Sazuna/UAT_keywords huggingface dataset:
Super-resolution of SDO/HMI Magnetograms Using Novel Deep Learning Methods. Image super-resolution is a technique of enhancing the resolution of an image where a high-resolution (HR) image is reconstructed from a low-resolution (LR) image. In this Letter, we apply two novel deep learning models (residual attention model and progressive GAN model) for enhancing Solar Dynamics Observatory (SDO)/Helioseismic and Magnetic Imager (HMI) magnetograms. For this, we consider line-of-sight (LOS) magnetograms taken by SDO/HMI as output and their degraded ones with 4 × 4 binning as input. Deep learning networks try to find internal relationships between LR and HR images from the given input and the corresponding output image. We consider SDO/HMI magnetograms from 2014 May to August for training, from 2014 October to December for validation, and 2015 January to March for test. We find that the deep learning models generate higher-quality results than the bicubic interpolation in terms of visual aspects and metrics. We apply this model to a full-resolution SDO/HMI magnetogram and then compare the generated magnetogram with the corresponding Hinode/The Solar Optical Telescope Narrowband Filtergrams (NFI) magnetogram. This comparison shows that the generated magnetogram is consistent with the Hinode one with a high correlation (CC: 0.94) and a high similarity (SSIM: 0.93), which are better than the bicubic method.

Keywords: Solar atmosphere, The Sun, Astronomy data analysis, Solar magnetic fields
Hadronic versus Leptonic Models for γ-Ray Emission from VER J2227+608. Recent observations of VER J2227+608 reveal a broken power γ-ray spectrum with the spectral index increasing from ∼1.8 in the GeV energy range to ∼2.3 in the TeV range. Such a spectral break can be attributed to radiative energy loss of energetic electrons in the leptonic scenario for the γ-ray emission, which, in combination with the characteristic age of the nearby pulsar, can be used to constrain the magnetic field in the emission region. We show that the radio and X-ray observations can also be explained in such a scenario. In the hadronic scenario, the spectral break can be attributed to diffusion of energetic ions in a turbulent medium and detailed spectral measurement can be used to constrain the diffusion coefficient. These two models, however, predict drastically different spectra above 100 TeV, which will be uncovered with future high-resolution observations, such as Large High Altitude Air Shower Observatory (LHAASO) and/or Cherenkov Telescope Array (CTA).

Keywords: Rotation powered pulsars, Gamma-ray sources, Extended radiation sources, Cosmic ray astronomy, Galactic cosmic rays, Diffuse nebulae, Supernova remnants, Non-thermal radiation sources, Astrophysical processes, Cosmic ray sources, Gamma-ray astronomy, Ultra-high-energy cosmic radiation
Generating Transverse Loop Oscillations through a Steady-flow Driver. In recent years, the decay-less regime of standing transverse oscillations in coronal loops has been the topic of many observational and numerical studies, focusing on their physical characteristics, as well as their importance for coronal seismology and wave heating. However, no definitive answer has yet been given on the driving mechanism behind these oscillations, with most studies focusing on the use of periodic footpoint drivers as a means to excite them. In this paper, our goal is to explore the concept of these standing waves being self-sustained oscillations, driven by a constant background flow. To that end, we use the PLUTO code, to perform 3D magnetohydrodynamic simulations of a gravitationally stratified straight flux tube in a coronal environment, in the presence of a weak flow around the loop, perpendicular to its axis. Once this flow is firmly set up, a transverse oscillation is initiated, dominated by the fundamental kink mode of a standing wave, while the existence of a second harmonic is revealed, with a frequency ratio to the fundamental mode near the observed ones in decay-less oscillations. The presence of vortex shedding is also established in our simulations, which is connected to the "slippery" interaction between the oscillator and its surrounding plasma. We thus present a proof-of-concept of a self-oscillation in a coronal loop, and we propose it as a mechanism that could interpret the observed decay-less transverse oscillations of coronal loops.

