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<jats:title>Abstract</jats:title> <jats:p>We report the discovery of a new ultra-faint stellar system found near the Magellanic Clouds in the DECam Local Volume Exploration Survey. This new system, DELVE J0155−6815 (DELVE 2), is located at a heliocentric distance of <jats:italic>D</jats:italic> <jats:sub>⊙</jats:sub> = 71 ± 4 kpc, which places it at a 3D physical separation of 12 ± 3 kpc from the center of the Small Magellanic Cloud and <jats:inline-formula> <jats:tex-math> <?CDATA ${28}_{-3}^{+4}\,\mathrm{kpc}$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabe1afieqn1.gif" xlink:type="simple" /> </jats:inline-formula> from the center of the Large Magellanic Cloud (LMC). DELVE 2 is identified as a resolved overdensity of old (<jats:italic>τ</jats:italic> &gt; 13.3 Gyr) and metal-poor (<jats:inline-formula> <jats:tex-math> <?CDATA $[\mathrm{Fe}/{\rm{H}}]=-{2.0}_{-0.5}^{+0.2}$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabe1afieqn2.gif" xlink:type="simple" /> </jats:inline-formula> dex) stars with a projected half-light radius of <jats:inline-formula> <jats:tex-math> <?CDATA ${r}_{1/2}={21}_{-3}^{+4}\,\mathrm{pc}$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabe1afieqn3.gif" xlink:type="simple" /> </jats:inline-formula> and an absolute magnitude of <jats:inline-formula> <jats:tex-math> <?CDATA ${M}_{V}=-{2.1}_{-0.5}^{+0.4}\,\mathrm{mag}$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabe1afieqn4.gif" xlink:type="simple" /> </jats:inline-formula>. The size and luminosity of DELVE 2 are consistent with both the population of recently discovered ultra-faint globular clusters and the smallest ultra-faint dwarf galaxies. However, its photometrically derived age and metallicity would place it among the oldest and most metal-poor globular clusters in the Magellanic system. In the absence of spectroscopic measurements of the system’s metallicity dispersion and internal kinematics, we are unable to conclusively classify this system at this time. DELVE 2 is detected in <jats:italic>Gaia</jats:italic> DR2 with a clear proper-motion signal, with multiple blue horizontal-branch stars near the centroid of the system with proper motions consistent with the systemic mean. We measure the system proper motion to be <jats:inline-formula> <jats:tex-math> <?CDATA $({\mu }_{\alpha }\cos \delta ,{\mu }_{\delta })$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabe1afieqn5.gif" xlink:type="simple" /> </jats:inline-formula> = <jats:inline-formula> <jats:tex-math> <?CDATA $({1.02}_{-0.25}^{+0.24},-{0.85}_{-0.19}^{+0.18})$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabe1afieqn6.gif" xlink:type="simple" /> </jats:inline-formula> mas yr<jats:sup>−1</jats:sup>. We compare the spatial position and proper motion of DELVE 2 with simulations of the accreted satellite population of the LMC and find that it is very likely to be associated with the LMC.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The changes of broad emission lines should be a crucial issue in understanding the physical properties of changing-look active galactic nuclei (CL-AGNs). Here we present the results of an intensive and homogeneous 6 month long reverberation mapping (RM) monitoring campaign during a low-activity state of the CL-AGN Seyfert galaxy NGC 3516. Photometric and spectroscopic monitoring was carried out during 2018–2019 with the Lijiang 2.4 m telescope. The sampling is 2 days in most nights, and the average sampling is ∼3 days. The rest-frame time lags of H<jats:italic>α</jats:italic> and H<jats:italic>β</jats:italic> are <jats:inline-formula> <jats:tex-math> <?CDATA ${\tau }_{{\rm{H}}\alpha }={7.56}_{-2.10}^{+4.42}$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabd851ieqn1.gif" xlink:type="simple" /> </jats:inline-formula> and <jats:inline-formula> <jats:tex-math> <?CDATA ${\tau }_{{\rm{H}}\beta }={7.50}_{-0.77}^{+2.05}$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabd851ieqn2.gif" xlink:type="simple" /> </jats:inline-formula> days, respectively. From an rms H<jats:italic>β</jats:italic> line dispersion of <jats:italic>σ</jats:italic> <jats:sub>line</jats:sub> = 1713.3 ± 46.7 km s<jats:sup>−1</jats:sup> and a virial factor of <jats:italic>f</jats:italic> <jats:sub> <jats:italic>σ</jats:italic> </jats:sub> = 5.5, the central black hole mass of NGC 3516 is estimated to be <jats:inline-formula> <jats:tex-math> <?CDATA ${M}_{\mathrm{BH}}={2.4}_{-0.3}^{+0.7}\times {10}^{7}{M}_{\odot }$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabd851ieqn3.gif" xlink:type="simple" /> </jats:inline-formula>, which is in agreement with previous estimates. The velocity-resolved delays show that the time lags increase toward negative velocity for both H<jats:italic>α</jats:italic> and H<jats:italic>β</jats:italic>. The velocity-resolved RM of H<jats:italic>α</jats:italic> is done for the first time. These RM results are consistent with other observations before the spectral-type change, indicating a basically constant broad-line region structure during the CL process. The CL model of changes of accretion rate seems to be favored by long-term H<jats:italic>β</jats:italic> variability and RM observations of NGC 3516.