adsabs/SciX_UAT_keywords huggingface dataset:
GRB 170817A Afterglow from a Relativistic Electron-Positron Pair Wind Observed Off-axis

A relativistic electron-positron (e+e-) pair wind from a rapidly rotating, strongly magnetized neutron star (NS) would interact with a gamma-ray burst (GRB) external shock and reshape afterglow emission signatures. Assuming that the merger remnant of GW170817 is a long-lived NS, we show that a relativistic e+e- pair wind model with a simple top-hat jet viewed off-axis can reproduce multiwavelength afterglow lightcurves and superluminal motion of GRB 170817A. The Markov Chain Monte Carlo (MCMC) method is adopted to obtain the best-fitting parameters, which give the jet half-opening angle θj ≈ 0.12 rad, and the viewing angle θv ≈ 0.23 rad. The best-fitting value of θv is close to the lower limit of the prior that is chosen based on the gravitational-wave and electromagnetic observations. In addition, we also derive the initial Lorentz factor Γ0 ≈ 49 and the isotropic kinetic energy EK,iso ≈ 1× 1052 erg. Consistency between the corrected on-axis values for GRB 170817A and typical values observed for short GRBs indicates that our model can also reproduce the prompt emission of GRB 170817A. An NS with a magnetic field strength Bp ≈ 1.6 × 1013 G is obtained in our fitting, indicating that a relatively low thermalization efficiency η ≲ 10-3 is needed to satisfy observational constraints on the kilonova. Furthermore, our model is able to reproduce a late-time shallow decay in the X-ray lightcurve, and predicts that the X-ray and radio flux will continue to decline in the coming years.

Keywords: gamma-ray bursts; neutron stars
Newly Discovered Source of Turbulence and Heating in the Solar Chromosphere

Above the Sun's luminous photosphere lies the solar chromosphere, where the temperature increases from below 4000 K to over 1 million K. Though physicists do not understand the origin of these increases, they know it powers the solar wind with enormous consequences for the entire solar system. This report describes a set of simulations and analytical theory showing that solar atmospheric flows originating in the photosphere will frequently drive a previously unidentified thermal plasma instability that rapidly develops into turbulence. Though this turbulence is small scale (centimeters to a few meters), it will modify the conductivity, temperatures, and energy flows through much of the chromosphere. Incorporating the effects of this turbulence, and other small-scale turbulence, into large-scale models of solar and stellar atmospheres will improve physicists' ability to model energy flows with important consequences for the predicted temperatures and radiation patterns.

Keywords: solar chromosphere; quiet solar chromosphere
Imaging the 511 keV Positron Annihilation Sky with COSI

The balloon-borne Compton Spectrometer and Imager (COSI) had a successful 46-day flight in 2016. The instrument is sensitive to photons in the energy range 0.2-5 MeV. Compton telescopes have the advantage of a unique imaging response and provide the possibility of strong background suppression. With its high-purity germanium detectors, COSI can precisely map γ-ray line emission. The strongest persistent and diffuse γ-ray line signal is the 511 keV emission line from the annihilation of electrons with positrons from the direction of the Galactic center. While many sources have been proposed to explain the amount of positrons, ${\dot{N}}_{{e}^{+}}\sim {10}^{50}\,{e}^{+}\,{\mathrm{yr}}^{-1}$ , the true contributions remain unsolved. In this study, we aim at imaging the 511 keV sky with COSI and pursue a full-forward modeling approach, using a simulated and binned imaging response. For the strong instrumental background, we describe an empirical approach to take the balloon environment into account. We perform two alternative methods to describe the signal: Richardson-Lucy deconvolution, an iterative method toward the maximum likelihood solution, and model fitting with predefined emission templates. Consistently with both methods, we find a 511 keV bulge signal with a flux between 0.9 and $3.1\times {10}^{-3}\,\mathrm{photons}\,{\mathrm{cm}}^{-2}\,{{\rm{s}}}^{-1}$ , confirming earlier measurements, and also indications of more extended emission. The upper limit we find for the 511 keV disk, $\lt 4.3\times {10}^{-3}\,\mathrm{photons}\,{\mathrm{cm}}^{-2}\,{{\rm{s}}}^{-1}$ , is consistent with previous detections. For large-scale emission with weak gradients, coded aperture mask instruments suffer from their inability to distinguish isotropic emission from instrumental background, while Compton telescopes provide a clear imaging response, independent of the true emission.

