intensity catalog, revealing that nearly one-third of our sample exhibits additional burst components at higher time resolutions. We observe no correlations between scattering time and dispersion measure, rotation measure, or linear polarization fraction, with the latter suggesting that depolarization due to multipath propagation is negligible in our sample. Bursts with narrower envelopes (≤1 ms) in our sample exhibit higher flux densities, indicating the potential presence of submillisecond FRBs that are being missed by our real-time system below a brightness threshold. Most multicomponent bursts in our sample exhibit subburst separations of ≤1 ms, with no bursts showing separations &lt; 41 μs, even at a time resolution of 2.56 μs, but both scattering and low signal-to-noise ratio can hinder the detection of additional components. Lastly, given the morphological diversity of our sample, we suggest that one-off and repeating FRBs can come from different classes but have overlapping property distributions.

Keywords: Radio bursts, Radio transient sources
Maximum Energy of Particles in Plasmas. Particles are accelerated to very high, nonthermal energies in space, solar, and astrophysical plasma environments. In cosmic-ray physics, the Hillas limit is often used as a rough estimate (or the necessary condition) of the maximum energy of particles. This limit is based on the concepts of one-shot direct acceleration by a system-wide motional electric field, as well as stochastic and diffusive acceleration in strongly turbulent environments. However, it remains unclear how well this limit explains the actual observed maximum energies of particles. Here, we show, based on a systematic review, that the observed maximum energy of particles—those in space, solar, astrophysical, and laboratory environments—often reach the energy predicted by the Hillas limit. We also found several exceptions, such as electrons in solar flares and jet-terminal lobes of radio galaxies, as well as protons in planetary radiation belts, where deviations from this limit occur. We discuss possible causes of such deviations, and we argue in particular that there is a good chance of detecting ultra-high-energy (∼100 GeV) solar flare electrons that have not yet been detected. We anticipate that this study will facilitate further interdisciplinary discussions on the maximum energy of particles and the underlying mechanisms of particle acceleration in diverse plasma environments.

Keywords: Space plasmas, Solar flares, Shocks, Plasma astrophysics, Interplanetary particle acceleration, Planetary magnetospheres, Cosmic rays
A Study of the Star Clusters' Population in the Giant Molecular Cloud G174+2.5. We study the structure, interstellar absorption, color–magnitude diagrams (CMDs), kinematics, and dynamical state of embedded star clusters in the star-forming region associated with the giant molecular cloud G174+2.5. Our investigation is based on photometric data from the UKIDSS Galactic Plane Survey catalog and astrometric data from the Gaia DR3 catalogs. First, we recover all the known embedded clusters and candidate clusters in the region using surface density maps. Then, for the detected clusters, we determine their general parameters: the center positions, radii, number of stars, and reddening. To evaluate the reddening, we use both the NICEST algorithm and the Q-method. Both methods produce consistent extinction maps in the regions of the four studied clusters. However, the Q-method yields a much smaller color scatter in the CMD. For four clusters in particular (S235 North-West, S235 A-B-C, S235 Central, and S235 East1+East2), we were able to compute individual membership probabilities, the cluster distances, the cluster masses, and their average proper motions. By building on these results, we have studied the clusters' kinematics and dynamics. Moreover, we estimate the mass of the gas component and the star formation efficiency in the regions of these four clusters. Finally, we provide an estimate of the total energy of the stellar and gas components in the area of these four clusters to determine whether the clusters are bound (here, we consider a "cluster" as the system "stars + gas"). The gravitational bound strongly depends on the region for which we estimate the gas mass. If we consider the mass of the entire cloud, all these four clusters turn out to be bound.

