Sazuna/UAT_keywords huggingface dataset:
Breaks in the X-Ray Spectra of High-redshift Blazars and the Intervening Medium. Flat-spectrum radio quasars (FSRQs) are a subclass of blazars characterized by prominent optical emission lines and a collimated large-scale jet along the observer's line of sight. An X-ray spectral flattening has been reported in FSRQs (at relatively high redshifts) that is attributable either to absorption from gas along the line of sight or to intrinsic jet-based radiative processes. We study a sample of 16 high-redshift FSRQs (z of 1.1-4.7; rest-frame energy up to 50 keV) observed with the XMM-Newton and Swift satellites spanning 29 epochs. The X-ray spectra are fit with a power law including free excess absorption and one multiplied by an exponential roll-off to account for the intrinsic jet-based processes. A statistical analysis is used to distinguish between these models to understand the origin of the spectral flattening. The model selection is unable to distinguish between them in 10 of the 16 FSRQs. Intrinsic jet-based radiative processes are indicated in 4 FSRQs, where we infer energy breaks consistent with their expectation from the external Compton-scattering of low-energy ambient photons. Two of the FSRQs indicate mixed results, supportive of either scenario, illustrating the difficulty of identifying X-ray absorption signatures. A clear detection can be employed to disentangle the relative contributions from the intergalactic medium and the intracluster medium, the method of which is outlined and applied to the latter two sources.

Keywords: Jets, High energy astrophysics, Blazars, Flat-spectrum radio quasars, Active galactic nuclei, Warm-hot intergalactic medium, Intracluster medium
The X-Ray Properties of Eta Carinae During Its 2020 X-Ray Minimum. The massive binary system Eta Carinae is characterized by intense colliding winds that form shocks and emit X-rays. The system is highly eccentric (e ≃ 0.9), resulting in modulated X-ray emission during its 5.54 yr orbit. The X-ray flux increases in the months prior to periastron passage, exhibiting strong flares, then rapidly declines to a flat minimum lasting a few weeks, followed by a gradual recovery. We present the Neutron Star Interior Composition Explorer telescope spectra obtained before, during, and after the 2020 X-ray minimum, and perform spectral analysis to establish the temporal behavior of the X-ray flux and X-ray-absorbing column density (NH(t)) for the 2-10 keV and 5-10 keV energy ranges. The latter range is dominated by the stellar wind-collision region and, therefore, these spectral parameters-in particular, NH(t)-serve as potentially stringent constraints on the binary orientation. We compare the observed NH(t) results to the behavior predicted by a simple geometrical model in an attempt to ascertain which star is closer to us at periastron: the more massive primary (ω ≃ 240°-270°) or the secondary (ω ≃ 90°). We find that the variations in column density, both far from periastron and around periastron passage, support the latter configuration (ω ≃ 90°). The 2020 X-ray minimum showed the fastest recovery among the last five minima, providing additional evidence for a recent weakening of the primary star's wind.

Keywords: Massive stars, Binary stars, X-ray stars, Stellar winds
Using Chandra Localizations and Gaia Distances and Proper Motions to Classify Hard X-Ray Sources Discovered by INTEGRAL. Here, we report on X-ray observations of ten 17-60 keV sources discovered by the International Gamma-Ray Astrophysics Laboratory satellite. The primary new information is sub-arcsecond positions obtained by the Chandra X-ray Observatory. In six cases (IGR J17040-4305, IGR J18017-3542, IGR J18112-2641, IGR J18434-0508, IGR J19504+3318, and IGR J20084+3221), a unique Chandra counterpart is identified with a high degree of certainty, and for five of these sources (all but J19504), Gaia distances or proper motions indicate that they are Galactic sources. For four of these, the most likely classifications are that the sources are magnetic cataclysmic variables (CVs). J20084 could be either a magnetic CV or a high-mass X-ray binary. We classify the sixth source (J19504) as a likely active galactic nucleus (AGN). In addition, we find likely Chandra counterparts to IGR J18010-3045 and IGR J19577+3339, and the latter is a bright radio source and probable AGN. The other two sources, IGR J12529-6351 and IGR J18013-3222, do not have likely Chandra counterparts, indicating that they are transient, highly variable, or highly absorbed.