Keywords: astronomy data modeling; gamma-rays; high altitude balloons; interstellar medium; cosmic rays
The Long-term Spectral Changes of Eta Carinae: Are they Caused by a Dissipating Occulter as Indicated by CMFGEN Models?

Eta Carinae (η Car) exhibits a unique set of P Cygni profiles with both broad and narrow components. Over many decades, the spectrum has changed-there has been an increase in observed continuum fluxes and a decrease in Fe II and H I emission-line equivalent widths. The spectrum is evolving toward that of a P Cygni star such as P Cygni itself and HDE 316285. The spectral evolution has been attributed to intrinsic variations such as a decrease in the mass-loss rate of the primary star or differential evolution in a latitudinal-dependent stellar wind. However, intrinsic wind changes conflict with three observational results: the steady long-term bolometric luminosity; the repeating X-ray light curve over the binary period; and the constancy of the dust-scattered spectrum from the Homunculus. We extend previous work that showed a secular strengthening of P Cygni absorptions by adding more orbital cycles to overcome temporary instabilities and by examining more atomic transitions. CMFGEN modeling of the primary wind shows that a time-decreasing mass-loss rate is not the best explanation for the observations. However, models with a small dissipating absorber in our line of sight can explain both the increase in brightness and changes in the emission and P Cygni absorption profiles. If the spectral evolution is caused by the dissipating circumstellar medium, and not by intrinsic changes in the binary, the dynamical timescale to recover from the Great Eruption is much less than a century, different from previous suggestions.

Keywords: stellar phenomena; stellar mass loss
Dust Temperature Uncertainties Hamper the Inference of Dust and Molecular Gas Masses from the Dust Continuum Emission of Quiescent High-redshift Galaxies

Single flux density measurements at observed-frame submillimeter and millimeter wavelengths are commonly used to probe dust and gas masses in galaxies. In this Letter, we explore the robustness of this method to infer dust mass, focusing on quiescent galaxies, using a series of controlled experiments on four massive halos from the Feedback in Realistic Environments project. Our starting point is four star-forming central galaxies at seven redshifts between z = 1.5 and z = 4.5. We generate modified quiescent galaxies that have been quenched for 100 Myr, 500 Myr, or 1 Gyr prior to each of the studied redshifts by reassigning stellar ages. We derive spectral energy distributions for each fiducial and modified galaxy using radiative transfer. We demonstrate that the dust mass inferred is highly dependent on the assumed dust temperature, T dust, which is often unconstrained observationally. Motivated by recent work on quiescent galaxies that assumed T dust ~ 25 K, we show that the ratio between dust mass and 1.3 mm flux density can be higher than inferred by up to an order of magnitude, due to the considerably lower dust temperatures seen in non-star-forming galaxies. This can lead to an underestimation of dust mass (and, when submillimeter flux density is used as a proxy for molecular gas content and gas mass). This underestimation is most severe at higher redshifts, where the observed-frame 1.3 mm flux density probes rest-frame wavelengths far from the Rayleigh-Jeans regime, and hence depends superlinearly on dust temperature. We fit relations between ratios of rest-frame far-infrared flux densities and mass-weighted dust temperature that can be used to constrain dust temperatures from observations and hence derive more reliable dust and molecular gas masses.

Keywords: galaxy evolution; galaxy quenching; radiative transfer simulations; submillimeter astronomy
Canonical X-Ray Fluorescence Line Intensities as Column Density Indicators

X-ray line fluorescence is ubiquitous around powerful accretion sources, namely, active galactic nuclei and X-ray binaries. The brightest and best-studied line is the Fe Kα line at λ = 1.937 Å(6.4 keV). This paper presents a survey of all well-measured Chandra/HETG grating spectra featuring several Kα fluorescence lines from elements between Mg and Ni. Despite the variety of sources and physical conditions, we identify a common trend that dictates the Kα line intensity ratios between elements. For the most part, the line intensities are well described by a simple, plane-parallel approximation of a near-neutral, solar-abundance, high column density (NH > 1024 cm-2) medium. This approximation gives canonical photon-intensity line ratios for the Kα fluorescence of all elements, e.g., 0.104:0.069:1.0:0.043 for Si:S:Fe:Ni, respectively. Deviations from these ratios are shown to be primarily due to excess column density along the line of sight beyond the Galactic column. Therefore, measured fluorescence line ratios provide an independent estimate of NH and insight into the environment of accretion sources. Residual discrepancies with the canonical ratios could be due to a variety of effects such as a fluorescing medium with NH < 1024 cm-2, a non-neutral medium, variations in the illuminating spectrum, nonsolar abundances, or an irregular source geometry. However, evidently and perhaps surprisingly, these are uncommon, and their effect remains minor.

