adsabs/SciX_UAT_keywords huggingface dataset:
An Energy Inventory of Tidal Disruption Events

Tidal disruption events (TDEs) offer a unique opportunity to study a single supermassive black hole (SMBH) under feeding conditions that change over timescales of days to months. However, the primary mechanism for generating luminosity during the flares remains debated. Despite the increasing number of observed TDEs, it is unclear whether most of the energy in the initial flare comes from accretion near the gravitational radius or from circularizing debris at larger distances from the SMBH. The energy dissipation efficiency increases with decreasing radius; therefore, by measuring the total energy emitted and estimating the efficiency, we can derive clues about the nature of the emission mechanism. Here we calculate the integrated energy, emission timescales, and average efficiencies for the TDEs using the Modular Open Source Fitter for Transients (MOSFiT). Our calculations of the total energy generally yield higher values than previous estimates. This is predominantly because, if the luminosity follows the mass fallback rate, TDEs release a significant fraction of their energy long after their light curve peaks. We use MOSFiT to calculate the conversion efficiency from mass to radiated energy and find that for many of the events, it is similar to efficiencies inferred for active galactic nuclei. There are, however, large systematic uncertainties in the measured efficiency due to model degeneracies between the efficiency and the mass of the disrupted star. These must be reduced before we can definitively resolve the emission mechanism of individual TDEs.

Keywords: tidal disruption; supermassive black holes; galaxy accretion disks; high energy astrophysics; black holes; active galactic nuclei; transient sources; time domain astronomy; ultraviolet transient sources; x-ray transient sources
Revisiting the Transit Timing Variations in the TrES-3 and Qatar-1 Systems with TESS Data

We present and analyze 58 transit light curves of TrES-3b and 98 transit light curves of Qatar-1b, observed by the Transiting Exoplanet Survey Satellite, plus two transit light curves of Qatar-1b, observed by us, using a ground-based 1.23 m telescope. These light curves are combined with the best-quality light curves taken from the Exoplanet Transit Database and the literature. The precisely determined midtransit times from these light curves enable us to obtain the refined orbital ephemerides, with improved precision, for both hot Jupiters. From the timing analysis, we find indications of the presence of transit timing variations (TTVs) in both systems. Since the observed TTVs are unlikely to be short-term and periodic, the possibility of additional planets in orbits close to TrES-3b and Qatar-1b is ruled out. The possible causes of long-term TTVs, such as orbital decay, apsidal precession, the Applegate mechanism, and line-of-sight acceleration, are also examined. However, none of these possibilities are found to explain the observed TTV of TrES-3b. In contrast to this, line-of-sight acceleration appears to be a plausible explanation for the observed TTV of Qatar-1b. In order to confirm these findings, further high-precision transit and radial velocity observations of both systems would be worthwhile.

Keywords: exoplanets; hot jupiters; tidal interaction; transit photometry; transit timing variation method; radial velocity
Clues to the Formation of Liller 1 from Modeling Its Complex Star Formation History

Liller 1 and Terzan 5 are two massive systems in the Milky Way bulge hosting populations characterized by significantly different ages (Δt > 7-8 Gyr) and metallicities (Δ[Fe/H] ~ 1 dex). Their origin is still strongly debated in the literature and all formation scenarios proposed so far require some level of fine-tuning. The detailed star formation histories of these systems may represent an important piece of information to assess their origin. Here we present the first attempt to perform such an analysis for Liller 1. The first key result we find is that Liller 1 has been forming stars over its entire lifetime. More specifically, three broad star formation episodes are clearly detected: (1) a dominant one, occurring some 12-13 Gyr ago with a tail extending for up to ~3 Gyr; (2) an intermediate burst, between 6 and 9 Gyr ago; and (3) a recent one, occurring between 1 and 3 Gyr ago. The old population contributes to about 70% of the total stellar mass, and the remaining fraction is almost equally split between the intermediate and young populations. If we take these results at face value, they would suggest that this system unlikely formed through the merger between an old globular cluster and a giant molecular cloud, as recently proposed. On the contrary, our findings provide further support to the idea that Liller 1 is the surviving relic of a massive primordial structure that contributed to the Galactic bulge formation, similarly to the giant clumps observed in star-forming high-redshift galaxies.

