Sazuna/UAT_keywords huggingface dataset:
Relativistic Envelopes and Gamma-Rays from Neutron Star Mergers. We suggest that neutron star mergers eject an ultrarelativistic envelope of mass m ∼ 10-7 M⊙, which helps explain the gamma-ray burst from GW170817. One ejection mechanism is the ablation of the neutron star surface by the burst of neutrinos in the first 30 μs of the merger. Another, more efficient mechanism for inflating the ultrarelativistic envelope is an internal shock in the massive ejecta from the merger. A strong shock is expected if the merger product is a magnetar, which emits a centrifugally accelerated wind. The shock propagates outward through the ejecta and accelerates in its outer layers at radii r ∼ 109-1010 cm, launching an ultrarelativistic opaque envelope filled with ∼104 photons per nucleon. The Lorentz factor profile of the envelope rises outward and determines its homologous expansion, which adiabatically cools the trapped photons. Once the magnetar loses its differential rotation and collapses into a black hole, a powerful jet forms. It drives a blast wave into the envelope, chasing its outer layers, and eventually catching up with the envelope photosphere at r ∼ 1012 cm. The ultrarelativistic photospheric breakout of the delayed blast wave emits a gamma-ray burst in a broad solid angle around the merger axis. This model explains the gamma-ray pulse from merger GW170817 with luminosity Lγ ∼ 1047 erg s-1, duration ${\rm{\Delta }}{t}_{\mathrm{obs}}\sim 0.5\,{\rm{s}}$ , and characteristic photon energy ∼100 keV. The blast-wave Lorentz factor at the envelope photosphere is consistent with Γ ≳ 5, which we derive from the observed light curve of the burst. We suggest future tests of the model.

Keywords: Neutron stars, Gamma-ray bursts, Gravitational wave sources, Non-thermal radiation sources, Shocks, Relativistic fluid dynamics
Orbital Evolution of Gas-driven Inspirals with Extreme Mass Ratios: Retrograde Eccentric Orbits. Using two-dimensional simulations, we compute the torque and rate of work (power) on a low-mass gravitational body, with softening length Rsoft, embedded in a gaseous disk when its orbit is eccentric and retrograde with respect to the disk. We explore orbital eccentricities e between 0 and 0.6. We find that the power has its maximum at e ≃ 0.25(h/0.05)2/3, where h is the aspect ratio of the disk. We show that the power and the torque converge to the values predicted in the local (nonresonant) approximation of the dynamical friction (DF) when Rsoft tends to zero. For retrograde inspirals with mass ratios ≲5 × 10-4 embedded in disks with h ≥ 0.025, our simulations suggest that (I) the rate of inspiral barely depends on the orbital eccentricity and (II) the local approximation provides the value of this inspiral rate within a factor of 1.5. The implications of the results for the orbital evolution of extreme mass ratio inspirals are discussed.

Keywords: Supermassive black holes, Hydrodynamical simulations, Galaxy accretion disks, Gravitational interaction, Active galactic nuclei, Compact objects, Tidal interaction, Dynamical friction
Diffuse Ionized Gas in Simulations of Multiphase, Star-forming Galactic Disks. It has been hypothesized that photons from young, massive star clusters are responsible for maintaining the ionization of diffuse warm ionized gas seen in both the Milky Way and other disk galaxies. For a theoretical investigation of the warm ionized medium (WIM), it is crucial to solve radiation-transfer equations where the interstellar medium (ISM) and clusters are modeled self-consistently. To this end, we employ a solar neighborhood model of Three-phase Interstellar Medium in Galaxies Resolving Evolution with Star Formation and Supernova Feedback (TIGRESS), a magnetohydrodynamic simulation of the multiphase, star-forming ISM, and post-process the simulation with an adaptive ray tracing method to transfer UV radiation from star clusters. We find that the WIM volume filling factor is highly variable, and sensitive to the rate of ionizing photon production and ISM structure. The mean WIM volume filling factor rises to ∼0.15 at $| z| \sim 1\,\,\mathrm{kpc}$ . Approximately half of ionizing photons are absorbed by gas and half by dust; the cumulative ionizing photon escape fraction is 1.1%. Our time-averaged synthetic ${\rm{H}}\alpha $ line profile matches Wisconsin ${\rm{H}}\alpha $ Mapper observations on the redshifted (outflowing) side, but has insufficient intensity on the blueshifted side. Our simulation matches the Dickey-Lockman neutral density profile well, but only a small fraction of snapshots have high-altitude WIM density consistent with Reynolds Layer estimates. We compute a clumping correction factor ${{ \mathcal C }}_{{n}_{{\rm{e}}}}\equiv \langle {n}_{{\rm{e}}}\rangle /\langle {n}_{{\rm{e}}}^{2}{\rangle }^{1/2}\sim 0.2$ that is remarkably constant with distance from the midplane and time; this can be used to improve estimates of ionized gas mass and mean electron density from observed ${\rm{H}}\alpha $ surface brightness profiles in edge-on galaxies.

