Sazuna/UAT_keywords huggingface dataset:
Kinematics of the Magellanic Stream and Implications for Its Ionization. The Magellanic Stream and the Leading Arm form a massive, filamentary system of gas clouds surrounding the Large and Small Magellanic Clouds. Here we present a new component-level analysis of their ultraviolet (UV) kinematic properties using a sample of 31 sightlines through the Magellanic System observed with the Hubble Space Telescope/Cosmic Origins Spectrograph. Using Voigt-profile fits to UV metal-line absorption, we quantify the kinematic differences between the low-ion ( $\mathrm{Si}\,\mathrm{II}$ and ${\rm{C}}\,\mathrm{II}$ ), intermediate-ion ( $\mathrm{Si}\,\mathrm{III}$ ), and high-ion ( $\mathrm{Si}\,\mathrm{IV}$ and ${\rm{C}}\,\mathrm{IV}$ ) absorption lines and compare the kinematics between the Stream and Leading Arm. We find that the Stream shows generally simple, single-phase kinematics, with statistically indistinguishable b-value distributions for the low-, intermediate-, and high-ion components, all dominated by narrow ( $b\lesssim 25$ km s-1) components that are well aligned in velocity. In contrast, we find tentative evidence that the Leading Arm shows complex, multi-phase kinematics, with broader high ions than low ions. These results suggest that the Stream is photoionized up to ${\rm{C}}\,\mathrm{IV}$ by a hard ionizing radiation field. This can be naturally explained by the Seyfert-flare model of Bland-Hawthorn, in which a burst of ionizing radiation from the Galactic Center photoionized the Stream as it passed below the south Galactic pole. The Seyfert flare is the only known source of radiation that is both powerful enough to explain the Hα intensity of the Stream and hard enough to photoionize $\mathrm{Si}\,\mathrm{IV}$ and ${\rm{C}}\,\mathrm{IV}$ to the observed levels. The flare's timescale of a few Myr suggests it is the same event that created the giant X-ray/γ-ray Fermi Bubbles at the Galactic Center. * Based on observations made with the NASA/ESA Hubble Space Telescope, obtained from the Data Archive at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555. These observations are associated with programs 11541, 11632, 11524, 11585, 11598, 11686, 12025, 12029, 12038, 12212, 12248, 12275, 12569, 12604, and 14687.

Keywords: Galactic center, High-velocity clouds, Galaxy evolution, Magellanic Stream, Circumgalactic medium, Milky Way evolution, Milky Way Galaxy
On the Shape of SEP Electron Spectra: The Role of Interplanetary Transport. We address the effect of particle scattering on the energy spectra of solar energetic electron events using (I) an observational and (II) a modeling approach. (I) We statistically study observations of the STEREO spacecraft, using directional electron measurements made with the Solar Electron and Proton Telescope in the range of 45-425 keV. We compare the energy spectra of the anti-Sunward propagating beam with that of the backward-scattered population and find that, on average, the backward-scattered population shows a harder spectrum with the effect being stronger at higher energies. (II) We use a numerical solar energetic particle (SEP) transport model to simulate the effect of particle scattering (both in terms of pitch angle and perpendicular to the mean field) on the spectrum. We find that pitch-angle scattering can lead to spectral changes at higher energies (E &gt; 100 keV) and further away from the Sun (r &gt; 1 au), which are also often observed. At lower energies, and closer to the Sun, the effect of pitch-angle scattering is much more reduced, so that the simulated energy spectra still resemble the injected power-law functions. When examining pitch-angle-dependent spectra, we find, in agreement with the observational results, that the spectra of the backward-propagating electrons are harder than that of the forward (from the Sun) propagating population. We conclude that Solar Orbiter and Parker Solar Probe will be able to observe the unmodulated omnidirectional SEP electron spectrum close to the Sun at higher energies, giving a direct indication of the accelerated spectrum.

