adsabs/SciX_UAT_keywords huggingface dataset:
Testing Physical Scenarios for the Reflection Features of Type-1 AGNs using XMM-Newton and NuSTAR Simultaneous Observations

Above ~3 keV, the X-ray spectrum of active galactic nuclei (AGNs) is characterized by the intrinsic continuum and Compton reflection features. For type-1 AGNs, several regions may contribute to the reflection. To investigate the nature of the reflecting medium, we perform a systematic analysis of the reflector using XMM-Newton and Nuclear Spectroscopic Telescope Array observations of a sample of 22 type-1 AGNs. We create a baseline model that includes Galactic absorption and an intrinsically absorbed power law, plus a reflection model. We test a set of nine reflection models in a subsample of five objects. Based on these results, we select three models to be tested on the entire sample, accounting for distinct physical scenarios: neutral/distant reflection, ionized/relativistic reflection, and neutral/distant+ionized/relativistic reflection, namely, a hybrid model. We find that 18 sources require the reflection component to fit their spectra. Among them, 67% prefer the hybrid model. Neutral and ionized models are equally preferred by three sources. We conclude that both the neutral/distant reflector most probably associated with the inner edges of the torus and the ionized/relativistic reflector associated with the accretion disk are required to describe the reflection in type-1 AGNs.

Keywords: x-ray active galactic nuclei
The Temporal and Spatial Behaviors of CME Occurrence Rate at Different Latitudes

The statistical study of the coronal mass ejections (CMEs) is a hot topic in solar physics. To further reveal the temporal and spatial behaviors of the CMEs at different latitudes and heights, we analyzed the correlation and phase relationships between the occurrence rate of CMEs, the coronal brightness index (CBI), and the 10.7 cm solar radio flux (F10.7). We found that the occurrence rate of the CMEs correlates with the CBI relatively stronger at high latitudes (≥60°) than at low latitudes (≤50°). At low latitudes, the occurrence rate of the CMEs correlates relatively weaker with the CBI than the F10.7. There is a relatively stronger correlation relationship between CMEs, the F10.7, and the CBI during Solar Cycle 24 (SC24) than Solar Cycle 23 (SC23). During SC23, the high-latitude CME occurrence rate lags behind the F10.7 by 3 months, and during SC24, the low-latitude CME occurrence rate leads the low-latitude CBI by 1 month. The correlation coefficient values turn out to be larger when the very faint CMEs are removed from the samples of the CDAW catalog. Based on our results, we may speculate that the source regions of the high/low-latitude CMEs may vary in height, and the process of magnetic energy accumulation and dissipation is from the lower to the upper atmosphere of the Sun. The temporal offsets between different indicators could help us better understand the physical processes responsible for the solar-terrestrial interactions.

Keywords: solar coronal mass ejections; solar activity; magnetic fields
Unique Chemical Composition of the Very Metal-poor Star LAMOST J1645+4357

We report on the chemical composition of the very metal-poor (VMP; [Fe/H] = -2.9) star LAMOST J1645+4357, which was identified as a red giant having peculiar abundance ratios by Li et al. The standard abundance analysis is carried out for this object and the well-studied metal-poor star HD 122563, which has similar atmospheric parameters. LAMOST J1645+4357 has a remarkable abundance set, highlighted by these features: (1) nitrogen is significantly enhanced ([N/Fe] = +1.4), and the total abundance of C and N is also very high ([(C+N)/Fe] = +0.9); (2) α-elements are overabundant with respect to iron, as generally found in VMP stars; (3) Ti, Sc, Co, and Zn are significantly deficient; (4) Cr and Mn are enhanced compared to most of VMP stars; and (5) Sr and Ba are deficient, and the Sr/Ba ratio ([Sr/Ba] = -1.0) is significantly lower than the value expected for the r-process. The overall abundance pattern of this object from C to Zn is well reproduced by a faint supernova model assuming spherical explosion, except for the excess of Cr and Mn, which requires enhancement of incomplete Si burning or small contributions from a Type Ia or pair-instability supernova. There remains, however, a question as to why the abundance pattern of this star is so unique among VMP stars.

