adsabs/SciX_UAT_keywords huggingface dataset:
Chandra Follow-up Observations of Swift-BAT-selected AGNs II

We present the combined Chandra and Swift-BAT spectral analysis of nine low-redshift (z ≤ 0.10), candidate heavily obscured active galactic nuclei (AGN) selected from the Swift-BAT 150 month catalog. We located soft (1-10 keV) X-ray counterparts to these BAT sources and joint fit their spectra with physically motivated models. The spectral analysis in the 1-150 keV energy band determined that all sources are obscured, with a line-of-sight column density N H ≥ 1022 cm-2 at a 90% confidence level. Four of these sources show significant obscuration with N H ≥ 1023 cm-2 and two additional sources are candidate Compton-thick Active Galactic Nuclei (CT-AGNs) with N H ≥ 1024 cm-2. These two sources, 2MASX J02051994-0233055 and IRAS 11058-1131, are the latest addition to the previous 3 CT-AGN candidates found using our strategy for soft X-ray follow-up of BAT sources. Here we present the results of our methodology so far, and analyze the effectiveness of applying different selection criteria to discover CT-AGN in the local Universe. Our selection criteria has an ~20% success rate of discovering heavily obscured AGN whose CT nature is confirmed by follow-up NuSTAR observations. This is much higher than the ~5% found in blind surveys.

Keywords: active galactic nuclei; high energy astrophysics; x-ray active galactic nuclei; seyfert galaxies
Filament Leg-Leg Reconnection as a Source of Prominent Supra-arcade Downflows

We report on the interaction of the legs of the erupting filament of 2012 August 31 and associated prominent supra-arcade downflows (P-SADs) as observed by the Atmospheric Imaging Assembly on board the Solar Dynamics Observatory. We employ a number of image processing techniques to enhance weak interacting features. As the filament erupts, both legs stretch outwards. The positive-polarity leg also untwists and splits into two parts. The first part runs into the conjugate (negative-polarity) leg, tearing it apart. The second part then converges into the remnant of the conjugate leg, after which both weaken and finally disappear. All these episodes of interaction of oppositely oriented filament legs are followed by the appearance of P-SADs, seen in the on-disk projection to be shaped as loop tops, along with many weaker SADs. All SADs are preceded by hot supra-arcade downflowing loops. This observed evolution is consistent with the three-dimensional rr-rf (leg-leg) reconnection, where the erupting flux rope reconnects with itself. In our observations, as well as in some models, the reconnection in this geometry is found to be long lasting. It plays a substantial role in the evolution of the flux rope of the erupting filament and leads to prominent SADs.

Keywords: solar flares; solar filament eruptions; solar magnetic reconnection; solar x-ray flares; solar ultraviolet emission
Classification of Fermi BCUs Using Machine Learning

The Fermi Large Area Telescope (LAT) has detected 6659 γ-ray sources in the incremental version (4FGL-DR3, for Data Release 3) of the fourth Fermi-LAT catalog of γ-ray sources and 3743 of them are blazars, including 1517 blazar candidates of uncertain type (BCUs). Blazars are generally classified by properties of emission lines into BL Lac objects and flat spectrum radio quasars (FSRQs). However, BCUs are difficult to classify because of the lack of spectrum. In this work we apply five different machine-learning algorithms (K-nearest neighbors, logistic regression, support vector machine, random forest, CatBoost) to evaluate the classification of 1517 BCUs based on the observational data of 4FGL-DR3. The results indicate that the use of recursive feature elimination cross-validation can effectively improve the accuracy of models and reduce computation time. We use our models to predict the BCUs from 4FGL-DR3 and the results of the overlapping of the five models are as follows: 811 BL Lac objects, 397 FSRQs, and 309 BCUs.