Keywords: Warm ionized medium, Galaxy structure, Radiative transfer, Interstellar medium
Gravitational Microlensing Event Statistics for the Zwicky Transient Facility. Microlensing surveys have discovered thousands of events, with almost all events discovered within the Galactic bulge or toward the Magellanic Clouds. The Zwicky Transient Facility (ZTF), while not designed to be a microlensing campaign, is an optical time-domain survey that observes the entire northern sky, including the Galactic plane, every few nights. The ZTF observes ∼109 stars in the g and r bands and can significantly contribute to the observed microlensing population. We predict that the ZTF will observe ∼1100 microlensing events in 3 yr of observing within 10° latitude of the Galactic plane, with ∼500 events in the outer Galaxy (ℓ ≥ 10°). This yield increases to ∼1400 (∼800) events by combining every three ZTF exposures, ∼1800 (∼900) events if the ZTF observes for a total of 5 yr, and ∼2400 (∼1300) events for a 5 yr survey with postprocessing image stacking. Using the microlensing modeling software PopSyCLE, we compare the microlensing populations in the Galactic bulge and the outer Galaxy. We also present an analysis of the microlensing event ZTF18abhxjmj to demonstrate how to leverage these population statistics in event modeling. The ZTF will constrain Galactic structure, stellar populations, and primordial black holes through photometric microlensing.

Keywords: Milky Way dark matter halo, Astronomy data modeling, Bayesian statistics, Black holes, Stellar populations, Transient sources, Galactic bulge, Stellar astronomy, Milky Way disk, Time domain astronomy, Gravitational microlensing, Microlensing event rate
Classification of Photospheric Emission in Short GRBs. In order to better understand the physical origin of short-duration gamma-ray bursts (GRBs), we perform a time-resolved spectral analysis on a sample of 70 pulses in 68 short GRBs with burst durations T90 ≲ 2 s detected by the Fermi/Gamma-ray Burst Monitor. We apply a Bayesian analysis to all spectra that have statistical significance S ≥ 15 within each pulse and apply a cutoff power-law model. We then select in each pulse the time bin that has the maximum value of the low-energy spectral index for further analysis. Under the assumption that the main emission mechanism is the same throughout each pulse, this analysis is indicative of pulse emission. We find that about 1/3 of the short GRBs are consistent with a pure, nondissipative photospheric model, at least around the peak of the pulse. This fraction is larger than the corresponding fraction (1/4) obtained for long GRBs. For these bursts, we find (I) a bimodal distribution in the values of the Lorentz factors and the hardness ratios and (II) an anticorrelation between T90 and the peak energy, Epk: ${T}_{90}\propto {E}_{\mathrm{pk}}^{-0.50\pm 0.19}$ . This correlation disappears when we consider the entire sample. Our results thus imply that the short GRB population may in fact be composed of two separate populations: one that is a continuation of the long GRB population to shorter durations, and another that is distinctly separate with different physical properties. Furthermore, thermal emission is initially ubiquitous, but is accompanied at longer times by additional radiation (likely synchrotron).