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We present the spatial analysis of five Compton thick (CT) active galactic nuclei (AGNs), including MKN 573, NGC 1386, NGC 3393, NGC 5643, and NGC 7212, for which high-resolution Chandra observations are available. For each source, we find hard X-ray emission (&gt;3 keV) extending to ∼kiloparsec scales along the ionization cone, and for some sources, in the cross-cone region. This collection represents the first, high-signal sample of CT AGN with extended hard X-ray emission for which we can begin to build a more complete picture of this new population of AGN. We investigate the energy dependence of the extended X-ray emission, including possible dependencies on host galaxy and AGN properties, and find a correlation between the excess emission and obscuration, suggesting a connection between the nuclear obscuring material and the galactic molecular clouds. Furthermore, we find that the soft X-ray emission extends farther than the hard X-rays along the ionization cone, which may be explained by a galactocentric radial dependence on the density of molecular clouds due to the orientation of the ionization cone with respect to the galactic disk. These results are consistent with other CT AGN with observed extended hard X-ray emission (e.g., ESO 428-G014 and the Ma et al. CT AGN sample), further demonstrating the ubiquity of extended hard X-ray emission in CT AGN.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Ultra-diffuse galaxies have generated significant interest due to their large optical extents and low optical surface brightnesses, which challenge galaxy formation models. Here we present resolved synthesis observations of 12 H <jats:sc>i</jats:sc>-bearing ultra-diffuse galaxies (HUDs) from the Karl G. Jansky Very Large Array, as well as deep optical imaging from the WIYN 3.5 m telescope at Kitt Peak National Observatory. We present the data processing and images, including total intensity H <jats:sc>i</jats:sc> maps and H <jats:sc>i</jats:sc> velocity fields. The HUDs show ordered gas distributions and evidence of rotation, important prerequisites for the detailed kinematic models of Mancera Piña et al. We compare the H <jats:sc>i</jats:sc> and stellar alignment and extent, and find that H <jats:sc>i</jats:sc> extends beyond the already extended stellar component and the H <jats:sc>i</jats:sc> disk is often misaligned with respect to the stellar one, emphasizing the importance of caution when approaching inclination measurements for these extreme sources. We explore the H <jats:sc>i</jats:sc> mass–diameter scaling relation, and find that, although the HUDs have diffuse stellar populations, they fall along the relation with typical global H <jats:sc>i</jats:sc> surface densities. This resolved sample forms an important basis for more detailed study of the H <jats:sc>i</jats:sc> distribution in this extreme extragalactic population.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>To date, about two dozen low-mass embedded protostars exhibit rich spectra with lines of complex organic molecules (COMs). These protostars seem to possess a different enrichment in COMs. However, the statistics of COM abundance in low-mass protostars are limited by the scarcity of observations. This study introduces the Perseus ALMA Chemistry Survey (PEACHES), which aims at unbiasedly characterizing the chemistry of COMs toward the embedded (Class 0/I) protostars in the Perseus molecular cloud. Of the 50 embedded protostars surveyed, 58% of them have emission from COMs. 56%, 32%, and 40% of the protostars have CH<jats:sub>3</jats:sub>OH, CH<jats:sub>3</jats:sub>OCHO, and N-bearing COMs, respectively. The detectability of COMs depends neither on the averaged continuum brightness temperature, a proxy of the H<jats:sub>2</jats:sub> column density, nor on the bolometric luminosity and the bolometric temperature. For the protostars with detected COMs, CH<jats:sub>3</jats:sub>OH has a tight correlation with CH<jats:sub>3</jats:sub>CN, spanning more than two orders of magnitude in column densities normalized by the continuum brightness temperature, suggesting a chemical relation between CH<jats:sub>3</jats:sub>OH and CH<jats:sub>3</jats:sub>CN and a large chemical diversity in the PEACHES samples at the same time. A similar trend with more scatter is also found between all identified COMs, which hints at a common chemistry for the sources with COMs. The correlation between COMs is insensitive to the protostellar properties, such as the bolometric luminosity and the bolometric temperature. The abundance of larger COMs (CH<jats:sub>3</jats:sub>OCHO and CH<jats:sub>3</jats:sub>OCH<jats:sub>3</jats:sub>) relative to that of smaller COMs (CH<jats:sub>3</jats:sub>OH and CH<jats:sub>3</jats:sub>CN) increases with the inferred gas column density, hinting at an efficient production of complex species in denser envelopes.