Keywords: Interplanetary turbulence, The Sun, Solar energetic particles
Blazar Variability: A Study of Nonstationarity and the Flux-Rms Relation. We analyze X-ray light curves of the blazars Mrk 421, PKS 2155-304, and 3C 273 using observations by the Soft X-ray Telescope on board AstroSat and archival XMM-Newton data. We use light curves of length 30-90 ks from three to four epochs for all three blazars. We apply the autoregressive integrated moving average model, which indicates the variability is consistent with short memory processes for most of the epochs. We show that the power spectral density (PSDs) of the X-ray variabilities of the individual blazars are consistent within uncertainties across the epochs. This implies that the construction of broadband PSD using light curves from different epochs is accurate. However, using certain properties of the variance of the light curves and its segments, we show that the blazars exhibit hints of nonstationarity beyond that due to their characteristic red-noise nature in some of those observations. We find a linear relationship between the root-mean-squared amplitude of variability at shorter timescales and the mean flux level at longer timescales for light curves of Mrk 421 across epochs separated by decades as well as light curves spanning 5 days and ∼10 yr. The presence of a flux-rms relation over very different timescales may imply that, similar to the X-ray binaries and Seyfert galaxies, longer and shorter timescale variabilities are connected in blazars.

Keywords: X-ray astronomy, Blazars, X-ray active galactic nuclei
Radiation Hydrodynamics Models of Active Galactic Nuclei: Beyond the Central Parsec. We produce radiation hydrodynamics models of an active galactic nucleus torus plus outflow on 1-100 pc scales. This large scale permits direct comparison with observations, provides justification for configurations used in radiation transfer models, and tests the sensitivity of results of smaller scale dynamical models. We find that anisotropic radiation from an active galactic nucleus accretion disk can cause an outflow to evolve to become more polar, agreeing with the ubiquity of polar extended mid-infrared emission, and the general geometry predicted by radiative transfer models. We also find that the velocity maps can reproduce many features of observations, including apparent counterrotation.

Keywords: Hydrodynamical simulations, Radiative transfer simulations, Active galactic nuclei, Infrared galaxies, Submillimeter astronomy, Astrophysical fluid dynamics
X-Ray Quasi-periodic Oscillations in the Lense-Thirring Precession Model. II. Variability of the Relativistic Iron Kα Line. The reprocessing of primary X-ray emission in the accretion disk of black hole X-ray binaries (BHXRBs) produces a reflection spectrum with the characteristic Fe Kα fluorescence line. Strong low-frequency quasi-periodic oscillations (QPOs) are observed from BHXRBs, and the dependence of QPO properties (e.g., phase lag) on the inclination angle suggests that the observed QPO may be associated with a geometrical effect, e.g., the precession of the X-ray source due to frame dragging near the spinning black hole. Here, in the scenario of the Lense-Thirring precession of the X-ray source, we use a Monte Carlo simulation of radiative transfer to study the irradiation/reflection and the resultant spectral properties including the Fe Kα line as a function of precession phase (time). We found that the reflection fraction, I.e., the ratio of incident flux toward the disk and the direct flux toward the observer at infinity, is modulated by the precession phase, which depends on the truncation radius (I.e., the spectral state in the truncated disk model) and the inclination angle. The Fe Kα line profile also changes as the primary X-ray source precesses, with the line luminosity and the flux-weighted centroid energy varying with the precession phase. The periodically modulated 2-10 keV continuum flux could apparently lag the line luminosity in phase, if the truncation radius is small enough for Doppler effects due to disk orbital motion to significantly affect the observed radiation.

Keywords: Accretion, Black hole physics
Near- and Mid-infrared Observations in the Inner Tenth of a Parsec of the Galactic Center Detection of Proper Motion of a Filament Very Close to Sgr A*. We analyze the gas and dust emission in the immediate vicinity of the supermassive black hole Sgr A* at the Galactic center (GC) with the ESO Very Large Telescope (Paranal/Chile) instruments SINFONI and VISIR. The SINFONI H+K data cubes show several emission lines with related line map counterparts. From these lines, the Brγ emission is the most prominent one and appears to be shaped as a bar extending along the north-south direction. With VISIR, we find a dusty counterpart to this filamentary emission. In this work, we present evidence that this feature could most be likely connected to the minispiral and potentially influenced by the winds of the massive stars in the central cluster or an accretion wind from Sgr A*. To this end, we coadd the SINFONI data between 2005 and 2015. The spectroscopic analysis reveals a range of Doppler-shifted emission lines. We also detect substructures in the shape of clumps that can be investigated in the channel maps of the Brγ bar. In addition, we compare the detection of the near-infrared Brγ feature to PAH1 mid-infrared observations and published 226 GHz radio data. These clumps show a proper motion of about 320 km s-1 that are consistent with other infrared continuum-detected filaments in the GC. Deriving a mass of $2.5\times {10}^{-5}\,{M}_{\odot }$ for the investigated Brγ feature shows an agreement with former derived masses for similar objects. Besides the north-south Brγ bar, we find a comparable additional east-west feature. Also, we identify several gas reservoirs that are located west of Sgr A* that may harbor dusty objects.