Keywords: blazars; bl lacertae objects; flat-spectrum radio quasars
SCUBA-2 Ultra Deep Imaging EAO Survey (STUDIES). IV. Spatial Clustering and Halo Masses of Submillimeter Galaxies

We analyze an extremely deep 450 μm image (1σ = 0.56 mJy beam-1) of a ≃300 arcmin2 area in the CANDELS/COSMOS field as part of the Sub-millimeter Common User Bolometric Array-2 Ultra Deep Imaging EAO Survey. We select a robust (signal-to-noise ratio ≥4) and flux-limited (≥4 mJy) sample of 164 submillimeter galaxies (SMGs) at 450 μm that have K-band counterparts in the COSMOS2015 catalog identified from radio or mid-infrared imaging. Utilizing this SMG sample and the 4705 K-band-selected non-SMGs that reside within the noise level ≤1 mJy beam-1 region of the 450 μm image as a training set, we develop a machine-learning classifier using K-band magnitude and color-color pairs based on the 13-band photometry available in this field. We apply the trained machine-learning classifier to the wider COSMOS field (1.6 deg2) using the same COSMOS2015 catalog and identify a sample of 6182 SMG candidates with similar colors. The number density, radio and/or mid-infrared detection rates, redshift and stellar-mass distributions, and the stacked 450 μm fluxes of these SMG candidates, from the S2COSMOS observations of the wide field, agree with the measurements made in the much smaller CANDELS field, supporting the effectiveness of the classifier. Using this SMG candidate sample, we measure the two-point autocorrelation functions from z = 3 down to z = 0.5. We find that the SMG candidates reside in halos with masses of ≃(2.0 ± 0.5) × 1013 h-1 M⊙ across this redshift range. We do not find evidence of downsizing that has been suggested by other recent observational studies.

Keywords: clustering; submillimeter astronomy; high-redshift galaxies; galaxy evolution; large-scale structure of the universe; galaxy formation
Understanding the Molecular Kinetics and Chemical Equilibrium Phase of Frozen CO during Bombardment by Cosmic Rays by Employing the PROCODA Code

Within the cold regions of space, ices that are enriched with carbon monoxide (CO) molecules are exposed to ionizing radiation, which triggers new reactions and desorption processes. Laboratory studies on astrochemical ices employing different projectiles have revealed the appearance of several new species. In this study, we employed the upgraded PROCODA code, which involves a calculation phase utilizing thermochemistry data, to map the chemical evolution of pure CO ice irradiated by cosmic-ray analogs. In the model, we have considered 18 different chemical species (six observed: CO, CO2, C3, O3, C2O, and C5O3; 12 unobserved: C, O, C2, O2, CO3, C3O, C4O, C5O, C2O2, C2O3, C3O2, and C4O2) coupled at 156 reaction routes. Our best-fit model provides effective reaction rates (effective rate constants, (ERCs)), branching ratios for reactions within reaction groups, several desorption parameters, and the characterization of molecular abundances at the chemical equilibrium (CE) phase. The most abundant species within the ice at the CE phase were atomic oxygen (68.2%) and atomic carbon (18.2%), followed by CO (11.8%) and CO2 (1.6%). The averaged modeled desorption yield and rate were 1.3e5 molecules ion-1 and 7.4e13 molecules s-1, respectively, while the average value of ERCs in the radiation-induced dissociation reactions was 2.4e-1 s-1 and for the bimolecular reactions it was 4.4e-24 cm3 molecule-1 s-1. We believe that the current kinetics study can be used in future astrochemical models to better understand the chemical evolution of embedded species within astrophysical ices under the presence of an ionizing radiation field.

Keywords: astrochemistry; laboratory astrophysics
Longitudinally Asymmetric Stratospheric Oscillation on a Tidally Locked Exoplanet

Using a three-dimensional general circulation model, we show that the atmospheric dynamics on a tidally locked Earth-like exoplanet, simulated with the planetary and orbital parameters of Proxima Centauri b, support a longitudinally asymmetric stratospheric wind oscillation (LASO), analogous to Earth's quasi-biennial oscillation (QBO). In our simulations, the LASO has a vertical extent of 35-55 km, a period of 5-6.5 months, and a peak-to-peak wind speed amplitude of -70 to +130 m s-1 with a maximum at an altitude of 41 km. Unlike the QBO, the LASO displays longitudinal asymmetries related to the asymmetric thermal forcing of the planet and to interactions with the resulting stationary Rossby waves. The equatorial gravity wave sources driving the LASO are localized in the deep convection region at the substellar point and in a jet exit region near the western terminator, unlike the QBO, for which these sources are distributed uniformly around the planet. Longitudinally, the western terminator experiences the highest wind speeds and undergoes reversals earlier than other longitudes. The antistellar point only experiences a weak oscillation with a very brief, low-speed westward phase. The QBO on Earth is associated with fluctuations in the abundances of water vapor and trace gases such as ozone, which are also likely to occur on exoplanets if these gases are present. Strong fluctuations in temperature and the abundances of atmospheric species at the terminators will need to be considered when interpreting atmospheric observations of tidally locked exoplanets.