Keywords: high mass x-ray binary stars; x-ray active galactic nuclei
Bayesian Time-resolved Spectroscopy of Multipulse GRBs: Variations of Emission Properties among Pulses

Gamma-ray bursts (GRBs) are highly variable and exhibit strong spectral evolution. In particular, the emission properties vary from pulse to pulse in multipulse bursts. Here we present a time-resolved Bayesian spectral analysis of a compilation of GRB pulses observed by the Fermi/Gamma-ray Burst Monitor. The pulses are selected to have at least four time bins with a high statistical significance, which ensures that the spectral fits are well determined and spectral correlations can be established. The sample consists of 39 bursts, 117 pulses, and 1228 spectra. We confirm the general trend that pulses become softer over time, with mainly the low-energy power-law index α becoming smaller. A few exceptions to this trend exist, with the hardest pulse occurring at late times. The first pulse in a burst is clearly different from the later pulses; three-fourths of them violate the synchrotron line of death, while around half of them significantly prefer photospheric emission. These fractions decrease for subsequent pulses. We also find that in two-thirds of the pulses, the spectral parameters (α and peak energy) track the light-curve variations. This is a larger fraction compared to what is found in previous samples. In conclusion, emission compatible with the GRB photosphere is typically found close to the trigger time, while the chance of detecting synchrotron emission is greatest at late times. This allows for the coexistence of emission mechanisms at late times.

Keywords: gamma-ray bursts; astronomy data analysis
Projected Rotational Velocities and Fundamental Properties of Low-mass Pre-main-sequence Stars in the Taurus-Auriga Star-forming Region

The projected stellar rotational velocity ( $v\sin i$ ) is critical for our understanding of processes related to the evolution of angular momentum in pre-main-sequence stars. We present $v\sin i$ measurements of high-resolution infrared and optical spectroscopy for 70 pre-main-sequence stars in the Taurus-Auriga star-forming region, in addition to effective temperatures measured from line-depth ratios, as well as stellar rotation periods determined from optical photometry. From the literature, we identified the stars in our sample that show evidence of residing in circumstellar disks or multiple systems. The comparison of infrared $v\sin i$ measurements calculated using two techniques shows a residual scatter of ∼1.8 km s-1, defining a typical error floor for the $v\sin i$ of pre-main-sequence stars from infrared spectra. A comparison of the $v\sin i$ distributions of stars with and without companions shows that binaries/multiples typically have a higher measured $v\sin i$ , which may be caused by contamination by companion lines, shorter disk lifetimes in binary systems, or tidal interactions in hierarchical triples. A comparison of optical and infrared $v\sin i$ values shows no significant difference regardless of whether the star has a disk or not, indicating that CO contamination from the disk does not impact $v\sin i$ measurements above the typical ∼1.8 km s-1 error floor of our measurements. Finally, we observe a lack of a correlation between the $v\sin i$ , presence of a disk, and H-R diagram position, which indicates a complex interplay between stellar rotation and evolution of pre-main-sequence stars.

Keywords: stellar rotation; pre-main sequence stars; low mass stars; fundamental parameters of stars; high resolution spectroscopy
ALMA Observations of the Molecular Clouds in NGC 625