Keywords: stellar abundances; nucleosynthesis; population ii stars
Controls of Atmospheric Methane on Early Earth and Inhabited Earth-like Terrestrial Exoplanets

Methane (CH4) is a primarily biogenic greenhouse gas. As such, it represents an essential biosignature to search for life on exoplanets. Atmospheric CH4 abundance on Earth-like inhabited exoplanets is likely controlled by marine biogenic production and atmospheric photochemical consumption. Such interactions have been previously examined for the case of the early Earth where primitive marine ecosystems supplied CH4 to the atmosphere, showing that the atmospheric CH4 response to biogenic CH4 flux variations is nonlinear, a critical property when assessing CH4 reliability as a biosignature. However, the contributions of atmospheric photochemistry, metabolic reactions, or solar irradiance to this nonlinear response are not well understood. Using an atmospheric photochemical model and a marine microbial ecosystem model, we show that the production of hydroxyl radicals from water vapor photodissociation is a critical factor controlling the atmospheric CH4 abundance. Consequently, atmospheric CH4 partial pressure (pCH4) on inhabited Earth-like exoplanets orbiting Sun-like stars (F-, G-, and K-type stars) would be controlled primarily by stellar irradiance. Specifically, irradiance at wavelengths of approximately 200–210 nm is a major controlling factor for atmospheric pCH4 when the carbon dioxide partial pressure is sufficiently high to absorb most stellar irradiance at 170–200 nm. Finally, we also demonstrated that inhabited exoplanets orbiting near the outer edge of K-type stars' habitable zones are better suited for atmospheric pCH4 buildup. Such properties will provide valuable support for future detection of life signatures.

Keywords: exoplanet atmospheric composition; earth atmosphere; atmospheric composition; astrobiology
An Anomalous Cosmic-Ray Mediated Termination Shock: Implications for Energetic Neutral Atoms

The Voyager 2 crossing of the termination shock indicated that most of the upstream energy from the thermal solar wind ions was transferred to pickup ions (PUIs) and other energetic particles downstream of the shock. We use hybrid simulations at the termination shock for the Voyager 2, flank, and tail directions to evaluate the distributions of different ion species downstream of the shock over the energy range of 0.52-55 keV. Here, we extend the work of Gkioulidou et al., which showed an energy-dependent discrepancy between modeled and energetic neutral atom (ENA) observations, and fit distributions to a hybrid model to show that a population of PUIs accelerated via diffusive shock acceleration (DSA) to become low-energy anomalous cosmic rays (ACRs) can bridge the gap between modeled and observed ENA fluxes. Our results with the inclusion of DSA via hybrid fitting give entirely new and novel evidence that DSA at the termination shock is likely to be an important physical process. These ACRs carry a significant fraction of the energy density at the termination shock (22%, 13%, and 19% in the Voyager 2, flank, and tail directions, respectively). Using these ACRs in global ENA modeling of the heliosphere from 0.52 to 55 keV, we find that scaling factors as large as 1.8-2.5 are no longer required to match ENA observations at energies of ~1-4 keV. Large discrepancies between modeled and observed ENAs only remain over energies of 4-20 keV, indicating that there may be a further acceleration mechanism in the heliosheath at these energies.

Keywords: heliosphere; solar system; heliopause; heliosheath; termination shock; pickup ions; solar wind; interstellar medium; magnetohydrodynamical simulations; magnetohydrodynamics
Brown Dwarf Retrievals on FIRE!: Atmospheric Constraints and Lessons Learned from High Signal-to-noise Medium-resolution Spectroscopy of a T9 Dwarf

Brown dwarf spectra offer vital testbeds for our understanding of the chemical and physical processes that sculpt substellar atmospheres. Recently, atmospheric retrieval approaches have been successfully applied to low-resolution (R ~ 100) spectra of L, T, and Y dwarfs, yielding constraints on the chemical abundances and temperature structures of these atmospheres. Medium-resolution (R ~ 103) spectra of brown dwarfs offer additional insight, as molecular features are more easily disentangled and the thermal structure of the upper atmosphere is better probed. We present results from a GPU-based retrieval analysis of a high signal-to-noise, medium-resolution (R ~ 6000) FIRE spectrum from 0.85 to 2.5 μm of the T9 dwarf UGPS J072227.51-054031.2. At 60× higher spectral resolution than previous brown dwarf retrievals, a number of novel challenges arise. We examine the effect of different opacity sources, in particular for CH4. Furthermore, we find that flaws in the data like errors from order stitching can bias our constraints. We compare these retrieval results to those for an R ~ 100 spectrum of the same object, revealing how constraints on atmospheric abundances and temperatures improve by an order of magnitude or more with increased spectral resolution. In particular, we can constrain the abundance of H2S, which is undetectable at lower spectral resolution. While these medium-resolution retrievals offer the potential of precise, stellar-like constraints on atmospheric abundances (~0.02 dex), our retrieved radius is unphysically small ( $R={0.50}_{-0.01}^{+0.01}$ R Jup), indicating shortcomings with our modeling framework. This work is an initial investigation into brown dwarf retrievals at medium spectral resolution, offering guidance for future ground-based studies and JWST observations.

