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<jats:title>Abstract</jats:title> <jats:p>Quality metrics for Spitzer–Härm and Grad closures are presented based on the percentage of the heat flux moment supported only by nonnegative, physical, phase space densities <jats:inline-formula> <jats:tex-math> <?CDATA ${\mathbb{F}}\gt 0$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabc475ieqn1.gif" xlink:type="simple" /> </jats:inline-formula> underlying the closure. The Spitzer and Grad qualities exceed 95% for the perturbative regimes where Spitzer’s formulation is analytically known to be convergent. Beyond this regime both heat flux qualities fall about 30% per decade increase of <jats:italic>ϵ</jats:italic> &gt; 0.01. In the solar corona the first decade’s decrease in quality straddles the radius of the coronal temperature maximum and spans the initial acceleration of the solar wind. By the end of the second decade of increase of <jats:italic>ϵ</jats:italic> the observer is between 5 and 10<jats:italic>R</jats:italic> <jats:sub>⊙</jats:sub>, already in conditions comparable to those at 1 au with ≃60% degradation of quality. These strong radial decays of closure quality show that integrating the fluid equations using such closures must represent a very poor assay of the role and effects of ∇ · <jats:bold>q</jats:bold> had the heat flux been described throughout with a uniformly high quality closure procedure. For small <jats:italic>ϵ</jats:italic>, <jats:inline-formula> <jats:tex-math> <?CDATA $\,{\mathbb{F}}\lt 0$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabc475ieqn2.gif" xlink:type="simple" /> </jats:inline-formula> occurs for cosine of pitch angle <jats:italic>μ</jats:italic> &lt; 0 opposed to <jats:bold>q</jats:bold> at speeds above 2 thermal speeds and are omnipresent (but ignorable) for truly perturbative closures. Above a computed threshold in <jats:italic>ϵ</jats:italic> unphysical <jats:inline-formula> <jats:tex-math> <?CDATA ${\mathbb{F}}\lt 0$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabc475ieqn3.gif" xlink:type="simple" /> </jats:inline-formula> occurs for speeds below 2 thermal speeds with <jats:italic>μ</jats:italic> &gt; 0. The present work graphically shows <jats:inline-formula> <jats:tex-math> <?CDATA ${\mathbb{F}}\lt 0$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabc475ieqn4.gif" xlink:type="simple" /> </jats:inline-formula> regimes becoming increasingly pervasive as <jats:italic>ϵ</jats:italic> increases, first crossing ≃4 thermal speeds at <jats:italic>μ</jats:italic> &lt; 0 and then representing ever larger unphysical incursions within the needed velocity sphere required to accurately determine the heat flux.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The coincident detection of GW170817 in gravitational waves and electromagnetic radiation spanning the radio to MeV gamma-ray bands provided the first direct evidence that short gamma-ray bursts (GRBs) can originate from binary neutron star (BNS) mergers. On the other hand, the properties of short GRBs in high-energy gamma-rays are still poorly constrained, with only ∼20 events detected in the GeV band, and none in the TeV band. GRB 160821B is one of the nearest short GRBs known at <jats:italic>z</jats:italic> = 0.162. Recent analyses of the multiwavelength observational data of its afterglow emission revealed an optical-infrared kilonova component, characteristic of heavy-element nucleosynthesis in a BNS merger. Aiming to better clarify the nature of short GRBs, this burst was automatically followed up with the MAGIC telescopes, starting from 24 s after the burst trigger. Evidence of a gamma-ray signal is found above ∼0.5 TeV at a significance of ∼ 3<jats:italic>σ</jats:italic> during observations that lasted until 4 hr after the burst. Assuming that the observed excess events correspond to gamma-ray emission from GRB 160821B, in conjunction with data at other wavelengths, we investigate its origin in the framework of GRB afterglow models. The simplest interpretation with one-zone models of synchrotron-self-Compton emission from the external forward shock has difficulty accounting for the putative TeV flux. Alternative scenarios are discussed where the TeV emission can be relatively enhanced. The role of future GeV–TeV observations of short GRBs in advancing our understanding of BNS mergers and related topics is briefly addressed.