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<jats:title>Abstract</jats:title> <jats:p>Under the hypothesis of gravitational redshift induced by a central supermassive black hole, and based on line widths and shifts of redward-shifted H<jats:italic>β</jats:italic> and Fe <jats:sc>ii</jats:sc> broad emission lines for a sample of 1973 <jats:italic>z</jats:italic> &lt; 0.8 Sloan Digital Sky Survey DR5 quasars, we measured the virial factor in determining supermassive black hole masses, usually estimated by the reverberation mapping method or the relevant secondary methods. The virial factor had been believed to be from the geometric effect of the broad-line region. The measured virial factor of Fe <jats:sc>ii</jats:sc> is larger than that of H<jats:italic>β</jats:italic> for 98% of these quasars. The virial factor is very different from object to object and for different emission lines. For most of these quasars, the virial factor of H<jats:italic>β</jats:italic> is larger than these averages that were usually used in determining the masses of black holes. There are three positive correlations among the measured virial factor of H<jats:italic>β</jats:italic>, dimensionless accretion rate, and Fe <jats:sc>ii</jats:sc>/H<jats:italic>β</jats:italic> line ratio. A positive three-dimensional correlation is found among these three quantities, and this correlation indicates that the virial factor is likely dominated by the dimensionless accretion rate and metallicity. A negative correlation is found between the redward shift of H<jats:italic>β</jats:italic> and the scaled size of the broad-line region radius in units of the gravitational radius of the black hole. This negative correlation will be expected naturally if the redward shift of H<jats:italic>β</jats:italic> is mainly from the gravity of the black hole. Radiation pressure from the accretion disk may be a significant contributor to the virial factor.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Photoionization modeling of active galactic nuclei (AGN) predicts that diffuse continuum (DC) emission from the broad-line region makes a substantial contribution to the total continuum emission from ultraviolet through near-infrared wavelengths. Evidence for this DC component is present in the strong Balmer jump feature in AGN spectra, and possibly from reverberation measurements that find longer lags than expected from disk emission alone. However, the Balmer jump region contains numerous blended emission features, making it difficult to isolate the DC emission strength. In contrast, the Paschen jump region near 8200 Å is relatively uncontaminated by other strong emission features. Here, we examine whether the Paschen jump can aid in constraining the DC contribution, using Hubble Space Telescope Space Telescope Imaging Spectrograph spectra of six nearby Seyfert 1 nuclei. The spectra appear smooth across the Paschen edge, and we find no evidence of a Paschen spectral break or jump in total flux. We fit multicomponent spectral models over the range 6800–9700 Å and find that the spectra can still be compatible with a significant DC contribution if the DC Paschen jump is offset by an opposite spectral break resulting from blended high-order Paschen emission lines. The fits imply DC contributions ranging from ∼10% to 50% at 8000 Å, but the fitting results are highly dependent on assumptions made about other model components. These degeneracies can potentially be alleviated by carrying out fits over a broader wavelength range, provided that models can accurately represent the disk continuum shape, Fe <jats:sc>ii</jats:sc> emission, high-order Balmer line emission, and other components.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We present Very Large Array observations toward the Class 0 protostar L1157 MMS at 6.8 and 9 mm with a resolution of ∼0.″04 (14 au). We detect two sources within L1157 MMS and interpret these sources as a binary protostar with a separation of ∼16 au. The material directly surrounding the binary system within the inner 50 au radius of the system has an estimated mass of 0.11 <jats:italic>M</jats:italic> <jats:sub>☉</jats:sub>, calculated from the observed dust emission. We interpret the observed binary system in the context of previous observations of its flattened envelope structure, low rates of envelope rotation from 5000 to 200 au scales, and an ordered, poloidal magnetic field aligned with the outflow. Thus, L1157 MMS is a prototype system for magnetically regulated collapse, and the presence of a compact binary within L1157 MMS demonstrates that multiple star formation can still occur within envelopes that likely have dynamically important magnetic fields.