Keywords: Interstellar dust, Galactic center, Interstellar atomic gas, Interstellar dust extinction, Interstellar clouds
Radial-velocity Variation of a Tertiary Star Orbiting a Binary Black Hole in Coplanar and Noncoplanar Triples: Short- and Long-term Anomalous Behavior. A number of ongoing surveys are likely to discover star-black hole binaries in our Galaxy in the near future. A fraction of them may be triple systems comprising an inner binary, instead of a single black hole, which might be progenitors of binary black holes (BBHs) routinely discovered now from the gravitational wave. We extend our previous proposal to locate inner BBHs from the short-term radial-velocity (RV) variation of a tertiary star in coplanar triples, and we consider noncoplanar triples and their long-term RV variations as well. Specifically, we assume coplanar and noncoplanar triples with an inner BBH of total mass 20 M⊙, whose outer and inner orbital periods are 80 days and 10 days, respectively. We perform a series of N-body simulations and compare the results with analytic approximate solutions based on quadrupole perturbation theory. For coplanar triples, the pericenter shift of the outer star can be used to detect the hidden inner BBH. For noncoplanar triples, the total RV semiamplitude of the outer star is modulated periodically on the order of 100 km s-1, due to its precession over roughly the Kozai-Lidov oscillation timescale. Such long-term modulations would be detectable within a decade, independent of the short-term RV variations on the order of 100 m s-1 at roughly twice the orbital frequency of the inner binary. Thus the RV monitoring of future star-black hole binary candidates offers a promising method for searching for their inner hidden BBHs in optical bands.

Keywords: Multiple stars, Black holes, Compact binary stars, Celestial mechanics, Three-body problem, Radial velocity
An Application of the Global ILC Algorithm over Large Angular Scales to Estimate the CMB Posterior Using Gibbs Sampling. In this work, we formalize a new technique to investigate the joint posterior density of the cosmic microwave background (CMB) signal and its theoretical angular power spectrum given the observed data, using the global internal-linear-combination method first proposed in a paper by Sudevan &amp; Saha in 2017. We implement the method on low-resolution CMB maps observed by the Wilkinson Microwave Anisotropy Probe (WMAP) and the Planck satellite missions, using Gibbs sampling, assuming that the detector noise is negligible on large angular scales of the sky. The main products of our analysis are a best-fit cleaned CMB map and its theoretical angular power spectrum, along with their error estimates. We validate the method by performing Monte Carlo simulations that include realistic foreground models and noise levels consistent with WMAP and Planck observations. Our method has a unique advantage: the posterior density is obtained without any need to explicitly model foreground components. Second, the power spectrum results with the error estimates can be directly used for cosmological parameter estimations.

Keywords: Diffuse radiation, Gibbs Sampler, Cosmic microwave background radiation, Observational cosmology
The Inner Disk of RY Tau: Evidence of Stellar Occultation by the Disk Atmosphere at the Sublimation Rim from K-band Continuum Interferometry. We present models of the inner region of the circumstellar disk of RY Tau that aim to explain our near-infrared (K-band: 2.1 μm) interferometric observations, while remaining consistent with the optical to near-infrared portions of the spectral energy distribution. Our submilliarcsecond-resolution CHARA Array observations are supplemented with shorter baseline, archival data from PTI, KI, and VLTI/GRAVITY and modeled using an axisymmetric Monte Carlo radiative transfer code. The K-band visibilities are well fit by models incorporating a central star illuminating a disk with an inner edge shaped by dust sublimation at 0.210 ± 0.005 au, assuming a viewing geometry adopted from millimeter interferometry (65° inclined with a disk major axis position angle of 23°). This sublimation radius is consistent with that expected of silicate grains with a maximum size of 0.36-0.40 μm contributing to the opacity, and is an order of magnitude further from the star than the theoretical magnetospheric truncation radius. The visibilities on the longest baselines probed by CHARA indicate that we lack a clear line of sight to the stellar photosphere. Instead, our analysis shows that the central star is occulted by the disk surface layers close to the sublimation rim. While we do not see direct evidence of temporal variability in our multiepoch CHARA observations, we suggest the aperiodic photometric variability of RY Tau is likely related temporal and/or azimuthal variations in the structure of the disk surface layers.

