adsabs/SciX_UAT_keywords huggingface dataset:
Searches for Modulated γ-Ray Precursors to Compact Binary Mergers in Fermi-GBM Data

GW170817 is the only gravitational-wave event for which a confirmed γ-ray counterpart, GRB 170817A, has been detected. Here, we present a method to search for another type of γ-ray signal, a γ-ray burst precursor, associated with a compact binary merger. If emitted shortly before the coalescence, a high-energy electromagnetic (EM) flash travels through a highly dynamical and relativistic environment, created by the two compact objects orbiting each other. Thus, the EM signal arriving at an Earth observer could present a somewhat predictable time-dependent modulation. We describe a targeted search method for light curves exhibiting such a modulation, parameterized by the observer-frame component masses and binary merger time, using Fermi-GBM data. The sensitivity of the method is assessed based on simulated signals added to GBM data. The method is then applied to a selection of potentially interesting compact binary mergers detected during the second (O2) and third (O3) observing runs of Advanced LIGO and Advanced Virgo. We find no significant modulated γ-ray precursor signal associated with any of the considered events.

Keywords: neutron stars; black holes; compact binary stars
Are Coronal Loops Projection Effects?

We report results of an in-depth numerical investigation of three-dimensional projection effects that could influence the observed loop-like structures in an optically thin solar corona. Several archetypal emitting geometries are tested, including collections of luminous structures with circular cross sections of fixed and random size, and light-emitting structures with highly anisotropic cross sections, as well as two-dimensional stochastic current density structures generated by fully developed magnetohydrodynamic turbulence. A comprehensive set of statistical signatures is used to compare the line-of-sight (LOS) integrated emission signals predicted by the constructed numerical models with the loop profiles observed by the extreme ultraviolet telescope on board the flight 2.1 of the High-Resolution Coronal Imager (Hi-C). The results suggest that typical cross-sectional emission envelopes of the Hi-C loops are unlikely to have high eccentricity, and that the observed loops cannot be attributed to randomly oriented quasi-two-dimensional emitting structures, some of which would produce anomalously strong optical signatures due to an accidental LOS alignment, as expected in the ''coronal veil" scenario proposed recently by Malanushenko et al. The possibility of apparent loop-like projections of very small (close to the resolution limit) or very large (comparable with the size of an active region) light-emitting sheets remains open, but the intermediate range of scales commonly associated with observed loop systems is most likely filled with true quasi-one-dimensional (roughly axisymmetric) structures embedded into the three-dimensional coronal volume.

Keywords: solar coronal loops; solar coronal heating; solar extreme ultraviolet emission; astronomy image processing
Emergent Nucleosynthesis from a 1.2 s Long Simulation of a Black Hole Accretion Disk

We simulate a black hole accretion disk system with full-transport general relativistic neutrino radiation magnetohydrodynamics for 1.2 s. This system is likely to form after the merger of two compact objects and is thought to be a robust site of r-process nucleosynthesis. We consider the case of a black hole accretion disk arising from the merger of two neutron stars. Our simulation time coincides with the nucleosynthesis timescale of the r-process (∼1 s). Because these simulations are time-consuming, it is common practice to run for a "short" duration of approximately 0.1–0.3 s. We analyze the nucleosynthetic outflow from this system and compare the results of stopping at 0.12 and 1.2 s. We find that the addition of mass ejected in the longer simulation as well as more favorable thermodynamic conditions from emergent viscous ejecta greatly impacts the nucleosynthetic outcome. We quantify the error in nucleosynthetic outcomes between short and long cuts.

Keywords: r-process; nucleosynthesis; nuclear astrophysics; nuclear physics; magnetohydrodynamical simulations; stellar accretion disks
Reconciling X-Ray and λ21 cm H I Absorption Gas Column Densities toward Obscured AGN

