adsabs/SciX_UAT_keywords huggingface dataset:
Discovery of a Fast Iron Low-ionization Outflow in the Early Evolution of the Nearby Tidal Disruption Event AT 2019qiz

We report the results of ultraviolet (UV) and optical photometric and spectroscopic analysis of the tidal disruption event (TDE) AT 2019qiz. Our follow-up observations started <10 days after the source began to brighten in the optical and lasted for a period of six months. Our late-time host-dominated spectrum indicates that the host galaxy likely harbors a weak active galactic nucleus. The initial Hubble Space Telescope (HST) spectrum of AT 2019qiz exhibits an iron and low-ionization broad absorption line (FeLoBAL) system that is seen for the first time in a TDE. This spectrum also bears a striking resemblance to that of Gaia16apd, a superluminous supernova. Our observations provide insights into the outflow properties in TDEs and show evidence for a connection between TDEs and engine-powered supernovae at early phases, as originally suggested by Metzger & Stone. In a time frame of 50 days, the UV spectra of AT 2019qiz started to resemble those of previous TDEs with only high-ionization broad absorption lines. The change in UV spectral signatures is accompanied by a decrease in the outflow velocity, which began at 15,000 km s-1 and decelerated to ~10,000 km s-1. A similar evolution in the H&agr; emission-line width further supports the speculation that the broad Balmer emission lines are formed in TDE outflows. In addition, we detect narrow absorption features on top of the FeLoBAL signatures in the early HST UV spectrum of AT 2019qiz. The measured H I column density corresponds to a Lyman-limit system, whereas the metal absorption lines (such as N V, C IV, Fe II, and Mg II) are likely probing the circumnuclear gas and interstellar medium in the host galaxy.

Keywords: black hole physics; high energy astrophysics; galaxy accretion disks
Characterizing the Optical Nature of the Blazar S5 1803+784 during Its 2020 Flare

We report the results from our study of the blazar S5 1803+784 carried out using quasi-simultaneous B, V, R, and I observations from 2020 May to 2021 July on 122 nights. Our observing campaign detected a historically bright optical flare during MJD 59,063.5-MJD 59,120.5. We also found the source in its brightest (R mag = 13.617) and faintest (R mag = 15.888) states to date. On 13 nights, covering both flaring and nonflaring periods, we searched for intraday variability using the power-enhanced F-test and the nested ANOVA test. We found significant variability in 2 of these 13 nights. However, no such variability was detected during the flaring period. From correlation analysis, we observed that the emission in all optical bands were strongly correlated with a time lag of ~0 days. To get insights into its dominant emission mechanisms, we generated the optical spectral energy distributions of the source on 79 nights and estimated the spectral indices by fitting a simple power law. The spectral index varied from 1.392 to 1.911 and showed significant variations with time and R-band magnitude. We detected a mild bluer-when-brighter (BWB) trend during the whole monitoring period and a much stronger BWB trend during the flare. We also carried out a periodicity search using four different methods and found no significant periodicity during our observation period. Based on the analysis during the flaring state of the source one can say that the emissions most likely originate from the jet rather than from the accretion disk.

Keywords: blazars; bl lacertae objects; active galactic nuclei; active galaxies
PIC Simulation of a Shock Tube: Implications for Wave Transmission in the Heliospheric Boundary Region

A shock tube problem is solved numerically by using one-dimensional full particle-in-cell simulations under the condition that a relatively tenuous and weakly magnetized plasma is continuously pushed by a relatively dense and strongly magnetized plasma having supersonic relative velocity. A forward and a reverse shock and a contact discontinuity are self-consistently reproduced. The spatial width of the contact discontinuity increases as the angle between the discontinuity normal and ambient magnetic field decreases. The inner structure of the discontinuity shows different profiles between magnetic field and plasma density, or pressure, which is caused by a non-MHD effect of the local plasma. The region between the two shocks is turbulent. The fluctuations in the relatively dense plasma are compressible and propagating away from the contact discontinuity, although the fluctuations in the relatively tenuous plasma contain both compressible and incompressible components. The source of the compressible fluctuations in the relatively dense plasma is in the relatively tenuous plasma. Only compressible fast mode fluctuations generated in the relatively tenuous plasma are transmitted through the contact discontinuity and propagate in the relatively dense plasma. These fast mode fluctuations are steepened when passing the contact discontinuity. This wave steepening and probably other effects may cause the broadening of the wave spectrum in the very local interstellar medium plasma. The results are discussed in the context of the heliospheric boundary region or heliopause.