Keywords: exoplanet atmospheres; exoplanet atmospheric variability; exoplanet dynamics; exoplanets; extrasolar rocky planets
Observations of Shock Propagation through Turbulent Plasma in the Solar Corona

Eruptive activity in the solar corona can often lead to the propagation of shock waves. In the radio domain the primary signature of such shocks are type II radio bursts, observed in dynamic spectra as bands of emission slowly drifting toward lower frequencies over time. These radio bursts can sometimes have an inhomogeneous and fragmented fine structure, but the cause of this fine structure is currently unclear. Here we observe a type II radio burst on 2019 March 20th using the New Extension in Nançay Upgrading LOFAR, a radio interferometer observing between 10-85 MHz. We show that the distribution of size scales of density perturbations associated with the type II fine structure follows a power law with a spectral index in the range of α = -1.7 to -2.0, which closely matches the value of -5/3 expected of fully developed turbulence. We determine this turbulence to be upstream of the shock, in background coronal plasma at a heliocentric distance of ~2 R⊙. The observed inertial size scales of the turbulent density inhomogeneities range from ~62 Mm to ~209 km. This shows that type II fine structure and fragmentation can be due to shock propagation through an inhomogeneous and turbulent coronal plasma, and we discuss the implications of this on electron acceleration in the coronal shock.

Keywords: solar physics; solar coronal mass ejections; solar coronal mass ejection shocks; solar particle emission
Gas-phase Hydrogen/Deuterium Exchange on Large, Astronomically Relevant Cationic PAHs

To examine the gas-phase hydrogen/deuterium exchange on large, astronomically relevant cationic polycyclic aromatic hydrocarbons (PAHs), the ion-molecule collision reaction between C42H18 + (hexa-peri-hexabenzocoronene cations, HBC+) and D atoms is studied. The experimental results show that the deuterated HBC cations ([C42H m D n ]+, m+2 ∗ n up to ~54) are efficiently formed, and an effective hydrogen/deuterium exchange is determined. The structure of newly formed deuterated HBC cations and the bonding energy for these reaction pathways are investigated with quantum theoretical calculations. The exothermic energy for each reaction pathway is relatively high, and the existence of competition between deuteration and dedeuteration and of hydrogen/deuterium exchange is confirmed. A kinetic model is constructed to simulate the deuteration and hydrogenation processes and the hydrogen/deuterium exchange on HBC+ as a function of the reaction time over the experimental and typical astrophysical conditions. We infer that if we do not consider other chemical evolution processes (e.g., photoevolution), then cationic PAHs will reach the final equilibrium state (reaction with H/D atoms) very quickly regardless of the initial state of PAHs, and deuterated cationic PAHs are scarce in the interstellar medium.

Keywords: astrochemistry; interstellar medium; polycyclic aromatic hydrocarbons; interstellar molecules
AstroSat UVIT Observations of Her X-1

An observation of the X-ray binary system Her X-1/HZ Her by the AstroSat UltraViolet Imaging Telescope (UVIT) was carried out in 2018. The observation was taken with the far ultra-violet (FUV) camera of UVIT with the CaF2 filter (125-175 nm band) and lasted ≃0.6 of one binary orbit. Her X-1 was in the late main high state at 35 day phase ≃0.20. Clear orbital modulation of the UVIT light curve is seen, showing egress from eclipse of the neutron star and double-peak shape half an orbital period later. The FUV emission from Her X-1 is shown to arise partly from the accretion disk and partly from the X-ray heated face of Hz Her. We carry out modeling of the system using the Shape code and fit the FUV light curve. New constraints are obtained for the geometric parameters of the tilted and twisted accretion disk around the neutron star, and improvements to the basic disk model are suggested.