Keywords: Surveys, Astrometry, High energy astrophysics
Expanded Atmospheres and Winds in Type I X-Ray Bursts from Accreting Neutron Stars. We calculate steady-state models of radiation-driven super-Eddington winds and static expanded envelopes of neutron stars caused by high luminosities in type I X-ray bursts. We use flux-limited diffusion to model the transition from optically thick to optically thin, and include effects of general relativity, allowing us to study the photospheric radius close to the star as the hydrostatic atmosphere evolves into a wind. We find that the photospheric radius evolves monotonically from static envelopes (rph ≲ 50-70 km) to winds (rph ≈ 100-1000 km). Photospheric radii of less than 100 km, as observed in most photospheric radius expansion bursts, can be explained by static envelopes, but only in a narrow range of luminosity. In most bursts, we would expect the luminosity to increase further, leading to a wind with photospheric radius ≳100 km. In the contraction phase, the expanded envelope solutions show that the photosphere is still ≈1 km above the surface when the effective temperature is only 3% away from its maximum value. This is a possible systematic uncertainty when interpreting the measured Eddington fluxes from bursts at touchdown. We also discuss the applicability of steady-state models to describe the dynamics of bursts. In particular, we show that the sub- to super-Eddington transition during the burst rise is rapid enough that static models are not appropriate. Finally, we analyze the strength of spectral shifts in our models. Expected shifts at the photosphere are dominated by gravitational redshift, and are therefore predicted to be less than a few percent.

Keywords: Neutron stars, Stellar winds, X-ray bursts
SN 2019yvq Does Not Conform to SN Ia Explosion Models. We present new photometric and spectroscopic observations of SN 2019yvq, a Type Ia supernova (SN Ia) exhibiting several peculiar properties including an excess of UV/optical flux within days of explosion, a high Si II velocity, and a low peak luminosity. Photometry near the time of first light places new constraints on the rapid rise of the UV/optical flux excess. A near-infrared spectrum at +173 days after maximum light places strict limits on the presence of H or He emission, effectively excluding the presence of a nearby nondegenerate star at the time of explosion. New optical spectra, acquired at +128 and +150 days after maximum light, confirm the presence of [Ca II] λ7300 and persistent Ca II NIR triplet emission as SN 2019yvq transitions into the nebular phase. The lack of [O I] $\lambda 6300$ emission disfavors the violent merger of two C/O white dwarfs (WDs) but the merger of a C/O WD with a He WD cannot be excluded. We compare our findings with several models in the literature postulated to explain the early flux excess including double-detonation explosions, 56Ni mixing into the outer ejecta during ignition, and interaction with H- and He-deficient circumstellar material. Each model may be able to explain both the early flux excess and the nebular [Ca II] emission, but none of the models can reconcile the high photospheric velocities with the low peak luminosity without introducing new discrepancies.

Keywords: Astrophysical explosive burning, White dwarf stars, Nuclear astrophysics, Type Ia supernovae
The Relative Importance of Faraday Rotation and QED Birefringence for the Linear Polarization of X-Rays from Mass-accreting Black Holes. The upcoming Imaging X-ray Polarimetry Explorer (IXPE; 2-8 keV) and XL-Calibur (15-75 keV) missions will make it possible to measure the linear polarization of X-rays from mass-accreting stellar mass black holes with unprecedented sensitivity, enabling the accurate measurement of percent-level and in some cases even sub-percent-level polarization fractions. The measurements are expected to constrain the spins, inclinations, and the structure of the accretion flows of the observed black holes. The effects of Faraday rotation and birefringence of the quantum electrodynamics vacuum may impact the observable polarization fractions and angles, complicating the interpretation of the results. We estimate the importance of both effects for X-rays from stellar mass and supermassive black holes and discuss the implications of the results for the upcoming IXPE and XL-Calibur observations.