Keywords: globular star clusters; star clusters; galactic bulge; star formation; photometry
The Evolution of Stellar X-Ray Activity and Angular Momentum as Seen by eROSITA, TESS, and Gaia

We have assembled a sample of ∼8200 stars with spectral types F5V–M5V, all having directly measured X-ray luminosities from eROSITA and rotation periods from TESS and having empirically estimated ages via their membership in stellar clusters and groups identified in Gaia astrometry (ages 3–500 Myr). This is the largest such study sample yet assembled for the purpose of empirically constraining the evolution of rotationally driven stellar X-ray activity. We observe rotation–age–activity correlations that are qualitatively as expected: stars of a given spectral type spin down with age, and they become less X-ray active as they do so. We provide simple functional representations of these empirical relationships that predict X-ray luminosity from basic observables to within 0.3 dex. Interestingly, we find that the rotation–activity relationship is far simpler and more monotonic in form when expressed in terms of stellar angular momentum instead of rotation period. We discuss how this finding may relate to the long-established idea that rotation–activity relationships are mediated by stellar structure (e.g., convective turnover time, surface area). Finally, we provide an empirical relation that predicts stellar angular momentum from basic observables, without requiring a direct measurement of stellar rotation, to within 0.5 dex.

Keywords: stellar rotation; stellar activity; stellar ages
Evolutionary and Observational Properties of Red Giant Acoustic Glitch Signatures

While solar-like oscillations in red giants have been observed at massive scales by the Kepler mission, few features of these oscillation mode frequencies, other than their global properties, have been exploited for stellar characterization. The signatures of acoustic glitches in mode frequencies have been used for studying main-sequence stars, but the validity of applying such techniques to evolved red giants, particularly pertaining to the inclusion of nonradial modes, has been less well examined. Making use of new theoretical developments, we characterize glitches using the π modes associated with red giant stellar models, and use our procedure to examine for the first time how the properties of the He II acoustic glitch-specifically its amplitude and associated acoustic depth-vary over the course of evolution up the red giant branch, and with respect to other fundamental stellar properties. We find that the acoustic depths of these glitches, in conjunction with other spectroscopic information, discriminate between red giants in the first-ascent and core-helium-burning phases. We critically reexamine previous attempts to constrain acoustic glitches from nonradial (in particular dipole) modes in red giants. Finally, we apply our fitting procedure to Kepler data, to evaluate its robustness to noise and other observational systematics.

Keywords: asteroseismology; red giant stars; computational methods
Physical Characterization of Near-Earth Asteroid (52768) 1998 OR2: Evidence of Shock Darkening/Impact Melt

We conducted photometric and spectroscopic characterization of near-Earth asteroid (52768) 1998 OR2 during a close approach to Earth in 2020 April. Our photometric measurements confirm the rotation period of the asteroid to be 4.126 ± 0.179 hr, consistent with the previously published value of 4.112 ± 0.001 hr. By combining our visible spectroscopic measurements (0.45-0.93 μm) with archival MITHNEOS near-infrared spectra (0.78-2.49 μm), we classify the asteroid as an Xn-type in the Bus-DeMeo taxonomy. The combined spectrum shows two weak absorption bands: Band I at 0.926 ± 0.003 μm and Band II at 2.07 ± 0.02 μm, with band depths of 4.5% ± 0.15% and 4.0% ± 0.21%, respectively. The band area ratio is 1.13 ± 0.05. These spectral band parameters plot at the tip of the S(IV) region of the Gaffey S-asteroid subtypes plot, suggesting an affinity to ordinary chondrite meteorites. We calculated the chemistry of olivine and pyroxene using the Band I center to be 20.1 ± 2.3 mol% fayalite and 18.2 ± 1.5 mol% ferrosilite, consistent with H-chondrites. Principal component analysis of 1998 OR2's combined visible-near-IR spectrum falls on the C/X-complex side of the α-line, near the end of the shock-darkening trend, consistent with its weak absorption bands (band depth <5%). We use an areal mixing model with lab measurements of the shock-darkened H5 chondrite, Chergach, to constrain the amount of shock-darkened material on the asteroid's surface at ~63% dark lithology and ~37% light lithology.