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The cusp–core problem is one of the main challenges of the cold dark matter paradigm on small scales; the density of a dark matter halo is predicted to rise rapidly toward the center as <jats:italic>ρ</jats:italic>(<jats:italic>r</jats:italic>) ∝ <jats:italic>r</jats:italic> <jats:sup> <jats:italic>α</jats:italic> </jats:sup> with <jats:italic>α</jats:italic> between −1 and −1.5, while such a cuspy profile has not been clearly observed. We have carried out the spatially resolved mapping of gas dynamics toward a nearby ultradiffuse galaxy (UDG), AGC 242019. The derived rotation curve of dark matter is well fitted by the cuspy profile as described by the Navarro–Frenk–White model, while the cored profiles including both the pseudo-isothermal and Burkert models are excluded. The halo has <jats:italic>α</jats:italic> = −(0.90 ± 0.08) at the innermost radius of 0.67 kpc, <jats:italic>M</jats:italic> <jats:sub>halo</jats:sub> = (3.5 ± 1.2) × 10<jats:sup>10</jats:sup> <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub>, and a small concentration of 2.0 ± 0.36. The UDG AGC 242019 challenges alternatives of cold dark matter by constraining the particle mass of fuzzy dark matter to be &lt;0.11 × 10<jats:sup>−22</jats:sup> or &gt;3.3 × 10<jats:sup>−22</jats:sup> eV, the cross section of self-interacting dark matter to be &lt;1.63 cm<jats:sup>2</jats:sup> g<jats:sup>−1</jats:sup>, and the particle mass of warm dark matter to be &gt;0.23 keV, all of which are in tension with other constraints. The modified Newtonian dynamics is also inconsistent with a shallow radial acceleration relationship of AGC 242019. For the feedback scenario that transforms a cusp to a core, AGC 242019 disagrees with the stellar-to-halo mass ratio dependent model but agrees with the star formation threshold dependent model. As a UDG, AGC 242019 is in a dwarf-sized halo with weak stellar feedback, late formation time, normal baryonic spin, and low star formation efficiency (SFR/gas).</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The microquasar MAXI J1820+070 went into outburst from 2018 mid-March until mid-July, with several faint rebrightenings afterward. With a peak flux of approximately 4 Crab in the 20–50 keV energy range, the source was monitored across the electromagnetic spectrum with detections from radio to hard X-ray frequencies. Using these multiwavelength observations, we analyzed quasi-simultaneous observations from April 12, near the peak of the outburst (∼March 23). Analysis of the X-ray spectrum found it indicative of an accreting black hole binary in the hard state, consistent with the flat/inverted radio spectrum and the accretion disk winds seen at optical wavelengths. Then, we constructed a spectral energy distribution spanning ∼12 orders of magnitude using modeling in <jats:monospace>JetSeT</jats:monospace>. The model is composed of an irradiated disk with a Compton hump and a leptonic jet with an acceleration region and a synchrotron-dominated cooling region. <jats:monospace>JetSeT</jats:monospace> finds that the spectrum is dominated by jet emission up to approximately 10<jats:sup>14</jats:sup> Hz, after which disk and coronal emission dominates. The acceleration region has a magnetic field of <jats:italic>B</jats:italic> ∼ 1.6 × 10<jats:sup>4</jats:sup> G, a cross section of <jats:italic>R</jats:italic> ∼ 2.8 × 10<jats:sup>9</jats:sup> cm, and a flat radio spectral shape naturally obtained from the synchroton cooling of the accelerated electrons. The jet luminosity is &gt;8 × 10<jats:sup>37</jats:sup> erg s<jats:sup>−1</jats:sup> (&gt;0.15<jats:italic>L</jats:italic> <jats:sub>Edd</jats:sub>), compared to an accretion luminosity of ∼6 × 10<jats:sup>37</jats:sup> erg s<jats:sup>−1</jats:sup>, assuming a distance of 3 kpc. Because these two values are comparable, it is possible that the jet is powered predominately via accretion with only a small contribution needed from the Blanford–Znajek mechanism from the reportedly slowly spinning black hole.