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We report results of optical imaging and low-resolution spectroscopic monitoring of supernova (SN) 2017iro that occurred in the nearby (∼31 Mpc) galaxy NGC 5480. The He <jats:sc>i</jats:sc> <jats:italic>λ</jats:italic>5876 feature present in the earliest spectrum (−7 days) classified it as a Type Ib SN. The follow-up observations span from −7 to +266 days with respect to the <jats:italic>B</jats:italic>-band maximum. With a peak absolute magnitude in <jats:italic>V</jats:italic> band <jats:italic>M</jats:italic> <jats:sub> <jats:italic>V</jats:italic> </jats:sub> = −17.76 ± 0.15 mag and bolometric luminosity log<jats:sub>10</jats:sub> <jats:italic>L</jats:italic> = 42.39 ± 0.09 erg s<jats:sup>−1</jats:sup>, SN 2017iro is a moderately luminous Type Ib SN. The overall light-curve evolution of SN 2017iro is similar to that of SN 2012au and SN 2009jf during the early (up to ∼100 days) and late phases (&gt;150 days), respectively. The line velocities of both Fe <jats:sc>ii</jats:sc> <jats:italic>λ</jats:italic>5169 and He <jats:sc>i</jats:sc> <jats:italic>λ</jats:italic>5876 are ∼9000 km s<jats:sup>−1</jats:sup> near the peak. The analysis of the nebular phase spectrum (∼+209 days) indicates an oxygen mass of ∼0.35 <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub>. The smaller [O <jats:sc>i</jats:sc>]/[Ca <jats:sc>ii</jats:sc>] flux ratio of ∼1 favors a progenitor with a zero-age main-sequence mass in the range ∼13–15 <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub>, most likely in a binary system, similar to the case of iPTF13bvn. The explosion parameters are estimated by applying different analytical models to the quasi-bolometric light curve of SN 2017iro. <jats:sup>56</jats:sup>Ni mass synthesized in the explosion has a range of ∼0.05–0.10 <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub>, ejecta mass ∼1.4–4.3 <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub>, and kinetic energy ∼(0.8–1.9) × 10<jats:sup>51</jats:sup> erg.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We study the kinetic plasma dynamics in collisionless relativistic jets with velocity shear, by carrying out particle-in-cell simulations in the transverse plane of a jet. It is discovered that intermittent magnetic reconnections (MRs) are driven by mushroom instability (MI), which is an important kinetic-scale plasma instability in the plasma shear flows with relativistic bulk speed. We refer to this sequence of kinetic plasma phenomena as “MI-driven MR.” The MI-driven MRs intermittently occur with moving the location of the reconnection points from the vicinity of the initial velocity-shear surface toward the center of the jet. As a consequence, the number density of high-energy electrons that are accelerated by MI-driven MRs increases with time in the region inside the initial velocity-shear surface with the accompanying generation and subsequent amplification of magnetic fields by MI. The maximum Lorentz factor of electrons increases with initial bulk Lorentz factor of the jet. A possible relation of MI-driven MR to the bright synchrotron emission in the jet spine of an active galactic nucleus jet is also discussed.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We present observations of ≳10–100 keV nucleon<jats:sup>−1</jats:sup> suprathermal (ST) H, He, O, and Fe ions associated with crossings of the heliospheric current sheet (HCS) at radial distances of &lt;0.1 au from the Sun. Our key findings are as follows: (1) very few heavy ions are detected during the first full crossing, the heavy-ion intensities are reduced during the second partial crossing and peak just after the second crossing; (2) ion arrival times exhibit no velocity dispersion; (3) He pitch-angle distributions track the magnetic field polarity reversal and show up to ∼10:1 anti-sunward, field-aligned flows and beams closer to the HCS that become nearly isotropic farther from the HCS; (4) the He spectrum steepens either side of the HCS, and the He, O, and Fe spectra exhibit power laws of the form ∼<jats:italic>E</jats:italic> <jats:sup>−4</jats:sup>–<jats:italic>E</jats:italic> <jats:sup>6</jats:sup>; and (5) maximum energies <jats:italic>E</jats:italic> <jats:sub> <jats:italic>X</jats:italic> </jats:sub> increase with the ion’s charge-to-mass (<jats:italic>Q</jats:italic>/<jats:italic>M</jats:italic>) ratio as <jats:inline-formula> <jats:tex-math> <?CDATA ${E}_{X}/{E}_{H}\propto {({Q}_{X}/{M}_{X})}^{\delta }$?