Keywords: brown dwarfs; t dwarfs; atmospheric composition; bayesian statistics
The Atmospheric Response to High Nonthermal Electron-beam Fluxes in Solar Flares. II. Hydrogen-broadening Predictions for Solar Flare Observations with the Daniel K. Inouye Solar Telescope

Redshifted components of chromospheric emission lines in the hard X-ray impulsive phase of solar flares have recently been studied through their 30 s evolution with the high resolution of the Interface Region Imaging Spectrograph. Radiative-hydrodynamic flare models show that these redshifts are generally reproduced by electron-beam-generated chromospheric condensations. The models produce large ambient electron densities, and the pressure broadening of the hydrogen Balmer series should be readily detected in observations. To accurately interpret the upcoming spectral data of flares with the DKIST, we incorporate nonideal, nonadiabatic line-broadening profiles of hydrogen into the RADYN code. These improvements allow time-dependent predictions for the extreme Balmer line wing enhancements in solar flares. We study two chromospheric condensation models, which cover a range of electron-beam fluxes (1 - 5 × 1011 erg s-1 cm-2) and ambient electron densities (1 - 60 × 1013 cm-3) in the flare chromosphere. Both models produce broadening and redshift variations within 10 s of the onset of beam heating. In the chromospheric condensations, there is enhanced spectral broadening due to large optical depths at Hα, Hβ, and Hγ, while the much lower optical depth of the Balmer series H12-H16 provides a translucent window into the smaller electron densities in the beam-heated layers below the condensation. The wavelength ranges of typical DKIST/ViSP spectra of solar flares will be sufficient to test the predictions of extreme hydrogen wing broadening and accurately constrain large densities in chromospheric condensations.

Keywords: solar flares; solar flare spectra
Solar-like to Antisolar Differential Rotation: A Geometric Interpretation

The solar convection zone rotates differentially, with its equatorial region rotating more rapidly than the polar regions. This form of differential rotation, also observed in many other low-mass stars, is understood to arise when Coriolis effects are stronger than those associated with buoyant driving of the convection. When buoyancy dominates, a so-called antisolar state of differential rotation results, characterized by rapidly rotating poles and a slow equator. The transition between these two states has been shown to occur when the intensity of these two forces is roughly equal or, equivalently, when the convective Rossby number of the system is unity. Here we consider an alternative view of the transition that relates this phenomenon to convective structure and convective-zone depth. Using a series of 3D rotating-convection-zone simulations, we demonstrate that the solar/antisolar transition occurs when the columnar convective structures characteristic of rotating convection attain a diameter roughly equivalent to the shell depth. When the characteristic convective wavelength exceeds twice the shell depth, we find that the coherent convective structures necessary to sustain an equatorward Reynolds stress are lost, and an antisolar state results. We conclude by presenting a force-balance analysis that relates this geometric interpretation of the transition to the convective Rossby-number criteria identified in previous studies.

