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<jats:title>Abstract</jats:title> <jats:p>The observed distributions of the source properties from gravitational-wave (GW) detections are biased due to the selection effects and detection criteria in the detections, analogous to the Malmquist bias. In this work, this observation bias is investigated through its fundamental statistical and physical origins. An efficient semi-analytical formulation for its estimation is derived, which is as accurate as the standard method of numerical simulations, with only a millionth of the computational cost. Then, the estimated bias is used for unmodeled inferences on the binary black hole population. These inferences show additional structures, specifically two peaks in the joint mass distribution around binary masses ∼10 <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub> and ∼30 <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub>. Example ready-to-use scripts and some produced data sets for this method are shared in an online repository.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We present optical follow-up imaging obtained with the Katzman Automatic Imaging Telescope, Las Cumbres Observatory Global Telescope Network, Nickel Telescope, Swope Telescope, and Thacher Telescope of the LIGO/Virgo gravitational wave (GW) signal from the neutron star–black hole (NSBH) merger GW190814. We searched the GW190814 localization region (19 deg<jats:sup>2</jats:sup> for the 90th percentile best localization), covering a total of 51 deg<jats:sup>2</jats:sup> and 94.6% of the two-dimensional localization region. Analyzing the properties of 189 transients that we consider as candidate counterparts to the NSBH merger, including their localizations, discovery times from merger, optical spectra, likely host galaxy redshifts, and photometric evolution, we conclude that none of these objects are likely to be associated with GW190814. Based on this finding, we consider the likely optical properties of an electromagnetic counterpart to GW190814, including possible kilonovae and short gamma-ray burst afterglows. Using the joint limits from our follow-up imaging, we conclude that a counterpart with an <jats:italic>r</jats:italic>-band decline rate of 0.68 mag day<jats:sup>−1</jats:sup>, similar to the kilonova AT 2017gfo, could peak at an absolute magnitude of at most −17.8 mag (50% confidence). Our data are not constraining for “red” kilonovae and rule out “blue” kilonovae with <jats:italic>M</jats:italic> &gt; 0.5 <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub> (30% confidence). We strongly rule out all known types of short gamma-ray burst afterglows with viewing angles &lt;17° assuming an initial jet opening angle of ∼5.°2 and explosion energies and circumburst densities similar to afterglows explored in the literature. Finally, we explore the possibility that GW190814 merged in the disk of an active galactic nucleus, of which we find four in the localization region, but we do not find any candidate counterparts among these sources.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We perform the first simultaneous Bayesian parameter inference and optimal reconstruction of the gravitational lensing of the cosmic microwave background (CMB), using 100 deg<jats:sup>2</jats:sup> of polarization observations from the SPTpol receiver on the South Pole Telescope. These data reach noise levels as low as 5.8 <jats:italic>μ</jats:italic>K arcmin in polarization, which are low enough that the typically used quadratic estimator (QE) technique for analyzing CMB lensing is significantly suboptimal. Conversely, the Bayesian procedure extracts all lensing information from the data and is optimal at any noise level. We infer the amplitude of the gravitational lensing potential to be <jats:inline-formula> <jats:tex-math> <?CDATA ${A}_{\phi }=0.949\,\pm \,0.122$?> </jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mrow> <mml:mi>A</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>ϕ</mml:mi> </mml:mrow> </mml:msub> <mml:mo>=</mml:mo> <mml:mn>0.949</mml:mn> <mml:mspace width="0.25em" /> <mml:mo>±</mml:mo> <mml:mspace width="0.25em" /> <mml:mn>0.122</mml:mn> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjac02bbieqn1.gif" xlink:type="simple" /> </jats:inline-formula> using the Bayesian pipeline, consistent with our QE pipeline result, but with 17% smaller error bars. The Bayesian analysis also provides a simple way to account for systematic uncertainties, performing a similar job as frequentist “bias hardening” or linear bias correction, and reducing the systematic uncertainty on <jats:italic>A</jats:italic> <jats:sub> <jats:italic>ϕ</jats:italic> </jats:sub> due