Keywords: Black hole physics, Supermassive black holes, High energy astrophysics, Stellar mass black holes, Particle astrophysics
Gyroresonance and Free-Free Radio Emissions from Multithermal Multicomponent Plasma. The solar atmosphere contains thermal plasma at a wide range of temperatures. This plasma is often quantified, in both observations and models, by a differential emission measure (DEM). The DEM is a distribution of the thermal electron density squared over temperature. In observations, the DEM is computed along a line of sight, while in the modeling it is over an elementary volume element (voxel). This description of the multithermal plasma is convenient and widely used in the analysis and modeling of extreme ultraviolet emission, which has an optically thin character. However, there is no corresponding treatment in the radio domain, where the optical depth of emission can be large, more than one emission mechanism is involved, and plasma effects are important. Here, we extend the theory of thermal gyroresonance and free-free radio emissions in the classical single-temperature Maxwellian plasma to the case of a multitemperature plasma. The free-free component is computed using the DEM and temperature-dependent ionization states of coronal ions, contributions from collisions of electrons with neutral atoms, the exact Gaunt factor, and the magnetic field effect. For the gyroresonant component, another measure of the multitemperature plasma is used, which describes the distribution of the thermal electron density over temperature. We give representative examples demonstrating important changes in the emission intensity and polarization due to the effects considered. The theory is implemented in available computer code.

Keywords: Solar coronal radio emission, Quiet sun, Active sun, Solar abundances, Solar radio emission, Solar magnetic fields
Robust Assessment of Clustering Methods for Fast Radio Transient Candidates. Fast radio transient search algorithms identify signals of interest by iterating and applying a threshold on a set of matched filters. These filters are defined by properties of the transient such as time and dispersion. A real transient can trigger hundreds of search trials, each of which has to be post-processed for visualization and classification tasks. In this paper, we have explored a range of unsupervised clustering algorithms to cluster these redundant candidate detections. We demonstrate this for REALFAST, the commensal fast-transient search system at the Karl G. Jansky Very Large Array. We use four features for clustering: sky position (l, m), time, and dispersion measure (DM). We develop a custom performance metric that makes sure that the candidates are clustered into a small number of pure clusters, i.e., clusters with either astrophysical or noise candidates. We then use this performance metric to compare eight different clustering algorithms. We show that using sky location along with DM/time improves clustering performance by ~10% as compared to the traditional DM/time-based clustering. Therefore, positional information should be used during clustering if it can be made available. We conduct several tests to compare the performance and generalizability of clustering algorithms to other transient data sets and propose a strategy that can be used to choose an algorithm. Our performance metric and clustering strategy can be easily extended to different single-pulse search pipelines and other astronomy and non-astronomy-based applications.

Keywords: Very Large Array, Extragalactic radio sources, Radio bursts, Random Forests, Clustering, Radio interferometry, Radio transient sources
Comparing the Inner and Outer Star-forming Complexes in the Nearby Spiral Galaxies NGC 628, NGC 5457, and NGC 6946 Using UVIT Observations. We present a far-UV (FUV) study of the star-forming complexes (SFCs) in three nearby galaxies using the Ultraviolet Imaging Telescope. The galaxies are close to face-on and show significant outer disk star formation. Two of them are isolated (NGC 628 and NGC 6946), and one is interacting with distant companions (NGC 5457). We compared the properties of the SFCs inside and outside the optical radius (R25). We estimated the sizes, star formation rates (SFRs), metallicities, and Toomre Q parameter of the SFCs. We find that the outer disk SFCs are at least 10 times smaller in area than those in the inner disk. The SFRs per unit area (ΣSFR) in both regions have similar mean values, but the outer SFCs have a much smaller range of ΣSFR. They are also metal-poor compared to the inner disk SFCs. The FUV emission is well correlated with the neutral hydrogen gas (H I) distribution and detected within and near several H I holes. Our estimation of the Q parameter in the outer disks of the two isolated galaxies suggests that their outer disks are stable (Q &gt; 1). However, their FUV images indicate that there is ongoing star formation in these regions. This suggests that there may be some nonluminous mass or dark matter in their outer disks, which increases the disk surface density and supports the formation of local gravitational instabilities. In the interacting galaxy, NGC 5457, the baryonic surface density is sufficient (Q &lt; 1) to trigger local disk instabilities in the outer disk.