Keywords: solar convective zone; stellar convective zones; hydrodynamics; solar differential rotation; astrophysical fluid dynamics; solar interior; solar rotation; stellar rotation; stellar interiors; helioseismology
A Quick Look at the 3 GHz Radio Sky. I. Source Statistics from the Very Large Array Sky Survey

The Very Large Array Sky Survey (VLASS) is observing the entire sky north of -40° in the S band (2 GHz < ν < 4 GHz), with the highest angular resolution (2"5) of any all-sky radio continuum survey to date. VLASS will cover its entire footprint over three distinct epochs, the first of which has now been observed in full. Based on Quick Look images from this first epoch, we have created a catalog of 1.9 × 106 reliably detected radio components. Due to the limitations of the Quick Look images, component flux densities are underestimated by ~15% at Speak > 3 mJy beam-1 and are often unreliable for fainter components. We use this catalog to perform statistical analyses of the ν ~ 3 GHz radio sky. Comparisons with the Faint Images of the Radio Sky at Twenty-Centimeters (FIRST) survey show the typical 1.4-3 GHz spectral index, α, to be ~-0.71. The radio color-color distribution of point and extended components is explored by matching with FIRST and the LOFAR Two-meter Sky Survey. We present the VLASS source counts, dN/dS, which are found to be consistent with previous observations at 1.4 and 3 GHz. Resolution improvements over FIRST result in excess power in the VLASS two-point correlation function at angular scales ≲7″, and in 18% of active galactic nuclei associated with a single FIRST component being split into multicomponent sources by VLASS.

Keywords: radio astronomy; radio galaxies; radio source catalogs; sky surveys
Resolved Velocity Profiles of Galactic Winds at Cosmic Noon

We study the kinematics of the interstellar medium (ISM) viewed "down the barrel" in 20 gravitationally lensed galaxies during cosmic noon (z = 1.5-3.5). We use moderate-resolution spectra (R ~ 4000) from Keck's Echellette Spectrograph and Imager and Magellan/MagE to spectrally resolve the ISM absorption in these galaxies into ~10 independent elements and use double Gaussian fits to quantify the velocity structure of the gas. We find that the bulk motion of gas in this galaxy sample is outflowing, with average velocity centroid $\left\langle {v}_{\mathrm{cent}}\right\rangle =-141$ km s-1 (±111 km s-1 scatter) measured with respect to the systemic redshift. A total of 16 out of the 20 galaxies exhibit a clear positive skewness, with a blueshifted tail extending to ~ -500 km s-1. We examine scaling relations in outflow velocities with galaxy stellar mass and star formation rate, finding correlations consistent with a momentum-driven wind scenario. Our measured outflow velocities are also comparable to those reported for FIRE-2 and TNG50 cosmological simulations at similar redshift and galaxy properties. We also consider implications for interpreting results from lower-resolution spectra. We demonstrate that while velocity centroids are accurately recovered, the skewness, velocity width, and probes of high-velocity gas (e.g., v 95) are subject to large scatter and biases at lower resolution. We find that R ≳ 1700 is required for accurate results for the gas kinematics of our sample. This work represents the largest available sample of well-resolved outflow velocity structure at z > 2 and highlights the need for good spectral resolution to recover accurate properties.

Keywords: galaxy winds; galaxy evolution; interstellar absorption; circumgalactic medium
Overview of the DESI Milky Way Survey

We describe the Milky Way Survey (MWS) that will be undertaken with the Dark Energy Spectroscopic Instrument (DESI) on the Mayall 4 m telescope at the Kitt Peak National Observatory. Over the next 5 yr DESI MWS will observe approximately seven million stars at Galactic latitudes |b| > 20°, with an inclusive target selection scheme focused on the thick disk and stellar halo. MWS will also include several high-completeness samples of rare stellar types, including white dwarfs, low-mass stars within 100 pc of the Sun, and horizontal branch stars. We summarize the potential of DESI to advance understanding of the Galactic structure and stellar evolution. We introduce the final definitions of the main MWS target classes and estimate the number of stars in each class that will be observed. We describe our pipelines for deriving radial velocities, atmospheric parameters, and chemical abundances. We use ≃500,000 spectra of unique stellar targets from the DESI Survey Validation program (SV) to demonstrate that our pipelines can measure radial velocities to ≃1 km s-1 and [Fe/H] accurate to ≃0.2 dex for typical stars in our main sample. We find the stellar parameter distributions from ≈100 deg2 of SV observations with ≳90% completeness on our main sample are in good agreement with expectations from mock catalogs and previous surveys.