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We present an updated model for the average cluster pressure profile, adjusted for hydrostatic mass bias by combining results from X-ray observations with cosmological simulations. Our model estimates this bias by fitting a power law to the relation between the “true” halo mass and X-ray cluster mass in hydrodynamic simulations (IllustrisTNG, BAHAMAS, and MACSIS). As an example application, we consider the REXCESS X-ray cluster sample and the universal pressure profile derived from scaled and stacked pressure profiles. We find adjusted masses, <jats:italic>M</jats:italic> <jats:sub>500c</jats:sub>, that are ≲15% higher and scaled pressures <jats:italic>P</jats:italic>/<jats:italic>P</jats:italic> <jats:sub>500c</jats:sub> that have ≲35% lower normalization than previously inferred. Our debiased pressure profile (DPP) is well-fit by a generalized Navarro–Frenk–White function, with parameters [<jats:italic>P</jats:italic> <jats:sub>0</jats:sub>, <jats:italic>c</jats:italic> <jats:sub>500</jats:sub>, <jats:italic>α</jats:italic>, <jats:italic>β</jats:italic>, <jats:italic>γ</jats:italic>] = [5.048, 1.217, 1.192, 5.490, 0.433] and does not require a mass-dependent correction term. When the DPP is used to model the Sunyaev–Zel’dovich (SZ) effect, we find that the integrated Compton <jats:italic>Y</jats:italic>–<jats:italic>M</jats:italic> relation has only minor deviations from self-similar scaling. The thermal SZ angular power spectrum is lower in amplitude by approximately 30%, assuming nominal cosmological parameters (e.g., Ω<jats:sub>m</jats:sub> = 0.3, <jats:italic>σ</jats:italic> <jats:sub>8</jats:sub> = 0.8), and is broadly consistent with recent Planck results without requiring additional bias corrections.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Interstellar dust grain alignment causes polarization from UV to mm wavelengths, allowing the study of the geometry and strength of the magnetic field. Over the last couple of decades, observations and theory have led to the establishment of the radiative alignment torque mechanism as a leading candidate to explain the effect. With a quantitatively well constrained theory, polarization can be used not only to study the interstellar magnetic field, but also the dust and other environmental parameters. Photodissociation regions, with their intense, anisotropic radiation fields, consequent rapid H<jats:sub>2</jats:sub> formation, and high spatial density-contrast provide a rich environment for such studies. Here we discuss an expanded optical, NIR, and mm-wave study of the IC 63 nebula, showing strong H<jats:sub>2</jats:sub> formation-enhanced alignment and the first direct empirical evidence for disalignment due to gas–grain collisions using high-resolution HCO<jats:sup>+</jats:sup>(<jats:italic>J</jats:italic> = 1-0) observations. We find that a relative amount of polarization is marginally anticorrelated with column density of HCO<jats:sup>+</jats:sup>. However, separating the lines of sight of optical polarimetry into those behind, or in front of, a dense clump as seen from <jats:italic>γ</jats:italic> Cas, the distribution separates into two well defined sets, with data corresponding to “shaded” gas having a shallower slope. This is expected if the decrease in polarization is caused by collisions since collisional disalignment rate is proportional to <jats:inline-formula> <jats:tex-math> <?CDATA ${R}_{C}\propto n\sqrt{T}$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabcb8eieqn1.gif" xlink:type="simple" /> </jats:inline-formula>. Ratios of the best-fit slopes for the “illuminated” and “shaded” samples of lines of sight agrees, within the uncertainties, with the square root of the two-temperature H<jats:sub>2</jats:sub> excitation in the nebula seen by Thi et al.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We present the first linear-polarization mosaicked observations performed by the Atacama Large Millimeter/submillimeter Array (ALMA). We mapped the Orion-KLeinmann-Low (Orion-KL) nebula using super-sampled mosaics at 3.1 and 1.3 mm as part of the ALMA Extension and Optimization of Capabilities program. We derive the magnetic field morphology in the plane of the sky by assuming that dust grains are aligned with respect to the ambient magnetic field. At the center of the nebula, we find a quasi-radial magnetic field pattern that is aligned with the explosive CO outflow up to a radius of approximately 12″ (∼5000 au), beyond which the pattern smoothly transitions into a quasi-hourglass shape resembling the morphology seen in larger-scale observations by the James-Clerk-Maxwell Telescope (JCMT). We estimate an average magnetic field strength <jats:inline-formula> <jats:tex-math> <?CDATA $\left\langle B\right\rangle =9.4$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabcafbieqn1.gif" xlink:type="simple" /> </jats:inline-formula> mG and a total magnetic energy of 2 × 10<jats:sup>45</jats:sup> erg, which is three orders of magnitude less than the energy in the explosive CO outflow. We conclude that the field has been overwhelmed by the outflow and that a shock is propagating from the center of the nebula, where the shock front is seen in the magnetic field lines at a distance of ∼5000 au from the explosion center.