Hydrogen column densities inferred from X-ray absorption are typically 5-30 times larger than the neutral atomic hydrogen column densities derived from λ21 cm H I absorption toward radio-loud active galactic nuclei (AGN). Some part of the difference is ascribed to uncertainty in the spin temperature Tsp = 100 K that is often used to convert λ21 cm H I absorption to N(H I). Here we propose another way to infer the gas column from H I absorption. In our Galaxy there is a nearly linear correlation between the inteferometrically measured integrated λ21 cm absorption ϒH I and reddening, ϒH I ∝ E(B-V)1.10 for ϒH I ≳ 0.7 km s-1 or E(B-V) ≳ 0.04 mag. Scaling E(B-V) then provides the total gas column density N(H) from the same dust column that is responsible for optical obscuration and X-ray absorption, without calculating N(H I). Values of N(H) so derived typically exceed N(H I) by a factor 4 because the ubiquitous Galactic λ21 cm H I absorption samples only a portion of the interstellar gas. If the well-studied case of Hydra-A is a guide, even very large disparities in X-ray and λ21 cm gas column densities can be explained by resolving the core radio continuum and inferring N(H) from λ21 cm absorption. Milky Way conditions are often invoked in discussion of obscured AGN, so the empirical relationship seen in the Milky Way should be a relevant benchmark.

Keywords: interstellar medium; interstellar dust extinction; interstellar atomic gas
Risks for Life on Proxima b from Sterilizing Impacts

We consider the implications that a debris belt located between Proxima b and Proxima c would pose for the rate of large asteroid impacts that could sterilize life from Proxima b. Future observations by the Atacama Large Millimeter/submillimeter Array or the James Webb Space Telescope could constrain the existence of an asteroid belt in the life-threatening regime. We generalize our rate calculation of sterilizing impacts for habitable planets in systems with an asteroid belt and an outer planet.

Keywords: m dwarf stars; habitable planets; debris disks; asteroid belt; impact phenomena
Destruction of Refractory Carbon Grains Drives the Final Stage of Protoplanetary Disk Chemistry

Here we aim to explore the origin of the strong C2H lines to reimagine the chemistry of protoplanetary disks. There are a few key aspects that drive our analysis. First, C2H is detected in young and old systems, hinting at a long-lived chemistry. Second, as a radical, C2H is rapidly destroyed, within <1000 yr. These two statements hint that the chemistry responsible for C2H emission must be predominantly in the gas phase and must be in equilibrium. Combining new and published chemical models, we find that elevating the total volatile (gas and ice) C/O ratio is the only natural way to create a long-lived, high C2H abundance. Most of the C2H resides in gas with an FUV/ngas ∼ 10-7 G0 cm3. To elevate the volatile C/O ratio, additional carbon has to be released into the gas to enable equilibrium chemistry under oxygen-poor conditions. Photoablation of carbon-rich grains seems the most straightforward way to elevate the C/O ratio above 1.5, powering a long-lived equilibrium cycle. The regions at which the conditions are optimal for the presence of high C/O ratio and elevated C2H abundances in the gas disk set by the FUV/ngas condition lie just outside the pebble disk as well as possibly in disk gaps. This process can thus also explain the (hints of) structure seen in C2H observations.

Keywords: astrochemistry; chemical abundances; protoplanetary disks
Bayesian Solar Wind Modeling with Pulsar Timing Arrays

Using Bayesian analyses we study the solar electron density with the NANOGrav 11 yr pulsar timing array (PTA) data set. Our model of the solar wind is incorporated into a global fit starting from pulse times of arrival. We introduce new tools developed for this global fit, including analytic expressions for solar electron column densities and open source models for the solar wind that port into existing PTA software. We perform an ab initio recovery of various solar wind model parameters. We then demonstrate the richness of information about the solar electron density, n E , that can be gleaned from PTA data, including higher order corrections to the simple 1/r 2 model associated with a free-streaming wind (which are informative probes of coronal acceleration physics), quarterly binned measurements of n E and a continuous time-varying model for n E spanning approximately one solar cycle period. Finally, we discuss the importance of our model for chromatic noise mitigation in gravitational-wave analyses of pulsar timing data and the potential of developing synergies between sophisticated PTA solar electron density models and those developed by the solar physics community.