Keywords: milky way stellar halo; dwarf galaxies; milky way evolution; milky way galaxy; milky way dark matter halo; milky way dynamics; surveys; milky way galaxy physics; spectroscopy; radial velocity; stellar abundances; galaxy formation
A Chandra and HST View of WISE-selected AGN Candidates in Dwarf Galaxies

Reliably identifying active galactic nuclei (AGNs) in dwarf galaxies is key to understanding black hole (BH) demographics at low masses and constraining models for BH seed formation. Here we present Chandra X-ray Observatory observations of 11 dwarf galaxies that were chosen as AGN candidates using Wide-field Infrared Survey Explorer (WISE) mid-infrared (mid-IR) color-color selection. Hubble Space Telescope images are also presented for 10 of the galaxies. Based on Sloan Digital Sky Survey spectroscopy, six galaxies in our sample have optical evidence for hosting AGNs and five are classified as star-forming. We detect X-ray point sources with luminosities above that expected from X-ray binaries in the nuclei of five of the six galaxies with optical evidence of AGNs. However, the X-ray emission from these AGNs is generally much lower than expected based on AGN scaling relations with infrared and optical tracers. We do not find compelling evidence for AGNs in the five optically-selected star-forming galaxies despite having red mid-IR colors. Only two are detected in X-rays and their properties are consistent with stellar-mass X-ray binaries. Based on this multiwavelength study, we conclude that two-color mid-IR AGN diagnostics at the resolution of WISE cannot be used to reliably select AGNs in optically-star-forming dwarf galaxies. Future observations in the infrared with the James Webb Space Telescope offer a promising path forward.

Keywords: dwarf galaxies; active galaxies; active galactic nuclei; x-ray active galactic nuclei; infrared sources
The Detection of Possible Transient Quasiperiodic Oscillations in the γ-Ray Light Curve of PKS 0244-470 and 4C+38.41

The continuous monitoring capability of Fermi-LAT has enabled the exploration of quasiperiodic oscillations (QPOs) in the γ-ray light curve of blazars that has given a new perspective to probe these sources over a wide range of timescales. We report the presence of transient QPOs in the long-term γ-ray light curve of blazars PKS 0244-470 and 4C +38.41. We first identified different flux states using the Bayesian block algorithm and then explored the possible transient QPOs in the segments of each flux phase where the flux level changes over fairly regular intervals. Combining this with the source's intrinsic variance, we identified two flux phases for PKS 0244-470: one activity (AP-1) and one quiescent phase (QP-1). For 4C+38.41, we similarly identified four activity (AP-1, AP-2, AP-3, and AP-4) and two quiescent (QP-1 and QP-2) phases. The AP-1 phase of PKS 0244-470 shows QPO of ~225 days persisting for eight cycles (~4.1σ). In 4C+38.41, AP-1 and AP-2 phases show QPO-like behavior of ~110 days and ~60 days, respectively, persisting for five cycles. In AP-3, we identified three subphases, and all show a ~7 day scale possible recurrent rise with five complete cycles, while in QP-1, we could identify two subphases (Q1 and Q2). The Q1 phase shows a period of ~104 days with six complete cycles. The Q2 phase also shows QPO but with only ~3.7 cycles. We discuss the possible origin and argue that the current-driven kink instability and curved jet model seem to be the most likely causes for shorter and longer QPOs.

Keywords: active galaxies; gamma-rays; blazars
Are There Terrestrial Planets Lurking in the Outer Solar System?

Motivated by recent measurements of the free-floating-planet mass function at terrestrial masses, we consider the possibility that the solar system may have captured a terrestrial planet early in its history. We show that ~1.2 captured free-floating planets with mass strictly greater than that of Mars may exist in the outer solar system, with a median predicted distance of ~1400 au. If we consider a logarithmic bin centered on the mass of Mars, rather than a cutoff, we find that ~2.7 captured free-floating planets with mass comparable to Mars may exist in the outer solar system. We derive an expectation value of ~0.9 for the number of captured free-floating planets with mass comparable to that of Mars (~1.4 for mass comparable to that of Mercury) that are currently brighter than the 10 yr coadded point-source detection limits of the Vera C. Rubin Observatory's Legacy Survey of Space and Time. Blind shift-and-stack searches could potentially enable the detection of such a planet if it is currently in the Southern sky. The theoretical argument presented here does not rely on the existence of posited patterns in the orbital elements of small bodies in and beyond the Kuiper Belt, in contrast with other hypothetical outer-solar-system planets motivated in recent years.

Keywords: planetary dynamics; free floating planets