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>At cosmic recombination, there was supersonic relative motion between baryons and dark matter, which originated from baryonic acoustic oscillations in the early universe. This motion has been considered to have a negligible impact on the late stage of cosmic reionization because the relative velocity quickly decreases. However, recent studies have suggested that the recombination in gas clouds smaller than the local Jeans mass (≲10<jats:sup>8</jats:sup> <jats:italic>M</jats:italic> <jats:sub>☉</jats:sub>) can affect the reionization history by boosting the number of ultraviolet photons required for ionizing the intergalactic medium. Motivated by this, we performed a series of radiation-hydrodynamic simulations to investigate whether the streaming motion can generate variation in the local reionization history by smoothing out clumpy small-scale structures and lowering the ionizing photon budget. We found that the streaming velocity can add a variation of Δ<jats:italic>z</jats:italic> <jats:sub> <jats:italic>e</jats:italic> </jats:sub> ∼ 0.05–0.5 in the end-of-reionization redshift, depending on the level of X-ray preheating and the time evolution of ionizing sources. The variation tends to be larger when the ionizing efficiency of galaxies decreases toward later times. Given the long spatial fluctuation scales of the streaming motion (≳100 Mpc), it can help to explain the Ly<jats:italic>α</jats:italic> opacity variation observed from quasars and leave large-scale imprints on the ionization field of the intergalactic medium during the reionization. The pre-reionization heating by X-ray sources is another critical factor that can suppress small-scale gas clumping and can diminish the variation in <jats:italic>z</jats:italic> <jats:sub> <jats:italic>e</jats:italic> </jats:sub> introduced by the streaming motion.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We perform Bayesian model selection with parameter estimation to identify potentially lensed gravitational-wave images from the second observing run (O2) of Advanced LIGO and Advanced Virgo. Specifically, we compute the Bayesian evidence for a pair of events being lensed or not lensed (unlensed) using nested sampling. We consider in the model selection the discrete coalescence phase shifts that can be induced if the gravitational-wave signal intersects with the lens caustics. We find that the pair of events, GW170104 and GW170814 with a <jats:italic>π</jats:italic>/2 coalescence phase shift, has a significant Bayes factor (<jats:inline-formula> <jats:tex-math> <?CDATA ${B}_{U}^{L}\sim 1.98\times {10}^{4}$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabd7ebieqn1.gif" xlink:type="simple" /> </jats:inline-formula>) favoring the lensing hypothesis. However, after taking into account the long time delay of approximately 7 months between events, the timing Bayes factor is significantly small (<jats:italic>B</jats:italic> <jats:sub> <jats:italic>t</jats:italic> </jats:sub> ∼ 8.7 × 10<jats:sup>−2</jats:sup>). The prior probability for detecting strongly lensed pairs at O2 sensitivity is exceedingly small for both galaxy and galaxy cluster lensing. Combining the lensing and timing Bayes factors with the prior odds on lensing gives an odds ratio of <jats:inline-formula> <jats:tex-math> <?CDATA ${O}_{U}^{L}\sim 20$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabd7ebieqn2.gif" xlink:type="simple" /> </jats:inline-formula>. With the value of the odds ratio after including model dependence of the timing and prior odds factors, we do not have strong evidence to demonstrate that the aforementioned pair is strongly lensed.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Far-infrared dust polarimetry enables the study of interstellar magnetic fields via tracing of the polarized emission from dust grains that are partially aligned with the direction of the field. The advent of high-quality polarimetric data has permitted the use of statistical methods to extract both the direction and magnitude of the magnetic field. In this work, the Davis–Chandrasekhar–Fermi technique is used to make maps of the plane-of-sky (POS) component of the magnetic field in the Orion Molecular Cloud (OMC-1) by combining polarization maps at 53, 89, 154 and 214 <jats:italic>μ</jats:italic>m from HAWC+/SOFIA with maps of density and velocity dispersion. In addition, maps of the local dispersion of polarization angles are used in conjunction with Zeeman measurements to estimate a map of the strength of the line-of-sight (LOS) component of the field. Combining these maps, information about the three-dimensional magnetic field configuration (integrated along the LOS) is inferred over the OMC-1 region. POS magnetic field strengths of up to 2 mG are observed near the BN/KL object, while the OMC-1 bar shows strengths of up to a few hundred <jats:italic>μ</jats:italic>G. These estimates of the magnetic field components are used to produce maps of the mass-to-magnetic-flux ratio (<jats:italic>M</jats:italic>/Φ)—a metric for probing the conditions for star formation in molecular clouds—and determine regions of sub- and supercriticality in OMC-1. Such maps can provide invaluable input and comparison to MHD simulations of star formation processes in filamentary structures of molecular clouds.</jats:p>