Keywords: Solar coronal loops, Magnetohydrodynamics, Solar coronal waves
Solar Flare-CME Coupling throughout Two Acceleration Phases of a Fast CME. Solar flares and coronal mass ejections (CMEs) are closely coupled through magnetic reconnection. CMEs are usually accelerated impulsively within the low solar corona, synchronized with the impulsive flare energy release. We investigate the dynamic evolution of a fast CME and its associated X2.8 flare occurring on 2013 May 13. The CME experiences two distinct phases of enhanced acceleration, an impulsive one with a peak value of ∼5 km s-2, followed by an extended phase with accelerations up to 0.7 km s-2. The two-phase CME dynamics is associated with a two-episode flare energy release. While the first episode is consistent with the "standard" eruption of a magnetic flux rope, the second episode of flare energy release is initiated by the reconnection of a large-scale loop in the aftermath of the eruption and produces stronger nonthermal emission up to γ-rays. In addition, this long-duration flare reveals clear signs of ongoing magnetic reconnection during the decay phase, evidenced by extended hard X-ray bursts with energies up to 100-300 keV and intermittent downflows of reconnected loops for &gt;4 hr. The observations reveal that the two-step flare reconnection substantially contributes to the two-phase CME acceleration, and the impulsive CME acceleration precedes the most intense flare energy release. The implications of this non-standard flare/CME observation are discussed.

Keywords: Solar flares, Solar coronal mass ejections, Solar magnetic reconnection
Hydroxylated Mg-rich Amorphous Silicates: A New Component of the 3.2 μm Absorption Band of Comet 67P/Churyumov-Gerasimenko. The VIRTIS imaging spectrometer on board Rosetta has shown that the nucleus surface of comet 67P/Churyumov-Gerasimenko (67P/CG) is characterized by a broad absorption band at around 3.2 μm. The feature is ubiquitous across the surface and its attribution to (a) specific material(s) has been challenging. In the present Letter, we report an experimental investigation showing that the interaction of hydrogen atoms with Mg-rich amorphous silicates determines the formation of hydroxyl groups. The resulting IR spectrum exhibits a broad feature around 3.2 μm similar to that of comet 67P/CG. Hapke's radiative transfer model was employed to estimate an upper limit contribution of 65% of hydroxylated silicates to the observed cometary band intensity. The presence of a hydroxylated fraction in silicates on the cometary surface would represent an evolutionary link between primitive objects of the solar system and dust in the interstellar medium (ISM), where silicate grains can be hydroxylated after having interacted with hydrogen atoms. This link is consistent with the detection of the aliphatic organics in 67P/CG that also originate in the ISM.

Keywords: Comets, Laboratory astrophysics, Astrochemistry
Carbon-grain Sublimation: A New Top-down Component of Protostellar Chemistry. Earth's carbon deficit has been an persistent problem in our understanding of the formation of our solar system. A possible solution would be the sublimation of carbon grains at the so-called soot line (∼300 K) early in the planet-formation process. Here, we argue that the most likely signatures of this process are an excess of hydrocarbons and nitriles inside the soot line, and a higher excitation temperature for these molecules compared to oxygen-bearing complex organics that desorb around the water snowline (∼100 K). Such characteristics have been reported in the literature, for example, in Orion KL, although not uniformly, potentially due to differences in the observational settings and analysis methods of different studies or the episodic nature of protostellar accretion. If this process is active, this would mean that there is a heretofore unknown component to the carbon chemistry during the protostellar phase that is acting from the top down—starting from the destruction of larger species—instead of from the bottom up from atoms. In the presence of such a top-down component, the origin of organic molecules needs to be re-explored.

Keywords: Protostars, Astrochemistry
(Non)radial Solar Wind Propagation through the Heliosphere. The solar wind nonradial velocity components observed beyond the Alfvén point are usually attributed to waves, the interaction of different streams, or other transient phenomena. However, Earth-orbiting spacecraft as well as monitors at L1 indicate systematic deviations of the wind velocity from the radial direction. Since these deviations are of the order of several degrees, the calibration of the instruments is often questioned. This paper investigates for the first time the evolution of nonradial components of the solar wind flow along the path from ≈0.17 to 10 au. A comparison of observations at 1 au with those closer to or farther from the Sun based on measurements of many spacecraft at different locations in the heliosphere (Parker Solar Probe, Helios 1 and 2, Wind, Advanced Composition Explorer, Spektr-R, Acceleration, Reconnection, Turbulence and Electrodynamics of the Moon's Interaction with the Sun (ARTEMIS) probes, Mars Atmosphere and Volatile Evolution (MAVEN), Voyagers 1 and 2) shows that (I) the average values of nonradial components are not zero and vary in a systematic manner with the distance from the Sun and (II) their values significantly depend on the solar wind radial velocity.