Keywords: Gamma-ray astronomy, Gamma-ray bursts, Astronomy data analysis
The Multiwavelength Counterparts of Fast Radio Bursts. The engines that produce extragalactic fast radio bursts (FRBs), and the mechanism by which the emission is generated, remain unknown. Many FRB models predict prompt multiwavelength counterparts, which can be used to refine our knowledge of these fundamentals of the FRB phenomenon. However, several previous targeted searches for prompt FRB counterparts have yielded no detections and have additionally not reached sufficient sensitivity with respect to the predictions. In this work, we demonstrate a technique to estimate the ratio, η, between the energy outputs of FRB counterparts at various wavelengths and the radio-wavelength emission. Our technique combines the fluence distribution of the FRB population with results from several wide-field blind surveys for fast transients from the optical to the TeV bands. We present constraints on η that improve upon previous observations even in the case where all unclassified transient events in existing surveys are FRB counterparts. In some scenarios for the FRB engine and emission mechanism, we find that FRB counterparts should have already been detected, thus demonstrating that our technique can successfully test predictions for η. However, it is possible that FRB counterparts are lurking among catalogs of unclassified transient events. Although our technique is robust to the present uncertainty in the FRB fluence distribution, its ultimate application to accurately estimate or bound η will require the careful analysis of all candidate fast transient events in multiwavelength survey data sets.

Keywords: Neutron stars, Radio bursts, Radio transient sources
Toward a General Parameterization of the Warm Dark Matter Halo Mass Function. Studies of flux anomalies statistics and perturbations in stellar streams have the potential to constrain models of warm dark matter (WDM), including sterile neutrinos. Producing these constraints requires a parameterization of the WDM mass function relative to that of the cold dark matter (CDM) equivalent. We use five WDM models with half-mode masses, Mhm = [1.3,35] × 108 ${M}_{\odot }$ , spread across simulations of the Local Group, lensing ellipticals and the z = 2 universe, to generate such a parameterization: we fit parameters to a functional form for the WDM-to-CDM halo mass function ratio, ${n}_{\mathrm{WDM}}({M}_{X})/{n}_{\mathrm{CDM}}({M}_{X})$ , of ${\left(1+{(\alpha {M}_{\mathrm{hm}}/{M}_{X})}^{\beta }\right)}^{\gamma }$ . For MX ≡ virial mass of central halos we obtain α = 2.3, β = 0.8, and γ = -1.0, and this fit is steeper than the extended Press-Schechter formalism predicts. For MX ≡ mass of subhalos we instead obtain α = 4.2, β = 2.5, and γ = -0.2; in both mass definitions the scatter is ∼20%. The second fit typically underestimates the relative abundance of z = 2 WDM subhalos at the tens of percent level. We caution that robust constraints will require bespoke simulations and a careful definition of halo mass, particularly for subhalos of mass &lt;108 ${M}_{\odot }$ .

Keywords: Warm dark matter
Verification of Radiative Transfer Schemes for the EHT. The Event Horizon Telescope (EHT) Collaboration has recently produced the first resolved images of the central supermassive black hole in the giant elliptical galaxy M87. Here we report on tests of the consistency and accuracy of the general relativistic radiative transfer codes used within the collaboration to model M87* and Sgr A*. We compare and evaluate (1) deflection angles for equatorial null geodesics in a Kerr spacetime; (2) images calculated from a series of simple, parameterized matter distributions in the Kerr metric using simplified emissivities and absorptivities; (3) for a subset of codes, images calculated from general relativistic magnetohydrodynamics simulations using different realistic synchrotron emissivities and absorptivities; (4) observables for the 2017 configuration of EHT, including visibility amplitudes and closure phases. The error in total flux is of order 1% when the codes are run with production numerical parameters. The dominant source of discrepancies for small camera distances is the location and detailed setup of the software "camera" that each code uses to produce synthetic images. We find that when numerical parameters are suitably chosen and the camera is sufficiently far away the images converge and that for given transfer coefficients, numerical uncertainties are unlikely to limit parameter estimation for the current generation of EHT observations. The purpose of this paper is to describe a verification and comparison of EHT radiative transfer codes. It is not to verify EHT models more generally.