Keywords: solar wind; pulsar timing method; radio pulsars; millisecond pulsars; gravitational waves
Tidal Circularization of Binaries by Resonance Locking. I. The Importance of the Pre-main Sequence

Although tidal dissipation in binary stars has been studied for over a century, theoretical predictions have yet to match the observed properties of binary populations. This work quantitatively examines the recent proposal of tidal circularization by resonance locking, where tidal dissipation arises from resonances between the star's natural oscillation frequencies and harmonics of the orbital frequency, and where resonances are "locked" for an extended period of time due to concurrent stellar evolution. We focus on tidal resonances with axi-symmetric gravity modes, and examine binaries with primary masses from one to two solar masses. We find that orbital evolution via resonance locking occurs primarily during the star's pre-main-sequence phase, with the main-sequence phase contributing negligibly. Resonance locking, ignoring nonlinearity, can circularize binaries with peri-center distances out to ∼10 stellar radii, corresponding to circular periods of ∼4-6 days. However, we find resonantly excited gravity modes will become nonlinear in stellar cores, which prevents them from reaching their full, linear amplitudes. We estimate that such a "saturated resonance lock" reduces the circularization period by about a third, but resonance locking remains much more effective than the cumulative actions of equilibrium tides. In a companion paper, we examine recent binary data to compare against theory.

Keywords: binary stars; tides; stellar oscillations; young stellar objects; stellar evolution
Discovery of Tidal Tails around the Old Open Cluster NGC 2506

We report the discovery of tidal tails around the old open cluster NGC 2506 based on Gaia-DR2. A principal-component analysis and Gaussian mixture model are used to identify extra-tidal stars. We identify 2301 stars closely related to the cluster, 147 of which are likely extra-tidal stars. We find clear evidence for the existence of two well-defined tidal tails emerging from the cluster. The tidal tails extend out to projected distances of at least 54 pc, which is nearly three times as large as the tidal radius of the cluster. Furthermore, we find that the extra-tidal stars have a wide mass spectrum (∼0.6-1.4 M⊙), which cannot be completely explained by mass segregation and the preferential evaporation of low-mass stars. An extra dynamical process such as disk-shocking may also play an important role in forming these extra-tidal stars. A mass-loss rate of ∼5.3 M⊙ Myr-1 is estimated for the cluster. This reveals that NGC 2506 is currently undergoing heavy mass-loss due to star escapes.

Keywords: open star clusters; parallax; proper motions; stellar kinematics; hertzsprung russell diagram
Numerical Convergence of 2D Solar Convection in Implicit Large-eddy Simulations

Large-eddy simulations (LES) and implicit LES (ILES) are wise and affordable alternatives to the unfeasible direct numerical simulations of turbulent flows at high Reynolds (Re) numbers. However, for systems with few observational constraints, it is a formidable challenge to determine if these strategies adequately capture the physics of the system. Here, we address this problem by analyzing numerical convergence of ILES of turbulent convection in 2D, with resolutions between 642 and 20482 grid points, along with the estimation of their effective viscosities, resulting in effective Reynolds numbers between 1 and ~104. The thermodynamic structure of our model resembles the solar interior, including a fraction of the radiative zone and the convection zone. In the convective layer, the ILES solutions converge for the simulations with ≥5122 grid points, as evidenced by the integral properties of the flow and its power spectra. Most importantly, we found that even a resolution of 1282 grid points, $\mathrm{Re}\,\sim \,10$ , is sufficient to capture the dynamics of the large scales accurately. This is a consequence of the ILES method allowing the energy contained in these scales to be the same in simulations with low and high resolution. Special attention is needed in regions with a small density scale height driving the formation of fine structures unresolved by the numerical grid. In the stable layer, we found the excitation of internal gravity waves, yet high resolution is needed to capture their development and interaction.

Keywords: solar convective zone; hydrodynamical simulations; computational methods
Ultra-short-period Planets Are Stable against Tidal Inspiral