Keywords: Solar wind, Heliosphere, Fast solar wind
A Magnetar-asteroid Impact Model for FRB 200428 Associated with an X-Ray Burst from SGR 1935+2154. Very recently, an extremely bright fast radio burst (FRB) 200428 with two submillisecond pulses was discovered coming from the direction of the Galactic magnetar SGR 1935+2154, and an X-ray burst (XRB) counterpart was detected simultaneously. These observations favor magnetar-based interior-driven models. In this Letter, we propose a different model for FRB 200428 associated with an XRB from SGR 1935+2154 in which a magnetar with high proper velocity encounters an asteroid of mass ∼1020 g. This infalling asteroid in the stellar gravitational field is first possibly disrupted tidally into a great number of fragments at a radius of ∼a few times 1010 cm, and then slowed around the Alfvén radius by an ultra-strong magnetic field, and in the meantime two major fragments of mass ∼1017 g that cross magnetic field lines produce two pulses of FRB 200428. The whole asteroid is eventually accreted onto the poles along magnetic field lines, impacting the stellar surface, creating a photon-e± pair fireball trapped initially in the stellar magnetosphere, and further leading to an XRB. We show that this gravitationally powered model can interpret all of the observed features self-consistently.

Keywords: Asteroids, Soft gamma-ray repeaters, Radio bursts, Magnetars
Electron Acceleration in One-dimensional Nonrelativistic Quasi-perpendicular Collisionless Shocks. We study diffusive shock acceleration (DSA) of electrons in nonrelativistic quasi-perpendicular shocks using self-consistent one-dimensional particle-in-cell simulations. By exploring the parameter space of sonic and Alfvénic Mach numbers we find that high Mach number quasi-perpendicular shocks can efficiently accelerate electrons to power-law downstream spectra with slopes consistent with DSA prediction. Electrons are reflected by magnetic mirroring at the shock and drive nonresonant waves in the upstream. Reflected electrons are trapped between the shock front and upstream waves, and undergo multiple cycles of shock-drift acceleration before the injection into DSA. Strong current-driven waves also temporarily change the shock obliquity and cause mild proton pre-acceleration even in quasi-perpendicular shocks, which otherwise do not accelerate protons. These results can be used to understand nonthermal emission in supernova remnants and intracluster medium in galaxy clusters.

Keywords: Plasma astrophysics, Supernovae, Particle astrophysics
Resolved Multi-element Stellar Chemical Abundances in the Brightest Quiescent Galaxy at z ∼ 2. Measuring the chemical composition of galaxies is crucial to our understanding of galaxy formation and evolution models. However, such measurements are extremely challenging for quiescent galaxies at high redshifts, which have faint stellar continua and compact sizes, making it difficult to detect absorption lines and nearly impossible to spatially resolve them. Gravitational lensing offers the opportunity to study these galaxies with detailed spectroscopy that can be spatially resolved. In this work, we analyze deep spectra of MRG-M0138, a lensed quiescent galaxy at z = 1.98, which is the brightest of its kind, with an H-band magnitude of 17.1. Taking advantage of full spectral fitting, we measure [Mg/Fe] = 0.51 ± 0.05, [Fe/H] = 0.26 ± 0.04, and, for the first time, the stellar abundances of six other elements in this galaxy. We further constrained, also for the first time in a z ∼ 2 galaxy, radial gradients in stellar age, [Fe/H], and [Mg/Fe]. We detect no gradient in age or [Mg/Fe] and a slightly negative gradient in [Fe/H], which has a slope comparable to that seen in local early-type galaxies. Our measurements show that not only is MRG-M0138 very Mg-enhanced compared to the centers of local massive early-type galaxies, it is also very iron rich. These dissimilar abundances suggest that even the inner regions of massive galaxies have experienced significant mixing of stars in mergers, in contrast to a purely inside-out growth model. The abundance pattern observed in MRG-M0138 challenges simple galactic chemical evolution models that vary only the star formation timescale and shows the need for more elaborate models.

