Sazuna/UAT_keywords huggingface dataset:
Discovery of ATLAS17jrp as an Optical-, X-Ray-, and Infrared-bright Tidal Disruption Event in a Star-forming Galaxy. We hereby report the discovery of ATLAS17jrp as an extraordinary tidal disruption event (TDE) in the star-forming galaxy SDSS J162034.99+240726.5 in our recent sample of mid-infrared outbursts in nearby galaxies. Its optical/UV light curves rise to a peak luminosity of ~1.06 × 1044 erg s-1 in about a month and then decay as t -5/3 with a roughly constant temperature around 19,000 K, and the optical spectra show a blue continuum and very broad Balmer lines with FWHM ~ 15,000 km s-1, which gradually narrowed to 1400 km s-1 within 4 yr, all agreeing well with other optical TDEs. A delayed and rapidly rising X-ray flare with a peak luminosity of ~1.27 × 1043 erg s-1 was detected ~170 days after the optical peak. The high MIR luminosity of ATLAS17jrp (~2 × 1043 erg s-1) has revealed a distinctive dusty environment with a covering factor as high as ~0.2, which is comparable to that of a torus in active galactic nuclei but at least one order of magnitude higher than normal optical TDEs. Therefore, ATLAS17jrp turns out to be one of the rare unambiguous TDEs found in star-forming galaxies, and its high dust-covering factor implies that dust extinction could play an important role in the absence of optical TDEs in star-forming galaxies.

Keywords: Accretion, Galaxy nuclei
Observations of a Flare-ignited Broad Quasiperiodic Fast-propagating Wave Train. Large-scale extreme-ultraviolet (EUV) waves are frequently observed as an accompanying phenomenon of flares and coronal mass ejections (CMEs). Previous studies mainly focused on EUV waves with single wave fronts that are generally thought to be driven by the lateral expansion of CMEs. Using high spatiotemporal resolution multi-angle imaging observations taken by the Solar Dynamics Observatory and the Solar Terrestrial Relations Observatory, we present the observation of a broad quasiperiodic fast-propagating (QFP) wave train composed of multiple wave fronts along the solar surface during the rising phase of a GOES M3.5 flare on 2011 February 24. The wave train transmitted through a lunate coronal hole (CH) with a speed of ~840 ± 67 km s-1, and the wave fronts showed an intriguing refraction effect when they passed through the boundaries of the CH. Due to the lunate shape of the CH, the transmitted wave fronts from the north and south arms of the CH started to approach each other and finally collided, leading to a significant intensity enhancement at the collision site. This enhancement might hint at the occurrence of interference between the two transmitted wave trains. The estimated magnetosonic Mach number of the wave train is about 1.13, which indicates that the observed wave train was a weak shock. Period analysis reveals that the period of the wave train was ~90 s, in good agreement with that of the accompanying flare. Based on our analysis results, we conclude that the broad QFP wave train was a large-amplitude fast-mode magnetosonic wave or a weak shock driven by some nonlinear energy release processes in the accompanying flare.

Keywords: Alfven waves, Solar coronal waves, Solar corona
Mantle Degassing Lifetimes through Galactic Time and the Maximum Age Stagnant-lid Rocky Exoplanets Can Support Temperate Climates. The ideal exoplanets to search for life are those within a star's habitable zone. However, even within the habitable zone, planets can still develop uninhabitable climate states. Sustaining a temperate climate over geologic (~gigayear) timescales requires a planet to contain sufficient internal energy to power a planetary-scale carbon cycle. A major component of a rocky planet's energy budget is the heat produced by the decay of radioactive elements, especially 40K, 232Th, 235U, and 238U. As the planet ages and these elements decay, this radiogenic energy source dwindles. Here we estimate the probability distribution of the amount of these heat-producing elements that enter into rocky exoplanets through Galactic history by combining the system-to-system variation seen in stellar abundance data with the results from Galactic chemical evolution models. From this, we perform Monte Carlo thermal evolution models that maximize the mantle cooling rate, thus allowing us to create a pessimistic estimate of lifetime a rocky, stagnant-lid exoplanet can support a global carbon cycle through Galactic history. We apply this framework to a sample of 17 likely rocky exoplanets with measured ages, seven of which we predict are likely to be actively degassing today, despite our pessimistic assumptions. For the remaining planets, including those orbiting TRAPPIST-1, we cannot confidently assume that they currently contain sufficient internal heat to support mantle degassing at a rate sufficient to sustain a global carbon cycle or temperate climate without additional tidal heating or undergoing plate tectonics.