> </jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mrow> <mml:mi>E</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>X</mml:mi> </mml:mrow> </mml:msub> <mml:mrow> <mml:mo stretchy="true">/</mml:mo> </mml:mrow> <mml:msub> <mml:mrow> <mml:mi>E</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>H</mml:mi> </mml:mrow> </mml:msub> <mml:mo>∝</mml:mo> <mml:msup> <mml:mrow> <mml:mo stretchy="false">(</mml:mo> <mml:msub> <mml:mrow> <mml:mi>Q</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>X</mml:mi> </mml:mrow> </mml:msub> <mml:mrow> <mml:mo stretchy="true">/</mml:mo> </mml:mrow> <mml:msub> <mml:mrow> <mml:mi>M</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>X</mml:mi> </mml:mrow> </mml:msub> <mml:mo stretchy="false">)</mml:mo> </mml:mrow> <mml:mrow> <mml:mi>δ</mml:mi> </mml:mrow> </mml:msup> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjac4961ieqn1.gif" xlink:type="simple" /> </jats:inline-formula>, where <jats:italic>δ</jats:italic> ∼ 0.65–0.76, assuming that the average <jats:italic>Q</jats:italic> states are similar to those measured in gradual and impulsive solar energetic particle events at 1 au. The absence of velocity dispersion in combination with strong field-aligned anisotropies closer to the HCS appears to rule out solar flares and near-Sun coronal-mass-ejection-driven shocks. These new observations present challenges not only for mechanisms that employ direct parallel electric fields and organize maximum energies according to <jats:italic>E</jats:italic>/<jats:italic>Q</jats:italic> but also for local diffusive and magnetic-reconnection-driven acceleration models. Reevaluation of our current understanding of the production and transport of energetic ions is necessary to understand this near-solar, current-sheet-associated population of ST ions.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Tidal disruption events (TDEs) can uncover the quiescent supermassive black holes (SMBHs) at the center of galaxies and also offer a promising method to study them. After the disruption of a star by an SMBH, the highly elliptical orbit of the debris stream will be gradually circularized due to the self-crossing, and then the circularized debris will form an accretion disk. The recent TDE candidate AT 2019avd has double peaks in its optical light curve, and the X-ray emerges near the second peak. The durations of the peaks are ∼400 and 600 days, respectively, and the separation between them is ∼700 days. We fit its spectral energy distribution and analyze its light curves in the optical/UV, mid-infrared, and X-ray bands. We find that this source can be interpreted as a two-phase scenario in which the first phase is dominated by the stream circularization, and the second phase is the delayed accretion. We use the succession of the self-crossing model and delayed accretion model to fit the first and second peaks, respectively. The fitting result implies that AT 2019avd can be interpreted by the partial disruption of a 0.9 <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub> star by a 7 × 10<jats:sup>6</jats:sup> <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub> SMBH, but this result is sensitive to the stellar model. Furthermore, we find that the large-amplitude (by factors up to ∼5) X-ray variability in AT 2019avd can be interpreted as the rigid-body precession of the misaligned disk due to the Lense–Thirring effect of a spinning SMBH, with a precession period of 10−25 days.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>For microlenses with sufficiently low mass, the angular radius of the source star can be much larger than the angular Einstein ring radius of the lens. For such extreme finite source effect (EFSE) events, finite source effects dominate throughout the duration of the event. Here, we demonstrate and explore a continuous degeneracy between multiple parameters of such EFSE events. The first component in the degeneracy arises from the fact that the directly observable peak change of the flux depends on both the ratio of the angular source radius to the angular Einstein ring radius and the fraction of the baseline flux that is attributable to the lensed source star. The second component arises because