Keywords: space plasmas; heliosphere; solar transition region; interstellar magnetic fields; interplanetary magnetic fields
Toward Extremely Precise Radial Velocities. II. A Tool for Using Multivariate Gaussian Processes to Model Stellar Activity

The radial velocity method is one of the most successful techniques for the discovery and characterization of exoplanets. Modern spectrographs promise measurement precision of 0.2-0.5 m s-1 for an ideal target star. However, the intrinsic variability of stellar spectra can mimic and obscure true planet signals at these levels. Rajpaul et al. and Jones et al. proposed applying a physically motivated, multivariate Gaussian process (GP) to jointly model the apparent Doppler shift and multiple indicators of stellar activity as a function of time, so as to separate the planetary signal from various forms of stellar variability. These methods are promising, but performing the necessary calculations can be computationally intensive and algebraically tedious. In this work, we present a flexible and computationally efficient software package, GPLinearODEMaker.jl, for modeling multivariate time series using a linear combination of univariate GPs and their derivatives. The package allows users to easily and efficiently apply various multivariate GP models and different covariance kernel functions. We demonstrate GPLinearODEMaker.jl by applying the Jones et al. model to fit measurements of the apparent Doppler shift and activity indicators derived from simulated active solar spectra time series affected by many evolving starspots. We show how GPLinearODEMaker.jl makes it easy to explore the effect of different choices for the GP kernel. We find that local kernels could significantly increase the sensitivity and precision of Doppler planet searches relative to the widely used quasiperiodic kernel.

Keywords: exoplanet detection methods; astronomy software; stellar activity; gaussian processes regression; time series analysis
Scattering of Lyα Photons through the Reionizing Intergalactic Medium: I. Spectral Energy Distribution

During reionization, a fraction of galactic Lyα emission is scattered in the intergalactic medium (IGM) and appears as diffuse light extending megaparsecs from the source. We investigate how to probe the properties of the early galaxies and their surrounding IGM using this scattered light. We create a Monte Carlo algorithm to track individual photons and reproduce several test cases from previous literature. Then, we run our code on the simulated IGM of the CoDaII simulation. We find that the scattered light can leave an observable imprint on the emergent spectrum if collected over several square arcminutes. Scattering can redden the emission by increasing the path lengths of photons, but it can also make the photons bluer by upscattering them according to the peculiar motion of the scatterer. The photons emitted on the far blue side of the resonance appear more extended in both frequency and space compared to those emitted near the resonance. This provides a discriminating feature for the blueward emission, which cannot be constrained from the unscattered light coming directly from the source. The ionization state of the IGM also affects the scattered light spectrum. When the source is in a small H II region, the emission goes through more scatterings in the surrounding H I region regardless of the initial frequency and ends up more redshifted and spatially extended. This can result in a weakening of the scattered light toward high z during reionization. Our results provide a framework for interpreting the scattered light to be measured by high-z integral-field-unit surveys.