It has been unambiguously shown both in individual systems and at the population level that hot Jupiters experience tidal inspiral before the end of their host stars' main-sequence lifetimes. Ultra-short-period (USP) planets have orbital periods P < 1 day, rocky compositions, and are expected to experience tidal decay on similar timescales to hot Jupiters if the efficiency of tidal dissipation inside their host stars parameterized as ${Q}_{* }^{{\prime} }$ is independent of P and/or secondary mass Mp. Any difference between the two classes of systems would reveal that a model with constant ${Q}_{* }^{{\prime} }$ is insufficient. If USP planets experience tidal inspiral, then USP planet systems will be relatively young compared to similar stars without USP planets. Because it is a proxy for relative age, we calculate the Galactic velocity dispersions of USP planet candidate host and non-host stars using data from Gaia Data Release 2 supplemented with ground-based radial velocities. We find that main-sequence USP planet candidate host stars have kinematics consistent with similar stars in the Kepler field without observed USP planets. This indicates that USP planet hosts have similar ages to field stars and that USP planets do not experience tidal inspiral during the main-sequence lifetimes of their host stars. The survival of USP planets requires that ${Q}_{* }^{{\prime} }$ ≳ 107 at P ≈ 0.7 day and ${M}_{{\rm{p}}}\approx 2.6\,{M}_{\oplus }$ . This result demands that ${Q}_{* }^{{\prime} }$ depend on the orbital period and/or mass of the secondary in the range $0.5\,\mathrm{day}\lesssim P\lesssim 5$ days and $1\,{M}_{\oplus }\lesssim {M}_{{\rm{p}}}\lesssim 1000\,{M}_{\oplus }$ .

Keywords: exoplanet dynamics; exoplanet evolution; exoplanet tides; stellar ages; tidal interaction; exoplanets
Highly Coherent Quasiperiodic Oscillations in the "Heartbeat" Black Hole X-Ray Binary IGR J17091–3624

IGR J17091–3624 is a black hole X-ray binary (BHXB), often referred to as the "twin" of GRS 1915+105 because it is the only other known BHXB that can show exotic "heartbeat"-like variability that is highly structured and repeated. Here, we report on observations of IGR J17091–3624 from its 2022 outburst, where we detect an unusually coherent quasiperiodic oscillation (QPO) when the broadband variability is low (total fractional rms ≲6%) and the spectrum is dominated by the accretion disk. Such spectral and variability behavior is characteristic of the soft state of typical BHXBs (i.e., those that do not show heartbeats), but we also find that this QPO is strongest when there is some exotic heartbeat-like variability (so-called Class V variability). This QPO is detected at frequencies between 5 and 8 Hz and has Q factors (defined as the QPO frequency divided by the width) ≳50, making it one of the most highly coherent low-frequency QPOs ever seen in a BHXB. The extremely high Q factor makes this QPO distinct from typical low-frequency QPOs that are conventionally classified into type-A/B/C QPOs. Instead, we find evidence that archival observations of GRS 1915+105 also showed a similarly high-coherence QPO in the same frequency range, suggesting that this unusually coherent and strong QPO may be unique to BHXBs that can exhibit "heartbeat"-like variability.

Keywords: astrophysical black holes; stellar mass black holes; accretion
The Morphology and Dynamics of Relativistic Jets with Relativistic Equation of State

We study the effect of plasma composition on the dynamics and morphology of relativistic astrophysical jets. Our work is based on a relativistic total variation diminishing simulation code. We use a relativistic equation of state in the simulation code that accounts for the thermodynamics of a multispecies plasma, which is a mixture of electrons, positrons, and protons. To study the effect of plasma composition, we consider various jet models. These models are characterized by the same injection parameters, same jet kinetic luminosity, and the same Mach numbers. The evolution of these models shows that the plasma composition affects the propagation speed of the jet head, the structure of the jet head, and the morphology, despite fixing the initial parameters. We conclude that electron-positron jets are the slowest and show more pronounced turbulent structures in comparison to other plasma compositions. The area and locations of the hot-spots also depend on the composition of the jet plasma. Our results also show that boosting mechanisms are an important aspect of multi-dimensional simulations, which are also influenced by the change in composition.