the directly observable duration of the event depends on both the impact parameter of the event and the relative lens-source proper motion. These two pairwise degeneracies become coupled when the detailed morphology of the light curve is considered, especially when including a limb-darkening profile of the source star. We derive these degeneracies mathematically through analytic approximations and investigate them further numerically with no approximations. We explore the likely physical situations in which these mathematical degeneracies may be realized and potentially broken. As more and more low-mass lensing events (with ever decreasing Einstein ring radii) are detected with improving precision and increasing cadence from microlensing surveys, one can expect that more of these EFSE events will be discovered. In particular, the detection of EFSE microlensing events could increase dramatically with the Roman Space Telescope Galactic Bulge Time Domain Survey.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We present an analytic approach to the dynamical friction (DF) acting on a circularly moving point mass perturber in a gaseous medium. We demonstrate that, when the perturber is turned on at <jats:italic>t</jats:italic> = 0, steady state (infinite time perturbation) is achieved after exactly one sound-crossing time. At low Mach numbers <jats:inline-formula> <jats:tex-math> <?CDATA ${ \mathcal M }\,\ll 1$?> </jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi mathvariant="italic"></mml:mi> <mml:mspace width="0.25em" /> <mml:mo>≪</mml:mo> <mml:mn>1</mml:mn> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjac5519ieqn1.gif" xlink:type="simple" /> </jats:inline-formula>, the circular-motion steady-state DF converges to the linear-motion, finite time perturbation expression. The analytic results describe both the radial and tangential forces on the perturbers caused by the backreaction of the wake propagating in the medium. The radial force is directed inward, toward the motion center, and is dominant at large Mach numbers. For subsonic motion, this component is negligible. For moderate and low Mach numbers, the tangential force is stronger and opposes the motion of the perturber. The analytic solution to the circular-orbit DF suffers from a logarithmic divergence in the supersonic regime. This divergence appears at short distances from the perturber solely (unlike the linear-motion result, which is also divergent at large distances) and can be encoded in a maximum multipole. This is helpful to assess the resolution dependence of numerical simulations implementing DF at the level of Liénard–Wiechert potentials. We also show how our approach can be generalized to calculate the DF acting on a compact circular binary.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We place empirical constraints on the yields from zero- and low-metallicity core-collapse supernovae (CCSNe) using abundances measured in very metal-poor (VMP; [Fe/H] ≤ −2) damped Ly<jats:italic>α</jats:italic> absorbers (DLAs). For some abundance ratios ([N,Al,S/Fe]), VMP DLAs constrain the metal yields of the first SNe more reliably than VMP stars. We compile a large sample of high-S/N VMP DLAs from over 30 yr of literature, most with high-resolution spectral measurements. We infer the initial-mass-function-averaged CCSNe yield from the median values from the DLA abundance ratios of C, N, O, Al, Si, S, and Fe (over Fe and O). We assume that the DLAs are metal-poor enough that they represent galaxies in their earliest stages of evolution, when CCSNe are the only nucleosynthetic sources of the metals we analyze. We compare five sets of zero- and low-metallicity theoretical yields to the empirical yields derived in this work. We find that the five models agree with the DLA yields for ratios containing Si and S. Only one model (Heger &amp; Woosley 2010, hereafter HW10) reproduced the DLA values for N, and one other model (Limongi &amp; Chieffi 2018, hereafter LC18) reproduced [N/O]. We found little change in the theoretical yields with the adoption of an SN explosion landscape (where certain progenitor masses collapse into black holes, contributing no yields) onto HW10, but fixing explosion energy to progenitor mass results in wide disagreements between the predictions and DLA abundances. We investigate the adoption of a simple, observationally motivated initial distribution of rotational velocities for LC18 and find a slight improvement.</jats:p>