Keywords: reionization; early universe; intergalactic medium; observational cosmology
Flare Statistics for Young Stars from a Convolutional Neural Network Analysis of TESS Data

All-sky photometric time-series missions have allowed for the monitoring of thousands of young (tage < 800 Myr) stars in order to understand the evolution of stellar activity. Here, we developed a convolutional neural network (CNN), stella, specifically trained to find flares in Transiting Exoplanet Survey Satellite (TESS) short-cadence data. We applied the network to 3200 young stars in order to evaluate flare rates as a function of age and spectral type. The CNN takes a few seconds to identify flares on a single light curve. We also measured rotation periods for 1500 of our targets and find that flares of all amplitudes are present across all spot phases, suggesting high spot coverage across the entire surface. Additionally, flare rates and amplitudes decrease for stars tage > 50 Myr across all temperatures Teff ≥ 4000 K, while stars from 2300 ≤ Teff < 4000 K show no evolution across 800 Myr. Stars of Teff ≤ 4000 K also show higher flare rates and amplitudes across all ages. We investigate the effects of high flare rates on photoevaporative atmospheric mass loss for young planets. In the presence of flares, planets lose 4%-7% more atmosphere over the first 1 Gyr. stella is an open-source Python toolkit hosted on GitHub and PyPI.

Keywords: pre-main sequence stars; convolutional neural networks; time series analysis; stellar activity; stellar rotation
The APOGEE Library of Infrared SSP Templates (A-LIST): High-resolution Simple Stellar Population Spectral Models in the H Band

Integrated light spectroscopy from galaxies can be used to study the stellar populations that cannot be resolved into individual stars. This analysis relies on stellar population synthesis (SPS) techniques to study the formation history and structure of galaxies. However, the spectral templates available for SPS are limited, especially in the near-infrared (near-IR). We present A-LIST (APOGEE Library of Infrared SSP Templates), a new set of high-resolution, near-IR SSP spectral templates spanning a wide range of ages (2-12 Gyr), metallicities ( - 2.2 < [M/H] < + 0.4) and α abundances ( - 0.2 < [α/M] < + 0.4). This set of SSP templates is the highest resolution (R ∼ 22, 500) available in the near-IR, and the first such based on an empirical stellar library. Our models are generated using spectra of ∼300,000 stars spread across the Milky Way, with a wide range of metallicities and abundances, from the APOGEE survey. We show that our model spectra provide accurate fits to M31 globular cluster spectra taken with APOGEE, with best-fit metallicities agreeing with those of previous estimates to within ∼0.1 dex. We also compare these model spectra to lower-resolution E-MILES models and demonstrate that we recover the ages of these models to within ∼1.5 Gyr. This library is available in https://github.com/aishashok/ALIST-library.

Keywords: galaxy stellar content; galaxy chemical evolution; stellar evolutionary models; stellar evolution
Radio Echo in the Turbulent Corona and Simulations of Solar Drift-pair Radio Bursts

Drift-pair bursts are an unusual type of solar low-frequency radio emission, which appear in the dynamic spectra as two parallel drifting bright stripes separated in time. Recent imaging spectroscopy observations allowed for the quantitative characterization of the drifting pairs in terms of source size, position, and evolution. Here, the drift-pair parameters are qualitatively analyzed and compared with the newly developed Monte Carlo ray-tracing technique simulating radio-wave propagation in the inhomogeneous anisotropic turbulent solar corona. The results suggest that drift-pair bursts can be formed due to a combination of refraction and scattering processes, with the trailing component being the result of turbulent reflection (turbulent radio echo). The formation of drift-pair bursts requires an anisotropic scattering with the level of plasma density fluctuations comparable to that in type III bursts, but with a stronger anisotropy at the inner turbulence scale. The anisotropic radio-wave scattering model can quantitatively reproduce the key properties of drift-pair bursts: the apparent source size and its increase with time at a given frequency, the parallel motion of the source centroid positions, and the delay between the burst components. The trailing component is found to be virtually cospatial and following the main component. The simulations suggest that drift-pair bursts are likely to be observed closer to the disk center and below 100 MHz due to the effects of free-free absorption and scattering. The exciter of drift pairs is consistent with propagating packets of whistlers, allowing for a fascinating way to diagnose the plasma turbulence and the radio emission mechanism.