We present the highest-resolution (1″) 12CO observations of molecular gas in the dwarf starburst galaxy NGC 625 to date, obtained with Atacama Large Millimeter/submillimeter Array. The molecular gas, which is distributed in discrete clouds within an area of 0.4 kpc2, does not have well-ordered large-scale motions. We measure a molecular mass in NGC 625 of 5.3 × 106 ${M}_{\odot }$ , assuming a Milky Way CO-to- ${{\rm{H}}}_{2}$ conversion factor. We use the CPROPS package to identify molecular clouds and measure their properties. The 19 resolved CO clouds have a median radius of 20 pc, a median linewidth 2.5 $\mathrm{km}\,{{\rm{s}}}^{-1}$ , and a median surface density of 169 ${M}_{\odot }\,{{\rm{pc}}}^{-2}$ . Larson scaling relations suggest that molecular clouds in NGC 625 are mostly in virial equilibrium. Comparison of our high-resolution CO observations with a star formation rate map, inferred from ancillary optical observations, suggests that about 40% of the molecular clouds coincide with the brightest H II regions. These bright H II regions have a range of molecular gas depletion timescales, all within a factor of ∼3 of the global depletion time in NGC 625 of 106-134 Myr. The highest surface density molecular clouds toward the southwest of the galaxy, in a region we call the Butterfly, do not show strong star formation activity and suggest a depletion timescale longer than 5 Gyr.

Keywords: dwarf galaxies; galaxies; star formation; interstellar medium; giant molecular clouds; molecular gas; interstellar molecules; starburst galaxies; metallicity; interferometry; submillimeter astronomy; molecular clouds
Revising Properties of Planet-Host Binary Systems. I. Methods and Pilot Study

To fully leverage the statistical strength of the large number of planets found by projects such as the Kepler survey, the properties of planets and their host stars must be measured as accurately as possible. One key population for planet demographic studies is circumstellar planets in close binaries (ρ < 50 au), where the complex dynamical environment of the binary inhibits most planet formation, but some planets nonetheless survive. Accurately characterizing the stars and planets in these complex systems is a key factor in better understanding the formation and survival of planets in binaries. Toward that goal, we have developed a new Markov Chain Monte Carlo fitting algorithm to retrieve the properties of binary systems using unresolved spectra, unresolved photometry, and resolved contrasts. We have analyzed eight Kepler Objects of Interest in M-star binary systems using literature data, and have found that the temperatures of the primary stars (and presumed planet hosts) are revised upward by an average of 200 K. The planetary radii should be revised upward by an average of 20% if the primary star is the host, and 80% if the secondary star is the planet host. The average contrast between stellar components in the Kepler band is 0.75 mag, which is small enough that neither star in any of the binaries can be conclusively ruled out as a potential planet host. Our results emphasize the importance of accounting for multiplicity when measuring stellar parameters, especially in the context of exoplanet characterization.

Keywords: close binary stars; binary stars; low mass stars; exoplanets; multiple stars
Time-dependent Numerical Model for Studying the Very-high-energy Emissions of Distant Gamma-Ray Burst GRB 201216C

Recently, the MAGIC Collaboration reported a ~5σ statistical significance of the very-high-energy (VHE) emission from a distant gamma-ray burst (GRB), GRB 201216C. Such distant GRB may be effectively absorbed by the extragalactic background light (EBL). The origin of the VHE emission from such distant objects is still unknown. Here, we propose a numerical model for studying the afterglow emission of this distant GRB. The model solves the continuity equation governing the temporal evolution of electron distribution, and the broadband observed data can be explained by the synchrotron plus synchrotron self-Compton (SSC) radiation of the forward shock. The predicted observed 0.1 TeV flux can reach ~10-9-10-10 erg cm-2 s-1 at t ~ 103-104 s, even with strong EBL absorption, such strong sub-teraelectronvolt (sub-TeV) emissions still can be observed by the MAGIC telescope. Using this numerical model, the shock parameters in the modeling are similar to two other sub-TeV GRBs (i.e., GRB 190114C and GRB 180720B), implying that the sub-TeV GRBs have some commonalities: they have energetic burst energy, low circumburst medium density, and a low magnetic equipartition factor. We regard GRB 201216C as a typical GRB, and estimate the maximum redshift of GRB that can be detected by the MAGIC telescope, i.e., z ~ 1.6. We also find that the VHE photon energy of such distant GRB can only reach ~0.1 TeV. Improving the low energy sensitivity of the VHE telescope is very important to detect the sub-TeV emissions of these distant GRBs.