to polarization calibration from almost half of the statistical error to effectively zero. Finally, we jointly constrain <jats:italic>A</jats:italic> <jats:sub> <jats:italic>ϕ</jats:italic> </jats:sub> along with <jats:italic>A</jats:italic> <jats:sub>L</jats:sub>, the amplitude of lensing-like effects on the CMB power spectra, demonstrating that the Bayesian method can be used to easily infer parameters both from an optimal lensing reconstruction and from the delensed CMB, while exactly accounting for the correlation between the two. These results demonstrate the feasibility of the Bayesian approach on real data, and pave the way for future analysis of deep CMB polarization measurements with SPT-3G, Simons Observatory, and CMB-S4, where improvements relative to the QE can reach 1.5 times tighter constraints on <jats:italic>A</jats:italic> <jats:sub> <jats:italic>ϕ</jats:italic> </jats:sub> and seven times lower effective lensing reconstruction noise.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We have carried out a detailed study of single-pulse emission from the pulsar J2048−1616 (B2045−16), observed at 732, 1369, and 3100 MHz frequencies using the Parkes 64 m radio telescope. The pulsar possesses three well-resolved emission components, with the central component resembling core emission. The single pulses show the presence of two distinct periodic modulations using fluctuation spectral analysis. About 12% nulls are found to create alternating bunches of nulls and bursts in a quasiperiodic manner with longer periodicities of 83, 28, and 14 rotational periods for simultaneous observations at 732 and 3100 MHz. At 1369 MHz, the quasiperiodic nulling is detected, as well, to modulate across the entire profile both in the core and conal components simultaneously with a fluctuation rate of about 50 rotational periods, and the nulling fraction is estimated to be around 10%. Additionally, the quasiperiodic modulations are significantly dependent on time. In addition to nulling, the pulsar also presents subpulse drifting in the single-pulse sequences with shorter periodicity. The subpulse drifting is presented in the conal components and is absent in the central core emission. The leading component is modulated in longitude with a period of three pulses. The trailing component remains phase stationary within the pulse window but periodically modulates in amplitude with a period of three pulses. Finally, possible physical mechanisms are discussed.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The polarization of X-ray emission is a unique tool used to investigate the magnetic field structure around astrophysical objects. In this paper, we study the linear polarization of X-ray emissions from gamma-ray binary systems based on pulsar scenarios. We discuss synchrotron emission from pulsar wind particles accelerated by a standing shock. We explore three kinds of axisymmetric magnetic field structures: (i) toroidal magnetic fields, (ii) poloidal magnetic fields, and (iii) tangled magnetic fields. Because of the axisymmetric structure, the polarization angle of integrated emission is oriented along or perpendicular to the shock-cone axis projected on the sky and swings around 360° in one orbit. For the toroidal case, the polarization angle is always directed along the shock-cone axis and smoothly changes along the orbital phase. For the poloidal/tangled magnetic field, the direction of the polarization angle depends on the system parameters and orbital phase. In one orbit, the polarization degree for the toroidal case can reach the maximum value of the synchrotron radiation (∼70%), while the maximum polarization degree for poloidal/tangled field cases is several 10%. We apply our model to bright gamma-ray binary LS 5039 and make predictions for future observations. With the expected sensitivity of the Imaging X-ray Polarimetry Explorer, linear polarization can be detected by an observation of several days if the magnetic field is dominated by the toroidal magnetic field. If the magnetic field is dominated by the poloidal/tangled field, significant detection is expected with an observation longer than 10 days.