Keywords: neutron stars; x-ray binary stars
An Empirical Bayesian Approach to Limb Darkening in Modeling WASP-121b Transit Light Curves

We present a novel, iterative method using an empirical Bayesian approach for modeling the limb-darkened WASP-121b transit from the TESS light curve. Our method is motivated by the need to improve Rp/R* estimates for exoplanet atmosphere modeling and is particularly effective with the limb-darkening (LD) quadratic law requiring no prior central value from stellar atmospheric models. With the nonlinear LD law, the method has all the advantages of not needing atmospheric models but does not converge. The iterative method gives a different Rp/R* for WASP-121b at a significance level of 1σ when compared with existing noniterative methods. To assess the origins and implications of this difference, we generate and analyze light curves with known values of the LD coefficients (LDCs). We find that noniterative modeling with LDC priors from stellar atmospheric models results in an inconsistent Rp/R* at a 1.5σ level when the known LDC values are the same as those previously found when modeling real data by the iterative method. In contrast, the LDC values from the iterative modeling yield the correct value of Rp/R* to within 0.25σ. For more general cases with different known inputs, Monte Carlo simulations show that the iterative method obtains unbiased LDCs and correct Rp/R* to within a significance level of 0.3σ. Biased LDC priors can cause biased LDC posteriors and lead to bias in the Rp/R* of up to 0.82%, 2.5σ for the quadratic law and 0.32%, 1.0σ for the nonlinear law. Our improvement in Rp/R* estimation is important when analyzing exoplanet atmospheres.

Keywords: exoplanet atmospheres; exoplanet detection methods; transit photometry; exoplanet systems
Photoionized Herbig-Haro Objects in the Orion Nebula through Deep High Spectral Resolution Spectroscopy. II. HH 204

We analyze the physical conditions, chemical composition, and other properties of the photoionized Herbig-Haro (HH) object HH 204 through Very Large Telescope echelle spectroscopy and Hubble Space Telescope imaging. We kinematically isolate the high-velocity emission of HH 204 from the emission of the background nebula and study the subarcsecond distribution of physical conditions and ionic abundances across the HH object. We find that low- and intermediate-ionization emission arises exclusively from gas at photoionization equilibrium temperatures, whereas the weak high-ionization emission from HH 204 shows a significant contribution from higher-temperature shock-excited gas. We separately derive the ionic abundances of HH 204, the emission of the Orion Nebula, and the fainter diffuse blue layer. In HH 204, the O+ abundance determined from collisional excited lines matches the one based on recombination lines, while the O2+ abundance is very low, so that the oxygen abundance discrepancy is zero. The ionic abundances of Ni and Fe in HH 204 have similar ionization and depletion patterns, with total abundances that are a factor of 3.5 higher than in the rest of the Orion Nebula due to dust destruction in the bow shock. We show that a failure to resolve the kinematic components in our spectra would lead to significant error in the determination of chemical abundances (for instance, a 40% underestimate of O), mainly due to incorrect estimation of the electron density.

Keywords: interstellar medium; chemical abundances; herbig-haro objects; h ii regions
Thermal and Nonthermal Emission from a Peculiar Long-duration GRB 211211A

Long-duration GRB 211211A that lacks a supernova emission even down to very stringent limits at such a low redshift z = 0.076 and is associated with kilonova emission, suggests that its physical origin is from a binary compact star merger. By reanalyzing its data observed with the Gamma-Ray Burst Monitor on board the Fermi mission, we find that both time-integrated and time-resolved spectra can be fitted well by using a 2SBPL plus blackbody (2SBPL+BB) model in the prompt emission. The bulk Lorentz factors (Γph) of the outflow can be inferred by invoking the observed thermal emission at the photosphere radius within a pure fireball model, and we find out that the temporal evolution of Γph seems to be tracking with the light curve. The derived values of Γph are also consistent with the Γph-L γ,iso/E γ,iso correlations that had been found in other bursts. Moreover, we also calculate the magnetization factor σ 0 in the central engine and σ ph at the photosphere radius within the framework of a hybrid jet model, and find that the values of both 1 + σ 0 and 1 + σ ph are larger than 1 for different time slices. It suggests that at least the Poynting-flux component is indeed existent in the outflow. If this is the case, one possible physical interpretation of thermal and nonthermal emissions in GRB 211211A is from the contributions of both $\nu \bar{\nu }$ annihilation and the Blandford-Znajek mechanisms in the relativistic jet when a stellar mass black hole resides in the central engine.