Keywords: Initial mass function, Galaxy chemical evolution, Abundance ratios, Stellar abundances, Chemical abundances, Chemical enrichment, Galaxy quenching, Metallicity, Early-type galaxies
Explaining the Galilean Satellites' Density Gradient by Hydrodynamic Escape. The Galilean satellites exhibit a monotonic decrease in density (and increase in ice mass fraction) with distance from Jupiter. Whether this is because of the background conditions when they formed, the process of accretion itself, or later loss due to tidal heating has been in dispute for 40 yr. We find that a hitherto largely neglected process—vapor loss driven by accretional heating—can reproduce the observed density trend for accretion timescales ≳300 kyr, consistent with gas-starved satellite formation models. In this model, both Io and Europa develop an early surface liquid water ocean. Vapor escape from this ocean causes the water inventories of Io and Europa to be completely and mostly lost, respectively. Isotopic fractionation arising from vapor loss means that Europa will develop a higher D/H ratio compared with Ganymede and Callisto. We make predictions that can be tested with in situ measurements of the D/H of potential Europa plumes by the Europa Clipper spacecraft or infrared spectroscopic determinations of D/H at all three bodies.

Keywords: Galilean satellites, Natural satellite formation
Intermediate Luminosity Red Transients by Black Holes Born from Erupting Massive Stars. We consider black hole formation in failed supernovae when a dense circumstellar medium (CSM) is present around the massive star progenitor. By utilizing radiation hydrodynamical simulations, we calculate the mass ejection of blue supergiants and Wolf-Rayet stars in the collapsing phase and the radiative shock occurring between the ejecta and the ambient CSM. We find that the resultant emission is redder and dimmer than normal supernovae (bolometric luminosity of 1040- ${10}^{41}\ \mathrm{erg}\ {{\rm{s}}}^{-1}$ , effective temperature of ∼5 × 103 K, and timescale of 10-100 days) and shows a characteristic power-law decay, which may comprise a fraction of intermediate luminosity red transients (ILRTs) including AT 2017be. In addition to searching for the progenitor star in the archival data, we encourage X-ray follow-up observations of such ILRTs ∼1-10 yr after the collapse, targeting the fallback accretion disk.

Keywords: Transient sources, Stellar mass black holes
How Is Helicity (and Twist) Partitioned in Magnetohydrodynamic Simulations of Reconnecting Magnetic Flux Tubes?. Magnetic helicity conservation provides a convenient way to analyze specific properties (namely, the linkage and twist) of reconnecting flux tubes and yield additional insight into the pre- and post-reconnection states of magnetic structures in the solar atmosphere. A previous study considered two flux tubes with footpoints anchored in two parallel planes. They showed that reconnection would add self-helicity equivalent to a half turn of twist to each flux tube. We address a related and fundamental question here: if two flux tubes anchored in a single plane reconnect, what are the resulting twists imparted to each of the reconnected tubes? Are they equal and do they have a simple exact value independent of footpoint location? To do this, we employ a new (computationally efficient) method which subdivides each flux tube into distinct elements and calculates the mutual helicity of many elemental pairs, the sum of which determines the self-helicity of the overall flux tube. Having tested the method using a simple analytical model, we apply the technique to a magnetohydrodynamic simulation where initially untwisted magnetic flux tubes are sheared and allowed to reconnect (based on a previous reconnection model). We recover values of self-helicity and twist in the final end state of the simulations which show excellent agreement with theoretical predictions.

Keywords: Magnetohydrodynamics, Magnetic fields, Solar magnetic reconnection, Magnetohydrodynamical simulations
Dynamical Masses for the Pleiades Binary System HII-2147. We report our long-term spectroscopic monitoring of the Pleiades member HII-2147, which was previously spatially resolved at radio wavelengths in very long baseline interferometry (VLBI) observations. It has also been claimed to be a (presumably short-period) double-lined spectroscopic binary with relatively sharp lines, although no orbit has ever been published. Examination of our new spectroscopic material and the historical radial velocities shows that the current and previous spectra are best interpreted as featuring only a single set of lines of a moderately rapidly rotating star with slowly variable radial velocity, which is one of the sources detected by VLBI. We combine our own and other velocities with the VLBI measurements and new adaptive optics observations to derive the first astrometric-spectroscopic orbit of the G5 + G9 pair, with a period of 18.18 ± 0.11 yr. We infer dynamical masses of 0.897 ± 0.022 M⊙ for the spectroscopically visible star and 0.978 ± 0.024 M⊙ for the other, along with a distance of ${136.78}_{-0.46}^{+0.50}$ pc. The lack of detection of the lines of the more massive component in our spectra can be adequately explained if it is rotating much more rapidly than the star we see. This is consistent with the observation that the lines of the secondary are shallower than expected for a star of its spectral type.