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We present estimates for the number of shadow-resolved supermassive black hole (SMBH) systems that can be detected using radio interferometers, as a function of angular resolution, flux density sensitivity, and observing frequency. Accounting for the distribution of SMBHs across mass, redshift, and accretion rate, we use a new semianalytic spectral energy distribution model to derive the number of SMBHs with detectable and optically thin horizon-scale emission. We demonstrate that (sub)millimeter interferometric observations with ∼0.1 <jats:italic>μ</jats:italic>as resolution and ∼1 <jats:italic>μ</jats:italic>Jy sensitivity could access &gt;10<jats:sup>6</jats:sup> SMBH shadows. We then further decompose the shadow source counts into the number of black holes for which we could expect to observe the first- and second-order lensed photon rings. Accessing the bulk population of first-order photon rings requires ≲2 <jats:italic>μ</jats:italic>as resolution and ≲0.5 mJy sensitivity, whereas doing the same for second-order photon rings requires ≲0.1 <jats:italic>μ</jats:italic>as resolution and ≲5 <jats:italic>μ</jats:italic>Jy sensitivity. Our model predicts that with modest improvements to sensitivity, as many as ∼5 additional horizon-resolved sources should become accessible to the current Event Horizon Telescope (EHT), whereas a next-generation EHT observing at 345 GHz should have access to ∼3 times as many sources. More generally, our results can help guide enhancements of current arrays and specifications for future interferometric experiments that aim to spatially resolve a large population of SMBH shadows or higher-order photon rings.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Magnetic reconnection is invoked as one of the primary mechanisms to produce energetic particles. We employ large-scale 3D particle-in-cell simulations of reconnection in magnetically dominated (<jats:italic>σ</jats:italic> = 10) pair plasmas to study the energization physics of high-energy particles. We identify an acceleration mechanism that only operates in 3D. For weak guide fields, 3D plasmoids/flux ropes extend along the <jats:italic>z</jats:italic>-direction of the electric current for a length comparable to their cross-sectional radius. Unlike in 2D simulations, where particles are buried in plasmoids, in 3D we find that a fraction of particles with <jats:italic>γ</jats:italic> ≳ 3<jats:italic>σ</jats:italic> can escape from plasmoids by moving along <jats:italic>z</jats:italic>, and so they can experience the large-scale fields in the upstream region. These “free” particles preferentially move in <jats:italic>z</jats:italic> along Speiser-like orbits sampling both sides of the layer and are accelerated linearly in time—their Lorentz factor scales as <jats:italic>γ</jats:italic> ∝ <jats:italic>t</jats:italic>, in contrast to <jats:inline-formula> <jats:tex-math> <?CDATA $\gamma \propto \sqrt{t}$?> </jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>γ</mml:mi> <mml:mo>∝</mml:mo> <mml:msqrt> <mml:mrow> <mml:mi>t</mml:mi> </mml:mrow> </mml:msqrt> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjac2e08ieqn1.gif" xlink:type="simple" /> </jats:inline-formula> in 2D. The energy gain rate approaches ∼<jats:italic>eE</jats:italic> <jats:sub>rec</jats:sub> <jats:italic>c</jats:italic>, where <jats:italic>E</jats:italic> <jats:sub>rec</jats:sub> ≃ 0.1<jats:italic>B</jats:italic> <jats:sub>0</jats:sub> is the reconnection electric field and <jats:italic>B</jats:italic> <jats:sub>0</jats:sub> the upstream magnetic field. The spectrum of free particles is hard, <jats:inline-formula> <jats:tex-math> <?CDATA ${{dN}}_{\mathrm{free}}/d\gamma \propto {\gamma }^{-1.5}$?> </jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mrow> <mml:mi mathvariant="italic">dN</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>free</mml:mi> </mml:mrow> </mml:msub> <mml:mrow> <mml:mo stretchy="true">/</mml:mo> </mml:mrow> <mml:mi>d</mml:mi> <mml:mi>γ</mml:mi> <mml:mo>∝</mml:mo> <mml:msup> <mml:mrow> <mml:mi>γ</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>1.5</mml:mn> </mml:mrow> </mml:msup> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjac2e08ieqn2.gif" xlink:type="simple" /> </jats:inline-formula>, contains ∼20% of the dissipated magnetic energy independently of domain size, and extends up to a cutoff energy scaling linearly with box size. Our results demonstrate that relativistic reconnection in GRB and AGN jets may be a promising mechanism for generating ultra-high-energy cosmic rays.