Keywords: asteroids; near-earth objects; spectroscopy; meteorites; observational astronomy; solar system astronomy; planetary science
UV Signatures of Magnetar Formation and Their Crucial Role for GW Detection

The emission from shock breakouts (SBOs) represents the earliest electromagnetic (EM) signal emitted by cataclysmic events involving the formation or the merger of neutron stars (NSs). As such, SBOs carry unique information on the structure of their progenitors and on the explosion energy. The characteristic SBO emission is expected in the UV range, and its detection is one of the key targets of the ULTRASAT satellite. Among SBO sources, we focus on a specific class involving the formation of fast-spinning magnetars in the core-collapse of massive stars. Fast-spinning magnetars are expected to produce a specific signature in the early UV supernova light curve, powered by the extra spin energy quickly released by the NS. Moreover, they are considered as optimal candidates for the emission of long-transient gravitational wave (GW) signals, the detection of which requires early EM triggers to boost the sensitivity of dedicated GW search pipelines. We calculate early supernova UV light curves in the presence of a magnetar central engine, as a function of the explosion energy, ejecta mass, and magnetar parameters. We then estimate the ULTRASAT detection horizon (z < 0.15) as a function of the same physical parameters, and the overall expected detection rate, finding that magnetar-powered SBOs may represent up to 1/5 of the total events detected by ULTRASAT. Moreover, at the expected sensitivity of the LIGO/Virgo/Kagra O5 science run, one such event occurring within 5 Mpc will provide an ideal trigger for a GW long-transient search. Future GW detectors like the Einstein Telescope will push the horizon for joint EM-GW detections to 35-40 Mpc.

Keywords: magnetars; near ultraviolet astronomy; core-collapse supernovae
Approximate Bayesian Uncertainties on Deep Learning Dynamical Mass Estimates of Galaxy Clusters

We study methods for reconstructing Bayesian uncertainties on dynamical mass estimates of galaxy clusters using convolutional neural networks (CNNs). We discuss the statistical background of approximate Bayesian neural networks and demonstrate how variational inference techniques can be used to perform computationally tractable posterior estimation for a variety of deep neural architectures. We explore how various model designs and statistical assumptions impact prediction accuracy and uncertainty reconstruction in the context of cluster mass estimation. We measure the quality of our model posterior recovery using a mock cluster observation catalog derived from the MultiDark simulation and UniverseMachine catalog. We show that approximate Bayesian CNNs produce highly accurate dynamical cluster mass posteriors. These model posteriors are log-normal in cluster mass and recover 68% and 90% confidence intervals to within 1% of their measured value. We note how this rigorous modeling of dynamical mass posteriors is necessary for using cluster abundance measurements to constrain cosmological parameters.

Keywords: cosmology; galaxy dynamics; astrostatistics; galaxy clusters
On the Initial Spin Period Distribution of Neutron Stars

We derive the initial spin period distribution of neutron stars by studying the population of young pulsars associated with supernova remnants. Our hierarchical Bayesian approach accounts for the measurement uncertainties of individual observations and selection effects. Without correcting for selection effects, as done in previous studies, we find that pulsar initial spin periods follow a Weibull distribution, peaking at 40 ms, which is favored against a lognormal distribution with a Bayes factor of 200. The known selection effects in radio pulsar surveys, including pulse broadening and period-dependent beaming fraction, have been quantitatively investigated. We show that, based on measurements of pulsar luminosity and spin period from the ATNF Pulsar Catalogue, the impact of pulse broadening on the inference of the pulsar period distribution is likely to be insignificant. Correcting for the beaming selection effect, the Weibull distribution remains the preferred model, while its peak slightly shifts to longer periods at 50 ms. Our method will prove useful in constraining the birth properties of neutron stars in the Square Kilometre Array era.

Keywords: pulsars; neutron stars; supernova remnants
Experimental Simulation of the Volatile Hydrocarbons Generated by the Long-UV Photoprocessing of (C6H6) Ices with Relevance to Titan's Southern Stratospheric Ice Clouds

Ice clouds containing benzene (C6H6) have recently been detected in the stratosphere at the south pole of Titan. Their subsequent aging process induced by long-UV solar photons could lead to a photoreactivity that may release some volatile organic compounds in the gas phase. The characterization of this volatile organic fraction coming from the photoprocessing (λ > 230 nm) of such icy C6H6 has been characterized by a gas chromatograph coupled to a mass spectrometer. Complex molecular diversity is observed through the identification of C3 to C8 photoproducts, which belong to the alkane, alkene, and alkyne families and aromatic derivatives. Thereafter, these hydrocarbons will potentially be transported down to the surface, as condensed ices. Because the energy of solar UV photons decrease with altitude, most of these solid-state hydrocarbons will not be photochemically degraded and may contribute at the end to the organic layer that covers Titan's surface. As these materials would be probed by DraMS, the mass spectrometer on board the future Dragonfly mission, these analyses could serve as benchmarks for future molecule detection on Titan's surface.