Keywords: Optical interferometry, Herbig Ae/Be stars, Infrared excess, T Tauri stars, Young stellar objects, Star formation, Radiative transfer, Circumstellar disks, Near infrared astronomy
Helium Enhancement in the Metal-rich Red Giants of ω Centauri. The helium-enriched (He-enriched) metal-rich red giants of ω Centauri, discovered by Hema &amp; Pandey using the low-resolution spectra from the Vainu Bappu Telescope, and confirmed by the analyses of the high-resolution spectra obtained from the HRS-South African Large Telescope for LEID 34225 and LEID 39048, are reanalyzed here to determine their degree of He enhancement/hydrogen deficiency (H deficiency). The observed MgH band combined with model atmospheres with differing He/H ratios are used for the analyses. The He/H ratios of these two giants are determined by enforcing the fact that the derived Mg abundances from the Mg I lines and from the subordinate lines of the MgH band must be same for the adopted model atmosphere. The estimated He/H ratios for LEID 34225 and LEID 39048 are 0.15 ± 0.04 and 0.20 ± 0.04, respectively, whereas the normal He/H ratio is 0.10. Following the same criteria for the analyses of the other two comparison stars (LEID 61067 and LEID 32169), a normal He/H ratio of 0.10 is obtained. The He/H ratio of 0.15-0.20 corresponds to a mass fraction of helium (Z(He) = Y) of about 0.375-0.445. The range of helium enhancement and the derived metallicity of the program stars are in line with those determined for ω Cen's blue main-sequence stars. Hence, our study provides the missing link for the evolutionary track of the metal-rich helium-enhanced population of ω Centuari. This research work is the very first spectroscopic determination of the amount of He enhancement in the metal-rich red giants of ω Centauri using the Mg I and MgH lines.

Keywords: Globular star clusters, Chemical abundances, Hydrogen deficient stars, Red giant stars, Stellar evolutionary tracks, Chemically peculiar stars
Detection of Small Flares from the Crab Nebula with Fermi-LAT. Gamma radiation from the Crab pulsar wind nebula (PWN) shows significant variability at ∼100 MeV energies, recently revealed with spaceborne gamma-ray telescopes. Here we report the results of a systematic search for gamma-ray flares using a 7.4 yr data set acquired with the Fermi Large Area Telescope. Analyzing the off-pulse phases of the Crab pulsar, we found seven previously unreported low-intensity flares ("small flares"). The "small flares" originate from the variable synchrotron component of the Crab PWN and show clearly different features from the steady component of the Crab PWN emission. They are characterized by larger fluxes and harder photon indices, similar to previously reported flares. These flares show day-scale time variability and imply a strong magnetic field of Bmin ≈ 1 mG at the site of the gamma-ray production. This result seems to be inconsistent with the typical values revealed with modeling of the nonthermal emission from the nebula. The detection of the "small flares" gives a hint of production of gamma-rays above 100 MeV in a part of the nebula with properties that are different from the main emitters, e.g., due to bulk relativistic motion.