Keywords: Black hole physics, High energy astrophysics, Relativity, Very long baseline interferometry, General relativity, Radiative transfer, Event horizons, Radio astronomy, Relativistic disks
Spatially Quasi-periodic Finger-like Auroras during Substorms. North-south-aligned discrete auroras are common phenomena during substorms. A quasi-periodic form of such auroras distributed azimuthally in the nightside resembles a finger-like morphology. They were noticed in sawtooth-time substorms, but have not yet been paid much attention during other geomagnetic times. In this study, we investigate the longitudinal periodicity of those auroras for different types of substorms using IMAGE data from 2001 to 2003. Using an automated detection algorithm, we identified multiple north-south-aligned thin auroral structures in auroral images and found that their average center and length in latitude, and longitudinal wavelength (∼25°) are essentially the same. However, the occurrent rates for finger-like auroras are substantially different, 83%, 64%, and 18% for sawtooth-time substorms, storm-time substorms and isolated substorms, respectively. Therefore, statistical results indicate that finger-like auroras occur in favor of geomagnetically more active times. Combining our data analysis and previous simulations, we suggest that interchange instability is a possible mechanism of spatially quasi-periodic finger-like auroras when a substorm injection happens in a very wide local time. It is particularly important to point out that the sawtooth events not only have a temporal periodicity as defined in particle injections, but also have a spatial periodicity as observed in auroral morphology for the majority of them.

Keywords: Space plasmas, Magnetohydrodynamics, Magnetic fields
Inclination Dependence of Kilonova Light Curves from Globally Aspherical Geometries. The merger of two neutron stars (NSs) or an NS and a black hole (BH) produces a radioactively powered transient known as a kilonova, first observed accompanying the gravitational wave event GW170817. While kilonovae are frequently modeled in spherical symmetry, the dynamical ejecta and disk outflows can be considerably asymmetric. We use Monte Carlo radiative transfer calculations to study the light curves of kilonovae with globally axisymmetric geometries (e.g., an ellipsoid and a torus). We find that the variation in luminosity in these models is most pronounced at early times and decreases until the light curves become isotropic in the late optically thin phase. The light-curve shape and peak time are not significantly modified by the global asymmetry. We show that the projected surface area along the line of sight captures the primary geometric effects, and we use this fact to provide a simple analytic estimate of the direction-dependent light curves of the aspherical ejecta. For the kilonova accompanying GW170817, accounting for asymmetry with an oblate (prolate) ellipsoid of axial ratio 2 (1/2) leads to an ∼40% decrease (increase) in the inferred ejecta mass compared to the spherical case. The pole-to-equator orientation effects are expected to be significantly larger (a factor of ∼5-10) for the more extreme asymmetries expected for some NS-BH mergers.

Keywords: Neutron stars, Radiative transfer simulations, Transient sources, High energy astrophysics
Evidence for a Rotational Component in the Circumgalactic Medium of Nearby Galaxies. We present results of a study comparing the relative velocity of Lyα absorbers to the rotation velocity of nearby galaxy disks in the local universe (z ≤ 0.03). We have obtained rotation curves via long-slit spectroscopy of eight galaxies with the Southern African Large Telescope, and combine this data set with an additional 16 galaxies with data from the literature. Each galaxy appears within 3Rvir of a QSO sightline with archival Cosmic Origin Spectrograph spectra. We study the velocity orientation of absorbers with respect to nearby galaxy's rotation, and compare with results from both the Steidel et al. monolithic halo model and a new cylindrical Navarro-Frenk-White galaxy halo model to interpret these data in the context of probing 3D galaxy halos via 1D QSO absorption-line spectroscopy. Relative to these models, we find that up to 59% ± 5% of Lyα absorbers have velocities consistent with corotation. We find the Lyα corotation fraction to decrease with galaxy luminosity (L*) and impact parameter in a model-independent fashion. We report that both anti-rotating absorbers and those found near luminous galaxies (L ≳ 0.5L*) mostly have low Doppler b-parameters (b ≲ 50 km s-1). Absorbers consistent with corotation show a wide range of Doppler b-parameters. Finally, we find a strong anticorrelation between corotation fraction and galaxy inclination, which is at odds with recent metal-line kinematic studies and suggests the kinematic and geometric distribution of the circumgalactic medium is complex and multiphase. * Based on observations made with the Southern African Large Telescope (SALT).