Keywords: Spiral galaxies, Star forming regions, Galaxy interactions, Ultraviolet astronomy, HI shells
The Jet-disk Boundary Layer in Black Hole Accretion. Magnetic fields lines are trapped in black hole event horizons by accreting plasma. If the trapped field lines are lightly loaded with plasma, then their motion is controlled by their footpoints on the horizon and thus by the spin of the black hole. In this paper, we investigate the boundary layer between lightly loaded polar field lines and a dense, equatorial accretion flow. We present an analytic model for aligned prograde and retrograde accretion systems and argue that there is significant shear across this jet-disk boundary at most radii for all black hole spins. Specializing to retrograde aligned accretion, where the model predicts the strongest shear, we show numerically that the jet-disk boundary is unstable. The resulting mixing layer episodically loads plasma onto trapped field lines where it is heated, forced to rotate with the hole, and permitted to escape outward into the jet. In one case we follow the mass loading in detail using Lagrangian tracer particles and find a time-averaged mass-loading rate $\sim 0.01\,\dot{M}$ .

Keywords: Accretion, Magnetohydrodynamics, Plasma astrophysics
On a Possible Solution to the Tidal Realignment Problem for Hot Jupiters. Hot stars with hot Jupiters have a wide range of obliquities, while cool stars with hot Jupiters tend to have low obliquities. An enticing explanation for this pattern is tidal realignment of the cool host stars, although this explanation assumes that obliquity damping occurs faster than orbital decay, an assumption that needs further exploration. Here we revisit this tidal realignment problem, building on previous work identifying a low-frequency component of the time-variable tidal potential that affects the obliquity but not the orbital separation. We adopt a recent empirically based model for the stellar tidal quality factor and its sharp increase with forcing frequency. This leads to enhanced dissipation at low frequencies, and efficient obliquity damping. We model the tidal evolution of 46 observed hot Jupiters orbiting cool stars. A key parameter is the stellar age, which we determine in a homogeneous manner for the sample, taking advantage of Gaia DR2 data. We explore a variety of tidal histories and futures for each system, finding in most cases that the stellar obliquity is successfully damped before the planet is destroyed. A testable prediction of our model is that hot Jupiter hosts with orbital periods shorter than 2-3 days should have obliquities much smaller than 1°. With the possible exception of WASP-19b, the predicted future lifetimes of the planets range from 108 yr to more than 1010 yr. Thus, our model implies that these hot Jupiters are probably not in immediate danger of being devoured by their host stars while they are on the main sequence.

Keywords: Exoplanet dynamics, Star-planet interactions, Exoplanet tides, Hot Jupiters
Strong Near-infrared Carbon Absorption in the Transitional Type Ia SN 2015bp. Unburned carbon is potentially a powerful probe of Type Ia supernova (SN) explosion mechanisms. We present comprehensive optical and near-infrared (NIR) data on the "transitional" Type Ia SN 2015bp. An early NIR spectrum ( $t=-9.9$ days with respect to B-band maximum) displays a striking C I 1.0693 μm line at 11.9 × 103 km s-1, distinct from the prominent Mg II 1.0927 μm feature, which weakens toward maximum light. SN 2015bp also displays a clear C II 6580 Å notch early ( $t=-10.9$ days) at 13.2 × 103 km s-1, consistent with our NIR carbon detection. At ${M}_{B}=-18.46$ , SN 2015bp is less luminous than a normal SN Ia and, along with iPTF 13ebh, is the second member of the transitional subclass to display prominent early-time NIR carbon absorption. We find it unlikely that the C I feature is misidentified He I 1.0830 μm because this feature grows weaker toward maximum light, while the helium line produced in some double-detonation models grows stronger at these times. Intrigued by these strong NIR carbon detections, but lacking NIR data for other SNe Ia, we investigated the incidence of optical carbon in the sample of nine transitional SNe Ia with early-time data (t ≲ -4 days). We find that four display C II 6580 Å, while two others show tentative detections, in line with the SN Ia population as a whole. We conclude that at least ~50% of transitional SNe Ia in our sample do not come from sub-Chandrasekhar-mass explosions due to the clear presence of carbon in their NIR and optical spectra. * This paper includes data gathered with the 6.5 m Magellan Telescope at Las Campanas Observatory, Chile.