Keywords: Supernova remnants, Magnetohydrodynamics, Gamma-ray sources, Pulsars
Delay-time Distributions of Solar Energetic Particles. II. Effects of Magnetic Focusing. We extend a recently published analytic model for the intensity-time profile of solar energetic particle (SEP) events, in which the dominant physical mechanism is turbulent pitch-angle scattering of a collimated distribution of particles accelerated at the Sun. The present model includes the effect of magnetic focusing in the expanding magnetic field geometry of the inner heliosphere. For a power-law variation of the magnetic field strength with distance (B ∼ s-α) that lacks a characteristic focusing length scale, the fundamental shape of the intensity-time profile (I.e., a Lévy distribution at times up to and just past the time of peak intensity, followed by an exponential decay) is preserved. The effect of magnetic focusing is essentially to produce a rescaling of the stochastic process describing the angular diffusion of the particles, making the typical time that characterizes the SEP time profile quantitatively lower by a factor of (α + 1), 3 for the radial field geometry B ∼ s-2.

Keywords: Solar activity, Solar flares, Solar energetic particles
2D and 3D Analysis of a Torus-unstable Quiet-Sun Prominence Eruption. The role of ideal-MHD instabilities in a prominence eruption is explored through 2D and 3D kinematic analysis of an event observed with the Solar Dynamics Observatory and the Solar Terrestrial Relations Observatory between 22:06 UT on 2013 February 26 and 04:06 UT on 2013 February 27. A series of 3D radial slits are used to extract height-time profiles ranging from the midpoint of the prominence leading edge to the southeastern footpoint. These height-time profiles are fit with a kinematic model combining linear and nonlinear rise phases, returning the nonlinear onset time (tnl) as a free parameter. A range (1.5-4.0) of temporal power indices (I.e., β in the nonlinear term ${(t-{t}_{\mathrm{nl}})}^{\beta }$ ) are considered to prevent prescribing any particular form of nonlinear kinematics. The decay index experienced by the leading edge is explored using a radial profile of the transverse magnetic field from a PFSS extrapolation above the prominence region. Critical decay indices are extracted for each slit at their own specific values of height at the nonlinear phase onset (h(tnl)) and filtered to focus on instances resulting from kinematic fits with ${\chi }_{\mathrm{red}}^{2}\lt 2$ (restricting β to 1.9-3.9). Based on this measure of the critical decay index along the prominence structure, we find strong evidence that the torus instability is the mechanism driving this prominence eruption. Defining any single decay index as being "critical" is not that critical because there is no single canonical or critical value of decay index through which all eruptions must succeed.

Keywords: Solar atmosphere, The Sun, Solar prominences, Solar magnetic fields
HD 62542: Probing the Bare, Dense Core of a Translucent Interstellar Cloud. We discuss the interstellar absorption from many atomic and molecular species seen in high-resolution Hubble Space Telescope/STIS UV and high signal-to-noise ratio optical spectra of the moderately reddened B3-5 V star HD 62542. This remarkable sight line exhibits both very steep far-UV extinction and a high fraction of hydrogen in molecular form, with strong absorption from CH, C2, CN, and CO, but weak absorption from CH+ and most of the commonly observed diffuse interstellar bands. Most of the material resides in a single narrow velocity component, offering a rare opportunity to probe the primarily molecular core of a single interstellar cloud with little associated diffuse atomic gas. Detailed analyses of the spectra indicate that (1) the molecular fraction in the main cloud is high (f(H2) ≳ 0.8); (2) the gas is fairly cold (Tk = 40-43 K; from the rotational excitation of H2 and C2); (3) the local hydrogen density nH ∼ 1500 cm-3 (from C2 excitation, fine-structure excitation of C0, and simple chemical models); (4) the unusually high excitation temperatures for 12CO and 13CO may be largely due to radiative excitation; (5) N(C+):N(CO):N(C) ∼ 100:10:1; (6) the depletions of many elements are more severe than those seen in any other sight line and the detailed pattern of depletions differs from those derived from larger samples of Galactic sight lines; and (7) the various neutral/first ion ratios do not yield consistent estimates for electron density, even when the effects of grain-assisted recombination and low-temperature dielectronic recombination are considered. * Based on observations made with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555. These observations are associated with program 12277. Based on observations made with ESO telescopes at the La Silla Paranal Observatory, under program IDs 064.I-0475, 072.C-0682, and 099.C-0637.

Keywords: Interstellar molecules, Interstellar line absorption, Interstellar abundances, Interstellar absorption, Interstellar clouds, Interstellar medium, Diffuse molecular clouds, Diffuse interstellar clouds