Keywords: Stellar kinematics, Open star clusters, Star formation, Intergalactic dust clouds
Residual Energy and Broken Symmetry in Reduced Magnetohydrodynamics. Alfvénic interactions that transfer energy from large to small spatial scales lie at the heart of magnetohydrodynamic turbulence. An important feature of the turbulence is the generation of negative residual energy—excess energy in magnetic fluctuations compared to velocity fluctuations. By contrast, an MHD Alfvén wave has equal amounts of energy in fluctuations of each type. Alfvénic quasi-modes that do not satisfy the Alfvén wave dispersion relation and exist only in the presence of a nonlinear term can contain either positive or negative residual energy, but until now, an intuitive physical explanation for why negative residual energy is preferred has remained elusive. This paper shows that the equations of reduced MHD are symmetric in that they have no intrinsic preference for one sign of the residual energy over the other. An initial state that is not an exact solution to the equations can break this symmetry in a way that leads to net-negative residual energy generation. Such a state leads to a solution with three distinct parts: nonresonant Alfvénic quasi-modes, normal modes produced to satisfy initial conditions, and resonant normal modes that grow in time. The latter two parts strongly depend on initial conditions; the resulting symmetry breaking leads to net-negative residual energy both in Alfvénic quasi-modes and ω = k ∥ V A = 0 modes. These modes have net-positive residual energy in the equivalent boundary value problem, suggesting that the initial value setup is a better match for solar wind turbulence.

Keywords: Interplanetary turbulence, Magnetohydrodynamics, Solar wind, Alfven waves
Discovery and Spectroscopic Confirmation of Aquarius III: A Low-mass Milky Way Satellite Galaxy. We present the discovery of Aquarius III, an ultra-faint Milky Way satellite galaxy identified in the second data release of the DECam Local Volume Exploration survey. Based on deeper follow-up imaging with DECam, we find that Aquarius III is a low-luminosity (   ), extended (  pc) stellar system located in the outer halo (D ⊙ = 85 ± 4 kpc). From medium-resolution Keck/DEIMOS spectroscopy, we identify 11 member stars and measure a mean heliocentric radial velocity of   for the system and place an upper limit of σ  v  &lt; 3.5 km s‑1 (σ  v  &lt; 1.6 km s‑1) on its velocity dispersion at the 95% (68%) credible level. Based on calcium-triplet metallicities of the six brightest red giant members, we find that Aquarius III is very metal-poor ([Fe/H]= ‑ 2.61 ± 0.21) with a statistically significant metallicity spread (  dex). We interpret this metallicity spread as strong evidence that the system is a dwarf galaxy as opposed to a star cluster. Combining our velocity measurement with Gaia proper motions, we find that Aquarius III is currently situated near its orbital pericenter in the outer halo (r peri = 78 ± 7 kpc) and that it is plausibly on first infall onto the Milky Way. This orbital history likely precludes significant tidal disruption from the Galactic disk, notably unlike other satellites with comparably low velocity dispersion limits in the literature. Thus, if further velocity measurements confirm that its velocity dispersion is truly below σ  v  ≲ 2 km s‑1, Aquarius III may serve as a useful laboratory for probing galaxy formation physics in low-mass halos.

Keywords: Local Group, Dwarf galaxies, Milky Way Galaxy
Dynamic Evolution of Magnetic Flux Ropes in Active Region 11429. II. Magnetic Characteristic of the Footpoints During the Preeruption and Eruption Phases. Magnetic flux rope (MFRs) are the most probable core structure of coronal mass ejections (CMEs). Their footpoints on the photosphere are the only feature for which their magnetic properties can be accurately measured, and to some extent, determine the property of the whole MFR. So far, the magnetic properties of footpoints are still a mystery. Y. Zhang et al. presented the dynamic evolution of four MFRs involved in a major eruption. For this special event, the footpoints of the MFRs could be identified by extreme-ultraviolet (EUV) brightening and MFR morphology during the preeruption phase, and by conjunction, EUV dimming during the eruption phase. This is a new effort to identify the footpoints of MFRs during the preeruption phase. This identification discloses that all footpoints originate out of the umbra and penumbra of the sunspots. The footpoint areas and magnetic flux during the preeruption phase are about 107 km2 and around the order of 1019–1020 Mx, respectively. During the eruption phase, the footpoint area and the magnetic flux are about 108 km2 and around the order of 1020 Mx, respectively. The temporal evolution of the magnetic features around the footpoints disclose that all footpoint pairs belong to nonconjugated opposite polarities. This observational evidence will help us to understand the magnetic nature of MFRs and their associated CMEs.

Keywords: Solar magnetic fields, Solar coronal mass ejections, Solar flares, Solar corona