Keywords: Exoplanet astronomy, Exoplanet evolution
Three-dimensional Magnetic and Thermodynamic Structures of Solar Microflares. Microflares, one of the small-scale solar activities, are believed to be caused by magnetic reconnection. Nevertheless, their three-dimensional (3D) magnetic structures, thermodynamic structures, and physical links to reconnection are unclear. In this Letter, based on a high-resolution 3D radiative magnetohydrodynamic simulation of the quiet Sun spanning from the upper convection zone to the corona, we investigate the 3D magnetic and thermodynamic structures of three homologous microflares. It is found that they originate from localized hot plasma embedded in the chromospheric environment at the height of 2-10 Mm above the photosphere and last for 3-10 minutes with released magnetic energy in the range of 1027-1028 erg. The heated plasma is almost cospatial with the regions where the heating rate per particle is maximal. The 3D velocity field reveals a pair of converging flows with velocities of tens of km s-1 moving toward and outflows with velocities of about 100 km s-1 moving away from the hot plasma. These features support magnetic reconnection playing a critical role in heating the localized chromospheric plasma to coronal temperature, giving rise to the observed microflares. The magnetic topology analysis further discloses that the reconnection region is located near quasi-separators where both current density and squashing factors are maximal although the specific topology may vary from a tether-cutting to fan-spine-like structure.

Keywords: Solar coronal heating, Solar flares, Solar magnetic reconnection
Origin of Intense Electron Heating in Relativistic Blast Waves. The modeling of gamma-ray burst afterglow emission bears witness to strong electron heating in the precursor of Weibel-mediated, relativistic collisionless shock waves propagating in unmagnetized electron-ion plasmas. In this Letter, we propose a theoretical model, which describes electron heating via a Joule-like process caused by pitch-angle scattering in the decelerating, self-induced microturbulence and the coherent charge-separation field induced by the difference in inertia between electrons and ions. The emergence of this electric field across the precursor of electron-ion shocks is confirmed by large-scale particle-in-cell (PIC) simulations. Integrating the model using a Monte Carlo-Poisson method, we compare the main observables to the PIC simulations to conclude that the above mechanism can indeed account for the bulk of electron heating.

Keywords: Gamma-ray bursts, High energy astrophysics, Relativistic jets, Plasma astrophysics, Shocks, Particle astrophysics
A New Member of the Milky Way's Family Tree: Characterizing the Pontus Merger of Our Galaxy. We study the Pontus structure-a recently discovered merger that brought in ~7 globular clusters in the course of the hierarchical buildup of the Milky Way's halo. Here, we analyze the stellar population of Pontus and examine (1) its phase-space distribution using the ESA/Gaia data set, (2) its metallicity and chemical abundances (i.e., [Fe/H], [α/Fe], [Mg/Fe], and [Al/Fe]) using the spectroscopic catalog of APOGEE DR17, and (3) the color-magnitude diagram that shows interesting features, including a possibly double horizontal branch and a small population of blue stragglers. In sum, the Pontus stars show some unique properties that suggest they likely originated from the merging of an independent satellite galaxy; however, future analysis will shed more light on the true nature of this structure. This chemodynamical analysis of Pontus stars is another step forward in our bigger quest to characterize all the merging events of our Milky Way.