Keywords: gamma-ray bursts
High-redshift Galaxies from Early JWST Observations: Constraints on Dark Energy Models

Early observations with JWST have led to the discovery of an unexpectedly large density (stellar-mass density ρ * ≈ 106 M ⊙ Mpc-3) of massive galaxies (stellar masses M * ≥ 1010.5 M ⊙) at extremely high redshifts z ≈ 10. While such a result is based on early measurements that are still affected by uncertainties currently under consideration by several observational groups, its confirmation would have a strong impact on cosmology. Here we show that-under the most conservative assumptions and independently of the baryon physics involved in galaxy formation-such galaxy abundance is not only in tension with the standard ΛCDM cosmology but provides extremely tight constraints on the expansion history of the universe and on the growth factors corresponding to a wide class of Dynamical Dark Energy (DDE) models. Adopting a parameterization w = w 0 + w a (1 - a) for the evolution of the DDE equation of the state parameter w with the expansion factor a, we derive constraints on combinations of (w 0, w a ) that rule out with confidence level >2σ a major portion of the parameter space (w 0, w a ) allowed (or even favored) by existing cosmological probes.

Keywords: cosmological parameters; high-redshift galaxies; galaxy evolution
The Spin-period History of Intermediate Polars

We report the detailed history of spin-period changes in five intermediate polars (DQ Herculis, AO Piscium, FO Aquarii, V1223 Sagittarii, and BG Canis Minoris) during the 30-60 yr since their original discovery. Most are slowly spinning up, although there are sometimes years-long episodes of spin-down. This is supportive of the idea that the underlying magnetic white dwarfs are near spin equilibrium. In addition to the ∼40 stars sharing many properties and defined by their strong, pulsed X-ray emission, there are a few rotating much faster (P < 80 s), whose membership in the class is still in doubt—and who are overdue for closer study.

Keywords: close binary stars; stellar accretion disks; cataclysmic variable stars; dq herculis stars; classical novae; stellar accretion; interacting binary stars; novae
Characteristics and Importance of "ICME-in-sheath" Phenomenon and Upper Limit for Geomagnetic Storm Activity

As an important source for large geomagnetic storms, an "ICME-in-sheath" is a completely shocked interplanetary coronal mass ejection (ICME) stuck in the sheath between a shock and host ejecta. Typical characteristics are identified from coordinated multi-sets of observations: (1) it is usually short in duration and lasts a few hours at 1 au; (2) its solar wind parameters, in particular the magnetic field, seem to keep enhanced for a large range of distances; and (3) common ICME signatures are often lost. The host ejecta could be a single ICME or a complex ejecta, being fast enough to drive a shock. These results clarify previous misinterpretations of this phenomenon as a normal part of a sheath region. The "ICME-in-sheath" phenomenon, together with a preconditioning effect, produced an extreme set of the magnetic field, speed, and density near 1 au in the 2012 July 23 case, all around their upper limits at the same time. This is probably the most extreme solar wind driving at 1 au and enables us to estimate the plausible upper limit for geomagnetic storm activity. With an appropriate modification in the southward field, we suggest that a geomagnetic storm with a minimum Dst of about -2000 nT could occur in principle. The magnetopause would be compressed to about 3.3 Earth radii from the Earth's center, well inside the geosynchronous orbit.

Keywords: solar-terrestrial interactions; solar coronal mass ejections; solar wind; shocks
Do the Periodic Activities of Repeating Fast Radio Bursts Represent the Spins of Neutron Stars?

Fast radio bursts (FRBs) are mysterious radio transients with millisecond durations. Recently, ~16 days of periodic activity and ~159 days of possible periodicity were detected to arise from FRB 180916.J0158+65 and FRB 121102, respectively, and the spin period of a slow-rotation magnetar was further considered to be one of the possible explanations of the periodic activities of repeating FRBs. For isolated neutron stars, the spin evolution suggests that it has difficulty reaching several hours. In this work, we mainly focus on the possible maximum spin period of isolated NSs/magnetars dominated by an interaction between a star's magnetic field and the disk. We find that the disk wind plays an important role in spin evolution, whose influence varies the power-law index in the evolution equation of mass flow rate. For a magnetar without disk wind, the longest spin period is tens of hours. When the disk wind with a classical parameter is involved, the maximum spin period can reach hundreds of hours. But for an extremely large index of mass flow rate due to disk wind or other angular momentum extraction processes, a spin period of ~(16-160) days is still possible.

Keywords: accretion; stellar accretion disks; magnetars; radio bursts; stellar rotation