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The hyperfine transitions of the ground-rotational state of the hydroxyl radical (OH) have emerged as a versatile tracer of the diffuse molecular interstellar medium. We present a novel automated Gaussian decomposition algorithm designed specifically for the analysis of the paired on-source and off-source optical depth and emission spectra of these OH transitions. In contrast to existing automated Gaussian decomposition algorithms, <jats:sc>Amoeba</jats:sc> (Automated Molecular Excitation Bayesian line-fitting Algorithm) employs a Bayesian approach to model selection, fitting all four optical-depth and four emission spectra simultaneously. <jats:sc>Amoeba</jats:sc> assumes that a given spectral feature can be described by a single centroid velocity and full width at half maximum, with peak values in the individual optical-depth and emission spectra then described uniquely by the column density in each of the four levels of the ground-rotational state, thus naturally including the real physical constraints on these parameters. Additionally, the Bayesian approach includes informed priors on individual parameters that the user can modify to suit different data sets. Here we describe <jats:sc>Amoeba</jats:sc> and establish its validity and reliability in identifying and fitting synthetic spectra with known (but hidden) parameters, finding that the code performs very well in a series of practical tests. <jats:sc>Amoeba’</jats:sc>s core algorithm could be adapted to the analysis of other species with multiple transitions interconnecting shared levels (e.g., the 700 MHz lines of the first excited rotational state of CH). Users are encouraged to adapt and modify <jats:sc>Amoeba</jats:sc> to suit their own use cases.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>In radio-loud active galactic nuclei (AGNs), ultra-fast outflows (UFOs) were detected at the inclination angle of ∼10°–70° away from jets. Except for the inclination angle of UFOs, the UFOs in radio-loud AGNs have similar properties to that in radio-quiet AGNs. The UFOs with such low inclination cannot be explained in the line-force mechanism. The magnetic-driving mechanism is suggested to explain the UFOs based on a self-similar solution with radiative transfer calculations. However, the energetics of self-similar solution need to be further confirmed based on numerical simulations. To understand the formation and acceleration of UFOs in radio-loud AGNs, this paper presents a model of the disk winds driven by both line force and magnetic field and implements numerical simulations. Initially, a magnetic field is set to 10 times stronger than the gas pressures at the disk surface. Simulation results imply that the disk winds driven by both line force and magnetic field could describe the properties of UFOs in radio-loud AGNs. Pure magnetohydrodynamics (MHDs) simulation is also implemented. When the initial conditions are the same, the hybrid models of magnetic fields and line force are more helpful to form UFOs than the pure MHD models. It is worth studying the case of a stronger magnetic field to confirm this result.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We report the results of near-infrared spectroscopic observations of 37 quasars in the redshift range 6.3 &lt; <jats:italic>z</jats:italic> ≤ 7.64, including 32 quasars at <jats:italic>z</jats:italic> &gt; 6.5, forming the largest quasar near-infrared spectral sample at this redshift. The spectra, taken with Keck, Gemini, VLT, and Magellan, allow investigations of central black hole mass and quasar rest-frame ultraviolet spectral properties. The black hole masses derived from the Mg <jats:sc>ii</jats:sc> emission lines are in the range (0.3–3.6) × 10<jats:sup>9</jats:sup> <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub>, which requires massive seed black holes with masses ≳10<jats:sup>3</jats:sup>–10<jats:sup>4</jats:sup> <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub>, assuming Eddington accretion since <jats:italic>z</jats:italic> = 30. The Eddington ratio distribution peaks at <jats:italic>λ</jats:italic> <jats:sub>Edd</jats:sub> ∼ 0.8 and has a mean of 1.08, suggesting high accretion rates for these quasars. The C <jats:sc>iv</jats:sc>–Mg <jats:sc>ii</jats:sc> emission-line velocity differences in our sample show an increase of C <jats:sc>iv</jats:sc> blueshift toward higher redshift, but the evolutionary trend observed from this sample is weaker than the previous results from smaller samples at similar redshift. The Fe <jats:sc>ii</jats:sc>/Mg <jats:sc>ii</jats:sc> flux ratios derived for these quasars up to <jats:italic>z</jats:italic> = 7.6, compared with previous measurements at different redshifts, do not show any evidence of strong redshift evolution, suggesting metal-enriched environments in these quasars. Using this quasar sample, we create a quasar composite spectrum for <jats:italic>z</jats:italic> &gt; 6.5 quasars and find no significant redshift evolution of quasar broad emission lines and continuum slope, except for a blueshift of the C <jats:sc>iv</jats:sc> line. Our sample yields a strong broad absorption line quasar fraction of ∼24%, higher than the fractions in lower-redshift quasar samples, although this could be affected by small sample statistics and selection effects.</jats:p>