Keywords: Galaxy formation, Quasar absorption line spectroscopy, Intergalactic medium, Galaxy evolution, Galaxy accretion, Circumgalactic medium, Extragalactic astronomy, Galaxy rotation curves
Gas Dynamics of the Nickel-56 Decay Heating in Pair-instability Supernovae. Very massive 140-260 M⊙ stars can die as highly energetic pair-instability supernovae (PI SNe) with energies of up to 100 times those of core-collapse SNe that can completely destroy the star, leaving no compact remnant behind. These explosions can synthesize 0.1-30 M⊙ of radioactive 56Ni, which can cause them to rebrighten at later times when photons due to 56Ni decay diffuse out of the ejecta. However, heat from the decay of such large masses of 56Ni could also drive important dynamical effects deep in the ejecta that are capable of mixing elements and affecting the observational signatures of these events. We have now investigated the dynamical effect of 56Ni heating on PI SN ejecta with high-resolution two-dimensional hydrodynamic simulations performed with the CASTRO code. We find that expansion of the hot 56Ni bubble forms a shell at the base of the silicon layer of the ejecta ∼200 days after the explosion but that no hydrodynamical instabilities develop that would mix 56Ni with the 28Si/16O-rich ejecta. However, while the dynamical effects of 56Ni heating may be weak they could affect the observational signatures of some PI SNe by diverting decay energy into internal expansion of the ejecta at the expense of rebrightening at later times.

Keywords: Hydrodynamical simulations, Supernova dynamics, Population III stars, Hypernovae, Massive stars, Shocks, Nuclear astrophysics, Astrophysical explosive burning, Astrophysical fluid dynamics, Supernovae
Spicule Jets in the Solar Atmosphere Modeled with Resistive MHD and Thermal Conduction. Using numerical simulations, we study the effects of magnetic resistivity and thermal conductivity in the dynamics and properties of solar jets with characteristics of Type II spicules and cool coronal jets. The dynamic evolution of the jets is governed by the resistive MHD equations with thermal conduction along the magnetic field lines on a 2.5D slice. The magnetic field configuration consists of two symmetric neighboring loops with opposite polarity, used to support reconnection and followed by the plasma jet formation. In total, 10 simulations were carried out with different values of resistivity and thermal conductivity that produce jets with different morphological and thermal properties we quantify. We find that an increase in magnetic resistivity does not produce significant effects on the morphology, velocity, and temperature of the jets. However, thermal conductivity affects both temperature and morphology of the jets. In particular, thermal conductivity causes jets to reach greater heights and increases the temperature of the jet-apex. Also, heat flux maps indicate the jet-apex and corona interchange energy more efficiently than the body of the jet. These results could potentially open a new avenue for plasma diagnostics in the Sun's atmosphere.

Keywords: Solar spicules, Solar atmosphere, Solar physics, Solar chromosphere
A Comparative Study of Long and Short GRBs. II. A Multiwavelength Method to Distinguish Type II (Massive Star) and Type I (Compact Star) GRBs. Gamma-ray bursts (GRBs) are empirically classified as long-duration GRBs (LGRBs, &gt;2 s) and short-duration GRBs (SGRBs, &lt;2 s). Physically they can be grouped into two distinct progenitor categories: those originating from collapse of massive stars (also known as Type II) and those related to mergers of compact stars (also known as Type I). Even though most LGRBs are Type II and most SGRBs are Type I, the duration criterion is not always reliable to determine the physical category of a certain GRB. Based on our previous comprehensive study of the multiwavelength properties of long and short GRBs, here we utilize the naive Bayes method to physically classify GRBs as Type I and Type II GRBs based on multiwavelength criteria. It results in a 0.5% training error rate and a 1% test error rate. Moreover, there is a gap [-1.2, -0.16] in the distribution of the posterior odds, $\mathrm{log}O(\mathrm{II}:{\rm{I}})$ , the Type II to Type I probability ratio. Therefore, we propose to use ${ \mathcal O }=\mathrm{log}O(\mathrm{II}:{\rm{I}})+0.7$ as the parameter to classify GRBs into Type I (&lt;0) or Type II (&gt;0). The only confirmed Type I GRB, GRB 170817A, has log $O(\mathrm{II}:{\rm{I}})=-10$ . According to this criterion, the supernova-less long GRBs 060614 and 060505 belong to Type I, and two controversial short GRBs 090426 and 060121 belong to Type II.

Keywords: Gamma-ray bursts