Keywords: Galaxy formation, Surveys, Chemical abundances, Milky Way stellar halo, Galaxy kinematics
Detection of Stellar-like Abundance Anomalies in the Slow Solar Wind. The elemental composition of the Sun's hot atmosphere, the corona, shows a distinctive pattern that is different from the underlying surface or photosphere. Elements that are easy to ionize in the chromosphere are enhanced in abundance in the corona compared to their photospheric values. A similar pattern of behavior is often observed in the slow-speed (&lt;500 km s-1) solar wind and in solar-like stellar coronae, while a reversed effect is seen in M dwarfs. Studies of the inverse effect have been hampered in the past because only unresolved (point-source) spectroscopic data were available for these stellar targets. Here we report the discovery of several inverse events observed in situ in the slow solar wind using particle-counting techniques. These very rare events all occur during periods of high solar activity that mimic conditions more widespread on M dwarfs. The detections allow a new way of connecting the slow wind to its solar source and are broadly consistent with theoretical models of abundance variations due to chromospheric fast-mode waves with amplitudes of 8-10 km s-1, sufficient to accelerate the solar wind. The results imply that M-dwarf winds are dominated by plasma depleted in easily ionized elements and lend credence to previous spectroscopic measurements.

Keywords: Slow solar wind, Solar abundances, Stellar abundances, Solar corona
Multipoint Analysis of the Interaction between a Shock and an ICME-like Structure around 2011 March 22. This work reports on the interaction between a fast forward shock and an interplanetary coronal-mass-ejection-like structure (ICMELS) as observed by in situ observations of radially aligned spacecraft. Around 2011 March 22, the Venus EXpress (VEX) and Solar TErrestrial RElations Observatory-A (STEREO-A) were nearly at the same longitude, providing us with an excellent opportunity to study the formation and evolution of the complex structures. The shock and ICMELS investigated in this paper are isolated near Venus, but when they approach STEREO-A, the shock nearly approaches the front edge of the ICMELS and forms a shock-ICMELS complex structure. The maximal magnetic field in the ICMELS increased 2.3 times due to shock compression, according to the observation. The recovery model, which restores the shocked portion of the shock-ICMELS to its uncompressed condition, likewise confirms this improvement. The interaction with the ICMELS, on the other hand, weakens shock 2. The magnetic compression ratio falls from 2.4 at Venus to 2.0 at STEREO-A. This research enables us to have a better physical knowledge of the impacts of the interaction between a shock and an ICME (or ICMELS), which will aid future space weather predictions.

Keywords: Solar-terrestrial interactions, Interplanetary shocks, Solar coronal mass ejections
First Sagittarius A* Event Horizon Telescope Results. I. The Shadow of the Supermassive Black Hole in the Center of the Milky Way. We present the first Event Horizon Telescope (EHT) observations of Sagittarius A* (Sgr A*), the Galactic center source associated with a supermassive black hole. These observations were conducted in 2017 using a global interferometric array of eight telescopes operating at a wavelength of λ = 1.3 mm. The EHT data resolve a compact emission region with intrahour variability. A variety of imaging and modeling analyses all support an image that is dominated by a bright, thick ring with a diameter of 51.8 ± 2.3 μas (68% credible interval). The ring has modest azimuthal brightness asymmetry and a comparatively dim interior. Using a large suite of numerical simulations, we demonstrate that the EHT images of Sgr A* are consistent with the expected appearance of a Kerr black hole with mass ~4 × 106 M ⊙, which is inferred to exist at this location based on previous infrared observations of individual stellar orbits, as well as maser proper-motion studies. Our model comparisons disfavor scenarios where the black hole is viewed at high inclination (i &gt; 50°), as well as nonspinning black holes and those with retrograde accretion disks. Our results provide direct evidence for the presence of a supermassive black hole at the center of the Milky Way, and for the first time we connect the predictions from dynamical measurements of stellar orbits on scales of 103-105 gravitational radii to event-horizon-scale images and variability. Furthermore, a comparison with the EHT results for the supermassive black hole M87* shows consistency with the predictions of general relativity spanning over three orders of magnitude in central mass.