Keywords: gamma-ray bursts; shocks
The APO-K2 Catalog. I. 7500 Red Giants with Fundamental Stellar Parameters from APOGEE DR17 Spectroscopy and K2-GAP Asteroseismology

We present a catalog of fundamental stellar properties for ~7500 evolved stars, including stellar radii and masses, determined from the combination of spectroscopic observations from the Apache Point Observatory Galactic Evolution Experiment, part of the Sloan Digital Sky Survey IV, and asteroseismology from K2. The resulting APO-K2 catalog provides spectroscopically derived temperatures and metallicities, asteroseismic global parameters, evolutionary states, and asteroseismically derived masses and radii. Additionally, we include kinematic information from Gaia. We investigate the multidimensional space of abundance, stellar mass, and velocity with an eye toward applications in Galactic archaeology. The APO-K2 sample has a large population of low-metallicity stars (~288 with [M/H] ≤ -1), and their asteroseismic masses are larger than astrophysical estimates. We argue that this may reflect offsets in the adopted fundamental temperature scale for metal-poor stars rather than metallicity-dependent issues with interpreting asteroseismic data. We characterize the kinematic properties of the population as a function of α enhancement and position in the disk and identify those stars in the sample that are candidate components of the Gaia-Enceladus merger. Importantly, we characterize the selection function for the APO-K2 sample as a function of metallicity, radius, mass, ${\nu }_{\max }$ , color, and magnitude referencing Galactic simulations and target selection criteria to enable robust statistical inferences with the catalog.

Keywords: galactic archaeology; stellar astronomy; asteroseismology; spectroscopy; stellar kinematics
PCAT-DE: Reconstructing Pointlike and Diffuse Signals in Astronomical Images Using Spatial and Spectral Information

Observational data from astronomical imaging surveys contain information about a variety of source populations and environments, and their complexity will increase substantially as telescopes become more sensitive. Even for existing observations, measuring the correlations between pointlike and diffuse emission can be crucial to correctly inferring the properties of any individual component. For this task, information is typically lost, because of conservative data cuts, aggressive filtering, or incomplete treatment of contaminated data. We present the code PCAT-DE, an extension of probabilistic cataloging, designed to simultaneously model pointlike and diffuse signals. This work incorporates both explicit spatial templates and a set of nonparametric Fourier component templates into a forward model of astronomical images, reducing the number of processing steps applied to the observed data. Using synthetic Herschel-SPIRE multiband observations, we demonstrate that point-source and diffuse emission can be reliably separated and measured. We present two applications of this model. For the first, we perform point-source detection/photometry in the presence of galactic cirrus and demonstrate that cosmic infrared background galaxy counts can be recovered in cases of significant contamination. In the second, we show that the spatially extended thermal Sunyaev-Zel'dovich effect signal can be reliably measured even when it is subdominant to the pointlike emission from individual galaxies.

Keywords: catalogs; diffuse radiation; markov chain monte carlo; bayesian statistics; astrostatistics tools
Separated Twins or Just Siblings? A Multiplanet System around an M Dwarf Including a Cool Sub-Neptune

We report the discovery of two TESS sub-Neptunes orbiting the early M dwarf TOI-904 (TIC 261257684). Both exoplanets, TOI-904 b and c, were initially observed in TESS Sector 12 with twin sizes of ${2.426}_{-0.157}^{+0.163}$ and ${2.167}_{-0.118}^{+0.130}$ R ⊕, respectively. Through observations in five additional sectors in the TESS primary mission and the first and second extended missions, the orbital periods of the planets were measured to be 10.887 ± 0.001 and 83.999 ± 0.001 days, respectively. Reconnaissance radial velocity measurements (taken with EULER/CORALIE and SMARTS/CHIRON) and high-resolution speckle imaging with adaptive optics (obtained from SOAR/HRCAM and Gemini South/ZORRO) show no evidence of an eclipsing binary or a nearby companion, which, together with the low false-positive probabilities calculated with the statistical validation software TRICERATOPS, establishes the planetary nature of these candidates. The outer planet, TOI-904 c, is the longest-period M dwarf exoplanet found by TESS, with an estimated equilibrium temperature of 217 K. As the three other validated planets with comparable host stars and orbital periods were observed by Kepler around much dimmer stars (J mag > 12), TOI-904 c, orbiting a brighter star (J mag = 9.6), is the coldest M dwarf planet easily accessible for atmospheric follow-up. Future mass measurements and transmission spectroscopy of the similar-sized planets in this system could determine whether they are also similar in density and composition, suggesting a common formation pathway, or whether they have distinct origins.

Keywords: exoplanet systems; exoplanet detection methods; exoplanet astronomy; m dwarf stars; cold neptunes; mini neptunes; transit photometry