Keywords: planetary atmospheres
Possible TeV Gamma-Ray Binary Origin of HESS J1828-099

The High Energy Stereoscopic System (H.E.S.S.) observatory has carried out a deep survey of the Galactic plane, in the course of which the existence of a significant number of (~78) TeV γ-ray sources was confirmed, many of which remain unidentified. HESS J1828-099 is a point-like (Gaussian standard deviation < 0.°07) unidentified source among the 17 confirmed point-like sources in the H.E.S.S. Galactic Plane Survey (HGPS) catalog. This source is also unique because it does not seem to have any apparent association with any object detected at other wavelengths. We investigate the nature and association of HESS J1828-099 with multiwavelength observational data. A high-mass X-ray binary (HMXB)-composed of the pulsar XTE J1829-098 and a companion Be star-has been observed earlier in the X-ray and infrared bands, 14' away from HESS J1828-099. With 12 yr of Fermi-LAT γ-ray data, we explore the possibility of 4FGL J1830.2-1005 being the GeV counterpart of HESS J1828-099. Within the RXTE confidence region, a steep-spectrum (α radio = -0.746 ± 0.284) plausible counterpart is detected in data from existing radio frequency surveys. In this Letter, we probe for the first time, using multiwavelength data, whether HESS J1828-099, 4FGL J1830.2-1005, and the HMXB system have a common origin. Our study indicates that HESS J1828-099 might be a TeV high-mass γ-ray binary source.

Keywords: galactic radio sources; massive stars; high mass x-ray binary stars; gamma-ray sources; high energy astrophysics
Relation of Cyclotron Resonant Energy and Luminosity in a Strongly Magnetized Neutron Star GRO J1008-57 Observed by Insight-HXMT

Cyclotron line scattering features are detected in a few tens of X-ray pulsars (XRPs) and used as direct indicators of a strong magnetic field at the surface of accreting neutron stars (NSs). In a few cases, cyclotron lines are known to be variable with the accretion luminosity of XRPs. It is accepted that the observed variations of cyclotron line scattering features are related to variations of geometry and dynamics of accretion flow above the magnetic poles of an NS. A positive correlation between the line centroid energy and luminosity is typical for subcritical XRPs, where the accretion results in hot spots at the magnetic poles. The negative correlation was proposed to be a specific feature of bright supercritical XRPs, where radiation pressure supports accretion columns above the stellar surface. The cyclotron line in the spectra of the Be-transient XRP GRO J1008-57 is detected at energies from ~75 to 90 keV, the highest observed energy of a cyclotron line feature in XRPs. We report the peculiar relation of cyclotron line centroid energies with luminosity in GRO J1008-57 during the Type II outburst in 2017 August observed by Insight-HXMT. The cyclotron line energy was detected to be negatively correlated with the luminosity at 3.2 × 1037 erg s-1 < L < 4.2 × 1037 erg s-1 and positively correlated at L ≳ 5 × 1037 erg s-1. We speculate that the observed peculiar behavior of a cyclotron line would be due to variations of accretion channel geometry.

Keywords: neutron stars; x-ray binary stars
The First High-contrast Images of Near High-mass X-Ray Binaries with Keck/NIRC2