Keywords: Heterodyne interferometry, Galactic center, Black holes, Kerr black holes, Rotating black holes
First Sagittarius A* Event Horizon Telescope Results. II. EHT and Multiwavelength Observations, Data Processing, and Calibration. We present Event Horizon Telescope (EHT) 1.3 mm measurements of the radio source located at the position of the supermassive black hole Sagittarius A* (Sgr A*), collected during the 2017 April 5-11 campaign. The observations were carried out with eight facilities at six locations across the globe. Novel calibration methods are employed to account for Sgr A*'s flux variability. The majority of the 1.3 mm emission arises from horizon scales, where intrinsic structural source variability is detected on timescales of minutes to hours. The effects of interstellar scattering on the image and its variability are found to be subdominant to intrinsic source structure. The calibrated visibility amplitudes, particularly the locations of the visibility minima, are broadly consistent with a blurred ring with a diameter of ~50 μas, as determined in later works in this series. Contemporaneous multiwavelength monitoring of Sgr A* was performed at 22, 43, and 86 GHz and at near-infrared and X-ray wavelengths. Several X-ray flares from Sgr A* are detected by Chandra, one at low significance jointly with Swift on 2017 April 7 and the other at higher significance jointly with NuSTAR on 2017 April 11. The brighter April 11 flare is not observed simultaneously by the EHT but is followed by a significant increase in millimeter flux variability immediately after the X-ray outburst, indicating a likely connection in the emission physics near the event horizon. We compare Sgr A*'s broadband flux during the EHT campaign to its historical spectral energy distribution and find that both the quiescent emission and flare emission are consistent with its long-term behavior.

Keywords: Black holes
First Sagittarius A* Event Horizon Telescope Results. III. Imaging of the Galactic Center Supermassive Black Hole. We present the first event-horizon-scale images and spatiotemporal analysis of Sgr A* taken with the Event Horizon Telescope in 2017 April at a wavelength of 1.3 mm. Imaging of Sgr A* has been conducted through surveys over a wide range of imaging assumptions using the classical CLEAN algorithm, regularized maximum likelihood methods, and a Bayesian posterior sampling method. Different prescriptions have been used to account for scattering effects by the interstellar medium toward the Galactic center. Mitigation of the rapid intraday variability that characterizes Sgr A* has been carried out through the addition of a "variability noise budget" in the observed visibilities, facilitating the reconstruction of static full-track images. Our static reconstructions of Sgr A* can be clustered into four representative morphologies that correspond to ring images with three different azimuthal brightness distributions and a small cluster that contains diverse nonring morphologies. Based on our extensive analysis of the effects of sparse (u, v)-coverage, source variability, and interstellar scattering, as well as studies of simulated visibility data, we conclude that the Event Horizon Telescope Sgr A* data show compelling evidence for an image that is dominated by a bright ring of emission with a ring diameter of ~50 μas, consistent with the expected "shadow" of a 4 × 106 M ⊙ black hole in the Galactic center located at a distance of 8 kpc.

Keywords: Galactic center, Aperture synthesis, Black holes, Very long baseline interferometry, Radio astronomy, High angular resolution
First Sagittarius A* Event Horizon Telescope Results. IV. Variability, Morphology, and Black Hole Mass. In this paper we quantify the temporal variability and image morphology of the horizon-scale emission from Sgr A*, as observed by the EHT in 2017 April at a wavelength of 1.3 mm. We find that the Sgr A* data exhibit variability that exceeds what can be explained by the uncertainties in the data or by the effects of interstellar scattering. The magnitude of this variability can be a substantial fraction of the correlated flux density, reaching ~100% on some baselines. Through an exploration of simple geometric source models, we demonstrate that ring-like morphologies provide better fits to the Sgr A* data than do other morphologies with comparable complexity. We develop two strategies for fitting static geometric ring models to the time-variable Sgr A* data; one strategy fits models to short segments of data over which the source is static and averages these independent fits, while the other fits models to the full data set using a parametric model for the structural variability power spectrum around the average source structure. Both geometric modeling and image-domain feature extraction techniques determine the ring diameter to be 51.8 ± 2.3 μas (68% credible intervals), with the ring thickness constrained to have an FWHM between ~30% and 50% of the ring diameter. To bring the diameter measurements to a common physical scale, we calibrate them using synthetic data generated from GRMHD simulations. This calibration constrains the angular size of the gravitational radius to be ${4.8}_{-0.7}^{+1.4}$ 4.8-0.7+1.4 μas, which we combine with an independent distance measurement from maser parallaxes to determine the mass of Sgr A* to be ${4.0}_{-0.6}^{+1.1}\times {10}^{6}$ 4.0-0.6+1.1×106 M ⊙.