Keywords: relativistic jets; relativistic fluid dynamics
Thermal Comptonization in a Changing Corona in the Changing-look Active Galaxy NGC 1566

We present broadband UV/X-ray spectral variability of the changing-look active galactic nucleus (AGN) NGC 1566, based on simultaneous near-ultraviolet and X-ray observations performed by the XMM-Newton, Swift, and NuSTAR satellites at five different epochs during the declining phase of the 2018 outburst. We found that the accretion disk, soft X-ray excess, and X-ray power-law components were extremely variable. Additionally, the X-ray power-law flux was correlated with both the soft excess plus disk and the pure disk fluxes. Our finding shows that at high-flux levels the soft X-ray excess and the disk emission both provided the seed photons for thermal Comptonization in the hot corona, whereas at low-flux levels, where the soft excess was absent, the pure disk emission alone provided the seed photons. The X-ray power-law photon index was only weakly variable (ΔΓhot ≤ 0.06), and it was not well correlated with the X-ray flux over the declining timescale. On the other hand, we found that the electron temperature of the corona increased from ~22 to ~200 keV with the decreasing numbers of seed photons from 2018 June to 2019 August. At the same time, the optical depth of the corona decreased from τ hot ~ 4 to ~0.7, and the scattering fraction increased from ~1% to ~10%. These changes suggest structural changes in the hot corona, such as it was growing in size and becoming hotter with the decreasing accretion rate during the declining phase. The AGN is most likely evolving with a decreasing accretion rate toward a state similar to the low/hard state of black hole X-ray binaries.

Keywords: active galactic nuclei; high energy astrophysics
Discovery of Stellar Streams around NGC 3614

In the course of surveying spiral galaxies in the Local Volume, long exposures of NGC 3416 show two probable stellar streams with the possible remnant of a satellite galaxy. I captured the discovery image using the Schulman Telescope at Steward Observatory's Mount Lemmon Sky Center (University of Arizona). I acquired the wide bandpass data over three nights under photometric conditions in 2015 February. Prominent theories of galaxy formation hold that the creation of present-day galaxies are the aggregation of many past minor mergers. Stellar streams lend credence to the idea by presenting evidence of extended low surface brightness tidally created signatures. NGC 3614 appears to be a good example the for kind of extended features expected by these theories.

Keywords: spiral galaxies; hierarchical cosmology; stellar streams
The Role of a Neutron Component in the Photospheric Emission of Long-duration Gamma-Ray Burst Jets

Long-duration gamma-ray bursts (LGRBs), thought to be produced during core-collapse supernovae, may have a prominent neutron component in the outflow material. If present, neutrons can change how photons scatter in the outflow by reducing its opacity, thereby allowing the photons to decouple sooner than if there were no neutrons present. Understanding the details of this process could therefore allow us to probe the central engine of LGRBs, which is otherwise hidden. Here, we present results of the photospheric emission from an LGRB jet, using a combination of relativistic hydrodynamic simulations and radiative transfer postprocessing using Monte Carlo radiation transfer code. We control the size of the neutron component in the jet material by varying the equilibrium electron fraction Y e , and we find that the presence of neutrons in the GRB fireball affects the Band parameters α and E 0, while the picture with the β parameter is less clear. In particular, the break energy E 0 is shifted to higher energies. Additionally, we find that increasing the size of the neutron component also increases the total radiated energy of the outflow across multiple viewing angles. Our results not only shed light on LGRBs but are also relevant to short-duration gamma-ray bursts associated with binary neutron star mergers due to the likelihood of a prominent neutron component in such systems.

Keywords: gamma-ray bursts; radiative transfer simulations; hydrodynamical simulations
Mapping the 3D Kinematical Structure of the Gas Disk of HD 169142

The disk around HD 169142 has been suggested to host multiple embedded planets due to the range of structures observed in the dust distributions. We analyze archival Atacama Large (sub-) Millimetre Array observations of 12CO (2-1), 13CO (2-1), and C18O (2-1) to search for large-scale kinematic structures associated with other embedded planets in the outer disk. At 125 au, we identify a coherent flow from the disk surface to the midplane, traced by all three CO isotopologues, and interpret it as a meridional flow potentially driven by an embedded planet. We use changes in the rotation speed of the gas to characterize the physical structure across this region, finding that at 125 au the CO emission traces regions of increased gas pressure, despite being at a surface density minimum. Developing a simple analytical model, we demonstrate that the physical structure of the gap can have non-trivial responses to changes in the surface density, consistent both with previous thermo-chemical models and the conditions inferred observationally. Applying this technique to a range of sources will allow us to directly confront theoretical models of gap-opening in protoplanetary disks.

Keywords: protoplanetary disks; submillimeter astronomy; planetary-disk interactions