Although the study of X-ray binaries has led to major breakthroughs in high-energy astrophysics, their circumbinary environment at scales of ∼100–10,000 au has not been thoroughly investigated. In this paper, we undertake a novel and exploratory study by employing direct and high-contrast imaging techniques on a sample of X-ray binaries, using adaptive optics and the vortex coronagraph on Keck/NIRC2. High-contrast imaging opens up the possibility to search for exoplanets, brown dwarfs, circumbinary companion stars, and protoplanetary disks in these extreme systems. Here we present the first near-infrared high-contrast images of 13 high-mass X-ray binaries located within ∼2–3 kpc. The key results of this campaign involve the discovery of several candidate circumbinary companions ranging from substellar (brown dwarf) to stellar masses. By conducting an analysis based on Galactic population models, we discriminate sources that are likely background/foreground stars and isolate those that have a high probability (≳60%–99%) of being gravitationally bound to the X-ray binary. This paper seeks to establish a preliminary catalog for future analyses of proper motion and subsequent observations. With our preliminary results, we calculate the first estimate of the companion frequency and the multiplicity frequency for X-ray binaries: ≈0.6 and 1.8 ± 0.9, respectively, considering only the sources that are most likely bound to the X-ray binary. In addition to extending our comprehension of how brown dwarfs and stars can form and survive in such extreme systems, our study opens a new window to our understanding of the formation of X-ray binaries.

Keywords: multiple stars; x-ray binary stars; near infrared astronomy; high mass x-ray binary stars; coronagraphic imaging; high contrast techniques; direct imaging; exoplanet detection methods; substellar companion stars
Evidence of a Dwarf Galaxy Stream Populating the Inner Milky Way Halo

Stellar streams produced from dwarf galaxies provide direct evidence of the hierarchical formation of the Milky Way. Here, we present the first comprehensive study of the LMS-1 stellar stream, that we detect by searching for wide streams in the Gaia EDR3 data set using the STREAMFINDER algorithm. This stream was recently discovered by Yuan et al. We detect LMS-1 as a 60° long stream to the north of the galactic bulge, at a distance of ~20 kpc from the Sun, together with additional components that suggest that the overall stream is completely wrapped around the inner Galaxy. Using spectroscopic measurements from the Large Sky Area Multi-Object Fiber Spectroscopic Telescope, the Sloan Digital Sky Survey, and the Apache Point Observatory Galactic Evolution Experiment, we infer that the stream is very metal-poor (⟨[Fe/H]⟩ = -2.1) with a significant metallicity dispersion (σ[Fe/H] = 0.4), and it possesses a large radial velocity dispersion (σv = 20 ± 4 km s-1). These estimates together imply that LMS-1 is a dwarf galaxy stream. The orbit of LMS-1 is close to polar, with an inclination of 75° to the galactic plane. Both the orbit and metallicity of LMS-1 are remarkably similar to the globular clusters NGC 5053, NGC 5024, and the stellar stream Indus. These findings make LMS-1 an important contributor to the stellar population of the inner Milky Way halo.

Keywords: milky way stellar halo; stellar streams; milky way formation; surveys; galaxy stellar content; galaxy structure
Ab-initio General-relativistic Neutrino-radiation Hydrodynamics Simulations of Long-lived Neutron Star Merger Remnants to Neutrino Cooling Timescales

We perform the first 3D ab-initio general-relativistic neutrino-radiation hydrodynamics of a long-lived neutron star merger remnant spanning a fraction of its cooling timescale. We find that neutrino cooling becomes the dominant energy loss mechanism after the gravitational-wave dominated phase (~20 ms postmerger). Electron flavor antineutrino luminosity dominates over electron flavor neutrino luminosity at early times, resulting in a secular increase of the electron fraction in the outer layers of the remnant. However, the two luminosities become comparable ~20-40 ms postmerger. A dense gas of electron antineutrinos is formed in the outer core of the remnant at densities ~1014.5 g cm-3, corresponding to temperature hot spots. The neutrinos account for ~10% of the lepton number in this region. Despite the negative radial temperature gradient, the radial entropy gradient remains positive, and the remnant is stably stratified according to the Ledoux criterion for convection. A massive accretion disk is formed from the material squeezed out of the collisional interface between the stars. The disk carries a large fraction of the angular momentum of the system, allowing the remnant massive neutron star to settle to a quasi-steady equilibrium within the region of possible, stable, rigidly rotating configurations. The remnant is differentially rotating, but it is stable against the magnetorotational instability. Other MHD mechanisms operating on longer timescales are likely responsible for the removal of the differential rotation. Our results indicate the remnant massive neutron star is thus qualitatively different from a protoneutron stars formed in core-collapse supernovae.

Keywords: gravitational waves; neutron stars; stellar mergers; stellar structures