Keywords: solar coronal radio emission; radio spectroscopy
Investigating Extreme Scattering Events by Volumetric Ray-tracing

Extreme scattering events (ESEs) are observed as dramatic (>50%) drops in flux density that occur over an extended period of weeks to months. Discrete plasma lensing structures are theorized to scatter the radio waves produced by distant sources such as pulsars, causing the signature decrease in flux density and characteristic caustic spikes in ESE light curves. While plasma lens models in the extant literature have reproduced key features of ESE light curves, they have all faced the problem of being highly overdense and overpressured relative to the surrounding interstellar medium by orders of magnitude. We model ESEs by numerically ray tracing through analytic, volumetric plasma lens models by solving the eikonal equation. Delaunay triangulation connecting the rays approximates the wave front, generating a mapping from the observer plane to the source plane to account for multiple imaging. This eikonal method of ray tracing is tested against known analytic solutions and is then applied to a three-dimensional Gaussian-distributed electron volume density lens and a filament model inspired by Grafton et al. We find convergence of our numerical results with established analytic solutions, validating our numerical method, and reproduce ESE-like light curves. Our numerical ray-tracing method lends itself well to exploring the lensing effects of volumetric turbulence as well as sheet-like lenses, which is currently in progress.

Keywords: interstellar plasma; computational astronomy; theoretical models; interstellar medium
Decay of Solar Pores Driven by Small-scale Magnetic Reconnection Episodes

We present the relationships between the disappearances of two small pores, magnetic cancellations, and magnetic reconnection episodes in the NOAA AR 12778 on 2020 October 26 with high-resolution observations of the New Vacuum Solar Telescope and the Solar Dynamics Observatory. Two emerging positive polarities (P1 and P2) approach a negative polarity (N1) with velocities of 0.26 and 0.42 km s-1, respectively. Then, two small-scale magnetic reconnection episodes occur between a series of magnetic loops that are rooted in these polarities. The reconnection inflow velocities are around 4.0 km s-1 which is faster than the movements of P1 and P2. Compared with the first magnetic reconnection episode, more magnetic free energy is released in the second reconnection episode due to the greater magnetic strength of P2. Subsequently, magnetic cancellation occurs first between P1 and N1, and then between P2 and N1. At the same time, the pores S1 (N1) and S2 (P2) decay and disappear. The area decay rate of the small pore S2 is estimated to be 7.3 Mm2 hr-1, which is larger than previously reported cases. And the flux decay rate of S2 is 5.1 × 1019 Mx hr-1, similar to the results obtained in the larger sunspots. We conclude that the magnetic reconnection episodes may be caused by both the movement of the magnetic polarities and the plasma dynamics themselves. The decay and disappearance of the small pores and the polarities are driven by magnetic reconnection episodes and then flux submergence. We suggest that a magnetic reconnection episode is a more efficient mechanism for the disappearance of solar pores.

Keywords: solar magnetic reconnection; solar magnetic fields; sunspots; solar activity
Physical Properties of the Supernova Remnant Population in the Small Magellanic Cloud

The X-ray emission from a supernova remnant is a powerful diagnostic of the state of its shocked plasma. The temperature and the emission measure are related to the energy of the explosion, the age of the remnant, and the density of the surrounding medium. Here we present the results of a study of the remnant population of the Small Magellanic Cloud. Progress in X-ray observations of remnants has resulted in a sample of 20 remnants in the Small Magellanic Cloud with measured temperatures and emission measures. We apply spherically symmetric supernova remnant evolution models to this set of remnants to estimate ages, explosion energies, and circumstellar medium densities. The distribution of ages yields a remnant birth rate of ~1/1200 yr. The energies and densities are well fit with log-normal distributions, with means of 1.6 × 1051 erg and 0.14 cm-3, and 1σ dispersions of a factor of 1.87 in energy and 3.06 in density, respectively.