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Neutron star mergers (NSMs) are promising astrophysical sites for the rapid neutron-capture (“<jats:italic>r</jats:italic>”) process, but can their integrated yields explain the majority of heavy-element material in the Galaxy? One method to address this question implements a forward approach that propagates NSM rates and yields along with stellar formation rates and compares those results with observed chemical abundances of <jats:italic>r</jats:italic>-process-rich, metal-poor stars. In this work, we take the inverse approach by utilizing <jats:italic>r</jats:italic>-process-element abundance ratios of metal-poor stars as input to reconstruct the properties—especially the masses—of their neutron star (NS) binary progenitors. This novel analysis provides an independent avenue for studying the population of the original NS binary systems that merged and produced the <jats:italic>r</jats:italic>-process material now incorporated in Galactic metal-poor halo stars. We use ratios of elements typically associated with the limited-<jats:italic>r</jats:italic>-process and the actinide region to those in the lanthanide region (i.e., Zr/Dy and Th/Dy) to probe the NS masses of the progenitor merger. We find that NSMs can account for all <jats:italic>r</jats:italic>-process material in metal-poor stars that display <jats:italic>r</jats:italic>-process signatures, while simultaneously reproducing the present-day distribution of double-NS systems. Notably, with our model assumptions and the studied stellar sample, we postulate that the most <jats:italic>r</jats:italic>-process enhanced stars (the <jats:italic>r</jats:italic>–II stars) on their own would require progenitor NSMs of asymmetric systems that are distinctly different from present ones in the Galaxy. We also explore variations to the model and find that the predicted degree of asymmetry is most sensitive to the electron fraction of the remnant disk wind.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The space gravitational wave (GW) detector Laser Interferometer Space Antenna (LISA) that is planned to be launched in the early 2030s will detect the low-frequency GW signals in the Galaxy. AM CVn stars were generally thought to be important low-frequency GW sources. Employing the MESA code, in this work we calculate the evolution of a great number of binary systems consisting of a white dwarf (WD) and a main sequence (MS) star, and diagnose whether their descendant-AM CVn stars will be visible with LISA. The simulated results show that the progenitors of these LISA sources, within a distance of 1 kpc, are WD–MS binaries with a donor star of 1.0–1.4 <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub> (for initial WD mass of 0.5 <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub>) or 1.0–2.0 <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub> (for initial WD mass of 0.7 <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub>), and an initial orbital period slightly smaller than the bifurcation period. Our simulations also indicate that 10 verification AM CVn sources can be reproduced by the standard magnetic braking model, and are potential LISA sources. Based on the birth rate of AM CVn stars simulated by the population synthesis, the birth rate of AM CVn-LISA sources evolving from the evolved donor star channel within a distance of 1 kpc can be estimated to be (0.6–1.4) × 10<jats:sup>−6</jats:sup> yr<jats:sup>−1</jats:sup>, and the predicted number of AM CVn-LISA sources is about 340–810. Therefore, the evolved donor star channel plays an important role in forming AM CVn-LISA sources in the Galaxy.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>In a previous paper, second- and fourth-order explicit symplectic integrators were designed for a Hamiltonian of the Schwarzschild black hole. Following this work, we continue to trace the possibility of construction of explicit symplectic integrators for a Hamiltonian of charged particles moving around a Reissner–Nordström black hole with an external magnetic field. Such explicit symplectic methods are still available when the Hamiltonian is separated into five independently integrable parts with analytical solutions as explicit functions of proper time. Numerical tests show that the proposed algorithms share desirable properties in their long-term stability, precision, and efficiency for appropriate choices of step size. For the applicability of one of the new algorithms, the effects of black hole’s charge, the Coulomb part of the electromagnetic potential and the magnetic parameter on the dynamical behavior are surveyed. Under some circumstances, the extent of chaos becomes strong with an increase of the magnetic parameter from a global phase-space structure. No variation of the black hole’s charge other than the Coulomb part affects the regular and chaotic dynamics of the particles’ orbits. A positive Coulomb part more easily induces chaos than a negative one.</jats:p>