Keywords: Black holes
First Sagittarius A* Event Horizon Telescope Results. V. Testing Astrophysical Models of the Galactic Center Black Hole. In this paper we provide a first physical interpretation for the Event Horizon Telescope's (EHT) 2017 observations of Sgr A*. Our main approach is to compare resolved EHT data at 230 GHz and unresolved non-EHT observations from radio to X-ray wavelengths to predictions from a library of models based on time-dependent general relativistic magnetohydrodynamics simulations, including aligned, tilted, and stellar-wind-fed simulations; radiative transfer is performed assuming both thermal and nonthermal electron distribution functions. We test the models against 11 constraints drawn from EHT 230 GHz data and observations at 86 GHz, 2.2 μm, and in the X-ray. All models fail at least one constraint. Light-curve variability provides a particularly severe constraint, failing nearly all strongly magnetized (magnetically arrested disk (MAD)) models and a large fraction of weakly magnetized models. A number of models fail only the variability constraints. We identify a promising cluster of these models, which are MAD and have inclination i ≤ 30°. They have accretion rate (5.2-9.5) × 10-9 M ☉ yr-1, bolometric luminosity (6.8-9.2) × 1035 erg s-1, and outflow power (1.3-4.8) × 1038 erg s-1. We also find that all models with i ≥ 70° fail at least two constraints, as do all models with equal ion and electron temperature; exploratory, nonthermal model sets tend to have higher 2.2 μm flux density; and the population of cold electrons is limited by X-ray constraints due to the risk of bremsstrahlung overproduction. Finally, we discuss physical and numerical limitations of the models, highlighting the possible importance of kinetic effects and duration of the simulations.

Keywords: Black hole physics, Galactic center
First Sagittarius A* Event Horizon Telescope Results. VI. Testing the Black Hole Metric. Astrophysical black holes are expected to be described by the Kerr metric. This is the only stationary, vacuum, axisymmetric metric, without electromagnetic charge, that satisfies Einstein's equations and does not have pathologies outside of the event horizon. We present new constraints on potential deviations from the Kerr prediction based on 2017 EHT observations of Sagittarius A* (Sgr A*). We calibrate the relationship between the geometrically defined black hole shadow and the observed size of the ring-like images using a library that includes both Kerr and non-Kerr simulations. We use the exquisite prior constraints on the mass-to-distance ratio for Sgr A* to show that the observed image size is within ~10% of the Kerr predictions. We use these bounds to constrain metrics that are parametrically different from Kerr, as well as the charges of several known spacetimes. To consider alternatives to the presence of an event horizon, we explore the possibility that Sgr A* is a compact object with a surface that either absorbs and thermally reemits incident radiation or partially reflects it. Using the observed image size and the broadband spectrum of Sgr A*, we conclude that a thermal surface can be ruled out and a fully reflective one is unlikely. We compare our results to the broader landscape of gravitational tests. Together with the bounds found for stellar-mass black holes and the M87 black hole, our observations provide further support that the external spacetimes of all black holes are described by the Kerr metric, independent of their mass.

Keywords: Black holes, Kerr black holes
Selective Dynamical Imaging of Interferometric Data. Recent developments in very long baseline interferometry (VLBI) have made it possible for the Event Horizon Telescope (EHT) to resolve the innermost accretion flows of the largest supermassive black holes on the sky. The sparse nature of the EHT's (u, v)-coverage presents a challenge when attempting to resolve highly time-variable sources. We demonstrate that the changing (u, v)-coverage of the EHT can contain regions of time over the course of a single observation that facilitate dynamical imaging. These optimal time regions typically have projected baseline distributions that are approximately angularly isotropic and radially homogeneous. We derive a metric of coverage quality based on baseline isotropy and density that is capable of ranking array configurations by their ability to produce accurate dynamical reconstructions. We compare this metric to existing metrics in the literature and investigate their utility by performing dynamical reconstructions on synthetic data from simulated EHT observations of sources with simple orbital variability. We then use these results to make recommendations for imaging the 2017 EHT Sgr A* data set.

Keywords: Very long baseline interferometry, Aperture synthesis, Astrophysical black holes, Interferometry