Keywords: supernova remnants; small magellanic cloud; magellanic clouds
A Detection of Cosmological 21 cm Emission from CHIME in Cross-correlation with eBOSS Measurements of the Lyα Forest

We report the detection of 21 cm emission at an average redshift $\bar{z}=2.3$ in the cross-correlation of data from the Canadian Hydrogen Intensity Mapping Experiment (CHIME) with measurements of the Lyα forest from eBOSS. Data collected by CHIME over 88 days in the 400–500 MHz frequency band (1.8 < z < 2.5) are formed into maps of the sky and high-pass delay filtered to suppress the foreground power, corresponding to removing cosmological scales with k ∥ ≲ 0.13 Mpc‑1 at the average redshift. Line-of-sight spectra to the eBOSS background quasar locations are extracted from the CHIME maps and combined with the Lyα forest flux transmission spectra to estimate the 21 cm–Lyα cross-correlation function. Fitting a simulation-derived template function to this measurement results in a 9σ detection significance. The coherent accumulation of the signal through cross-correlation is sufficient to enable a detection despite excess variance from foreground residuals ∼6–10 times brighter than the expected thermal noise level in the correlation function. These results are the highest-redshift measurement of 21 cm emission to date, and they set the stage for future 21 cm intensity mapping analyses at z > 1.8.

Keywords: cosmology; h i line emission; lyman alpha forest
The Hobby–Eberly Telescope VIRUS Parallel Survey (HETVIPS)

The Hobby–Eberly Telescope (HET) VIRUS Parallel Survey (HETVIPS) is a blind spectroscopic program that sparsely covers approximately two-thirds of the celestial sphere and consists of roughly 252 million fiber spectra. The spectra were taken in parallel mode with the Visible Integral-field Replicable Unit Spectrograph (VIRUS) instrument when the HET was observing a primary target with other HET facility instruments. VIRUS can simultaneously obtain approximately 35,000 spectra covering 3470–5540 Å at a spectral resolution of ≈800. Although the vast majority of these spectra cover blank sky, we used the Pan-STARRS1 Data Release 2 Stacked Catalog to identify objects encompassed in the HETVIPS pointings and extract their spectra. This paper presents the first HETVIPS data release, containing 493,012 flux-calibrated spectra obtained through 2023 March 31, as well as a description of the data processing technique. Each of the object spectra were classified, resulting in a catalog of 74,196 galaxies, 4,087 quasars, 259,396 stars, and 154,543 unknown sources. *Based on observations obtained with the Hobby–Eberly Telescope (HET), which is a joint project of the University of Texas at Austin, the Pennsylvania State University, Ludwig-Maximilians-Universität München, and Georg-August-Universität Göttingen. The HET is named in honor of its principal benefactors, William P. Hobby and Robert E. Eberly.

Keywords: surveys; redshift surveys; stellar classification; sky surveys; catalogs
Tracing Young Star-forming Clumps in the Nearby Flocculent Spiral Galaxy NGC 7793 with UVIT Imaging

Star formation in galaxies is a hierarchical process with a wide range of scales, from smaller clusters to larger stellar complexes. Here, we present an ultra-violet imaging study of the nearby flocculent spiral galaxy NGC 7793, observed using the Ultra-Violet Imaging Telescope (UVIT). We find that the disk scale length estimated in Far-UV (2.64 ± 0.16 kpc) is larger than that in Near-UV (2.21 ± 0.21 Kpc) and optical (1.08 Kpc), which supports the inside-out growth scenario of the galaxy disk. The star-forming UV disk is also found to be contained within the extent of H I gas, with a column density greater than 1021 cm-2. With the spatial resolution of UVIT (1 pixel ∼ 6.8 pc), we identified 2046 young star-forming clumps in the galaxy with radii between ∼12 and 70 pc, which matches well with the size of giant molecular clouds (GMCs) detected in the galaxy. Around 61% of the regions identified in our study are younger than 20 Myr, which points to a recent enhancement in star formation across the galaxy. We also note that the youngest star-forming regions, with an age of <10 Myr, distinctly trace the flocculent arms of the galaxy. The estimated mass of the clumps covers a range between 103 and 106M⊙. We notice a gradient in the mass distribution of identified clumps along the spiral arms. We also study the nuclear star cluster of the galaxy, finding that the stellar populations in the cluster outskirts are younger than the inner part.

Keywords: spiral galaxies; star formation; ultraviolet astronomy