Keywords: Observational astronomy, White dwarf stars, Type Ia supernovae, Supernovae
Chandra Observations of the Planck Early Sunyaev-Zeldovich Sample: A Reexamination of Masses and Mass Proxies. Using Chandra observations, we derive the YX proxy and associated total mass measurement, ${M}_{500}^{{Y}_{{\rm{X}}}}$ , for 147 clusters with z &lt; 0.35 from the Planck early Sunyaev-Zeldovich catalog, and for 80 clusters with z &lt; 0.22 from an X-ray flux-limited sample. We reextract the Planck YSZ measurements and obtain the corresponding mass proxy, ${M}_{500}^{\mathrm{SZ}}$ , from the full Planck mission maps, minimizing Malmquist bias due to observational scatter. The masses reextracted using the more precise X-ray position and characteristic size agree with the published PSZ2 values, but yield a significant reduction in the scatter (by a factor of two) in the ${M}_{500}^{\mathrm{SZ}}$ - ${M}_{500}^{{Y}_{{\rm{X}}}}$ relation. The slope is 0.93 ± 0.03, and the median ratio, ${M}_{500}^{\mathrm{SZ}}/{M}_{500}^{{Y}_{{\rm{X}}}}=0.91\pm 0.01$ , is within the expectations from known X-ray calibration systematics. YSZ/YX is 0.88 ± 0.02, in good agreement with predictions from cluster structure, and implying a low level of clumpiness. In agreement with the findings of the Planck Collaboration, the slope of the YSZ- ${D}_{{\rm{A}}}^{-2}{Y}_{{\rm{X}}}$ flux relation is significantly less than unity (0.89 ± 0.01). Using extensive simulations, we show that this result is not due to selection effects, intrinsic scatter, or covariance between quantities. We demonstrate analytically that changing the YSZ-YX relation from apparent flux to intrinsic properties results in a best-fit slope that is closer to unity and increases the dispersion about the relation. The redistribution resulting from this transformation implies that the best-fit parameters of the ${M}_{500}^{\mathrm{SZ}}$ - ${M}_{500}^{{Y}_{{\rm{X}}}}$ relation will be sample-dependent.

Keywords: Galaxy clusters, Intracluster medium, Sunyaev-Zeldovich effect
The Transition from Diffuse Molecular Gas to Molecular Cloud Material in Taurus. We study four lines of sight that probe the transition from diffuse molecular gas to molecular cloud material in Taurus. Measurements of atomic and molecular absorption are used to infer the distribution of species and the physical conditions toward stars behind the Taurus Molecular Cloud (TMC). New high-resolution spectra at visible and near-IR wavelengths of interstellar Ca II, Ca I, K I, CH, CH+, C2, CN, and CO toward HD 28975 and HD 29647 are combined with data at visible wavelengths and published CO results from ultraviolet measurements for HD 27778 and HD 30122. Gas densities and temperatures are inferred from C2, CN, and CO excitation and CN chemistry. Our results for HD 29647 are noteworthy because the CO column density is 1018 cm-2 while C2 and CO excitation reveals a temperature of 10 K and a density of ~1000 cm-3, more like conditions found in dark molecular clouds. Similar results arise from our chemical analysis for CN through reactions involving observations of CH, C2, and NH. Enhanced potassium depletion and a reduced CH/H2 column density ratio also suggest the presence of a dark cloud. The directions toward HD 27778 and HD 30122 probe molecule-rich diffuse clouds, which can be considered CO-dark gas, while the sight line toward HD 28975 represents an intermediate case. Maps of dust temperature help refine the description of the material along the four sight lines and provide an estimate of the distance between HD 29647 and a clump in the TMC. An appendix provides results for the direction toward HD 26571; this star also probes diffuse molecular gas.

Keywords: Interstellar dust, Interstellar molecules, Interstellar line absorption, Chemical abundances, Astrochemistry, Interstellar clouds