Keywords: Binary stars, Fundamental parameters of stars, Radial velocity, Spectroscopic binary stars, Astrometric binary stars, Open star clusters
The Mass-Size Relation and the Constancy of GMC Surface Densities in the Milky Way. We use two existing molecular cloud catalogs derived from the same CO survey and two catalogs derived from local dust extinction surveys to investigate the nature of the giant molecular cloud (GMC) mass-size relation in the Galaxy. We find that the four surveys are well described by MGMC ∼ R2, implying a constant mean surface density, ${{\rm{\Sigma }}}_{\mathrm{GMC}}$ , for the cataloged clouds. However, the scaling coefficients and scatter differ significantly between the CO- and extinction-derived relations. We find that the additional scatter seen in the CO relations is due to a systematic variation in ${{\rm{\Sigma }}}_{\mathrm{GMC}}$ with Galactic radius that is unobservable in the local extinction data. We decompose this radial variation of ${{\rm{\Sigma }}}_{\mathrm{GMC}}$ into two components, a linear negative gradient with Galactic radius and a broad peak coincident with the molecular ring and superposed on the linear gradient. We show that the former may be due to a radial dependence of XCO on metallicity, while the latter likely results from a combination of increased surface densities of individual GMCs and a systematic upward bias in the measurements of ${{\rm{\Sigma }}}_{\mathrm{GMC}}$ due to cloud blending in the molecular ring. We attribute the difference in scaling coefficients between the CO and extinction data to an underestimate of XCO. We recalibrate the CO observations of nearby GMCs using extinction measurements to find that locally XCO = 3.6 ± 0.3 × 1020 cm-2 (K km s-1)-1. We conclude that outside the molecular ring, the GMC population of the Galaxy can be described to relatively good precision by a constant ${{\rm{\Sigma }}}_{\mathrm{GMC}}$ of 35 ${M}_{\odot }$ pc-2.

Keywords: Giant molecular clouds
Quantifying the Stellar Halo's Response to the LMC's Infall with Spherical Harmonics. The vast majority of the mass in the Milky Way (MW) is in dark matter (DM); we therefore cannot directly observe the MW mass distribution and have to use tracer populations in order to infer properties of the MW DM halo. However, MW halo tracers do not only feel the gravitational influence of the MW itself. Tracers can also be affected by MW satellites; Garavito-Camargo et al. (2109) demonstrate that the Large Magellanic Cloud (LMC) induces a density wake in the MW DM, resulting in large-scale kinematic patterns in the MW stellar halo. In this work, we use spherical harmonic expansion (SHE) of the velocity fields of simulated stellar halos in an effort to disentangle perturbations on large scales (e.g., due to the LMC itself, as well as the LMC-induced DM wake) and small scales (due to substructure). Using the Garavito-Camargo et al. simulations, we demonstrate how the different terms in the SHE of the stellar velocity field reflect the different wake components and show that these signatures are a strong function of the LMC mass. An exploration of model halos built from accreted dwarfs suggests that stellar debris from massive, recent accretion events can produce much more power in the velocity angular power spectra than the perturbation from the LMC-induced wake. We therefore consider two models for the Sagittarius (Sgr) stream—the most recent, massive accretion event in the MW apart from the LMC—and find that the angular power on large scales is generally dominated by the LMC-induced wake, even when Sgr is included. We conclude that SHE of the MW stellar halo velocity field may therefore be a useful tool in quantifying the response of the MW DM halo to the LMC's infall.

Keywords: Milky Way dark matter halo, Milky Way stellar halo, Milky Way dynamics, Milky Way Galaxy