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<jats:title>Abstract</jats:title> <jats:p>The detection of the binary events GW170817 and GW190814 has provided invaluable constraints on the maximum mass of nonrotating configurations of neutron stars, <jats:italic>M</jats:italic> <jats:sub>TOV</jats:sub>. However, the large differences in the neutron-star masses measured in GW170817 and GW190814 has also lead to significant tension between the predictions for such maximum masses, with GW170817 suggesting that <jats:italic>M</jats:italic> <jats:sub>TOV</jats:sub> ≲ 2.3 <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub>, and GW190814 requiring <jats:italic>M</jats:italic> <jats:sub>TOV</jats:sub> ≳ 2.5 <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub> if the secondary was a (non- or slowly rotating) neutron star at merger. Using a genetic algorithm, we sample the multidimensional space of parameters spanned by gravitational-wave and astronomical observations associated with GW170817. Consistent with previous estimates, we find that all of the physical quantities are in agreement with the observations if the maximum mass is in the range of <jats:inline-formula> <jats:tex-math> <?CDATA ${M}_{\mathrm{TOV}}={2.210}_{-0.123}^{+0.116}\,{M}_{\odot }$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjlabdfc6ieqn1.gif" xlink:type="simple" /> </jats:inline-formula> within a 2<jats:italic>σ</jats:italic> confidence level. By contrast, maximum masses with <jats:italic>M</jats:italic> <jats:sub>TOV</jats:sub> ≳ 2.5 <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub>, not only require efficiencies in the gravitational-wave emission that are well above the numerical-relativity estimates, but they also lead to a significant underproduction of the ejected mass. Hence, the tension can be released by assuming that the secondary in GW190814 was a black hole at merger, although it could have been a rotating neutron star before.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We report the discovery of a spectroscopically confirmed strong Ly<jats:italic>α</jats:italic> emitter at <jats:italic>z</jats:italic> = 7.0281 ± 0.0003, observed as part of the Reionization Lensing Cluster Survey (RELICS). This galaxy, dubbed “Dichromatic Primeval Galaxy” at <jats:italic>z</jats:italic> ∼ 7 (DP7), shows two distinct components. While fairly unremarkable in terms of its ultraviolet (UV) luminosity (<jats:inline-formula> <jats:tex-math> <?CDATA $\sim 0.3{L}_{\mathrm{UV}}^{* }$?> </jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mo>∼</mml:mo> <mml:mn>0.3</mml:mn> <mml:msubsup> <mml:mrow> <mml:mi>L</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>UV</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>*</mml:mo> </mml:mrow> </mml:msubsup> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjlabdf56ieqn1.gif" xlink:type="simple" /> </jats:inline-formula>, where <jats:inline-formula> <jats:tex-math> <?CDATA ${L}_{\mathrm{UV}}^{* }$?> </jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msubsup> <mml:mrow> <mml:mi>L</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>UV</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>*</mml:mo> </mml:mrow> </mml:msubsup> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjlabdf56ieqn2.gif" xlink:type="simple" /> </jats:inline-formula> is the characteristic luminosity), DP7 has one of the highest observed Ly<jats:italic>α</jats:italic> equivalent widths (EWs) among Ly<jats:italic>α</jats:italic> emitters at <jats:italic>z</jats:italic> &gt; 6 (&gt;200 Å in the rest frame). The strong Ly<jats:italic>α</jats:italic> emission generally suggests a young metal-poor, low-dust galaxy; however, we find that the UV slope <jats:italic>β</jats:italic> of the galaxy as a whole is redder than typical star-forming galaxies at these redshifts, −1.13 ± 0.84, likely indicating, on average, a considerable amount of dust obscuration, or an older stellar population. When we measure <jats:italic>β</jats:italic> for the two components separately, however, we find evidence of differing UV colors, suggesting two separate stellar populations. Also, we find that Ly<jats:italic>α</jats:italic> is spatially extended and likely larger than the galaxy size, hinting to the possible existence of a Ly<jats:italic>α</jats:italic> halo. Rejuvenation or merging events could explain these results. Either scenario requires an extreme stellar population, possibly including a component of Population III stars, or an obscured active galactic nucleus. DP7, with its low UV luminosity and high Ly<jats:italic>α</jats:italic> EW, represents the typical galaxies that are thought to be the major contribution to the reionization of the universe, and for this reason DP7 is an excellent target for follow-up with the James Webb Space Telescope.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We report the detection of a rapid occultation event in the nearby Seyfert galaxy NGC 6814, simultaneously captured in a transient light curve and spectral variability. The intensity and hardness ratio curves capture distinct ingress and egress periods that are symmetric in duration. Independent of the selected continuum model, the changes can be simply described by varying the fraction of the central engine that is covered by transiting obscuring gas. Together, the spectral and timing analyses self-consistently reveal the properties of the obscuring gas, its location to be in the broad-line region, and the size of the X-ray source to be ∼ 25 <jats:italic>r</jats:italic> <jats:sub>g</jats:sub>. Our results demonstrate that obscuration close to massive black holes can shape their appearance, and can be harnessed to measure the active region that surrounds the event horizon.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The recent detection of TeV photons from two gamma-ray bursts (GRBs), GRB 190114C and GRB 180720B, has opened a new window for multimessenger and multiwavelength astrophysics of high-energy transients. We study the origin of very high energy (VHE) <jats:italic>γ</jats:italic>-rays from the short GRB 160821B, for which the MAGIC Collaboration reported a ∼3<jats:italic>σ</jats:italic> statistical significance. Short GRBs are often accompanied by extended and plateau emission, which is attributed to internal dissipation resulting from activities of a long-lasting central engine, and Murase et al. (2018) recently suggested the external inverse-Compton (EIC) scenario for VHE counterparts of short GRBs and neutron star mergers. Applying this scenario to GRB 160821B, we show that the EIC flux can reach ∼10<jats:sup>−12</jats:sup> erg cm<jats:sup>−2</jats:sup> s<jats:sup>−1</jats:sup> within a time period of ∼10<jats:sup>3</jats:sup>–10<jats:sup>4</jats:sup> s, which is consistent with the MAGIC observations. EIC <jats:italic>γ</jats:italic>-rays expected during the extended and plateau emission will be detectable with greater significance by future detectors such as the Cerenkov Telescope Array. The resulting light curve has a distinguishable feature, where the VHE emission is predicted to reach the peak around the end of the seed photon emission.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We investigate the possible dynamical origin of GW190814, a gravitational wave (GW) source discovered by the LIGO-Virgo-Kagra collaboration (LVC) associated with a merger between a stellar black hole (BH) with mass 23.2 <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub> and a compact object, either a BH or a neutron star (NS), with mass 2.59 <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub>. Using a database of 240,000 <jats:italic>N</jats:italic>-body simulations modeling the formation of NS–BH mergers via dynamical encounters in dense clusters, we find that systems like GW190814 are likely to form in young, metal-rich clusters. Our model suggests that a little excess (∼2%–4%) of objects with masses in the range of 2.3–3 <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub> in the compact remnants’ mass spectrum leads to a detection rate for dynamically formed “GW190814 -like” mergers of Γ<jats:sub>GW190814</jats:sub> ≃ 1–6 yr<jats:sup>−1</jats:sup> Gpc<jats:sup>−3</jats:sup>, i.e., within the observational constraints set by the GW190814 discovery, Γ<jats:sub>LVC</jats:sub> ∼ 1–23 yr<jats:sup>−1</jats:sup> Gpc<jats:sup>−3</jats:sup>. Additionally, our model suggests that ∼1.8%–4.8% of dynamical NS–BH mergers are compatible with GW190426_152155, the only confirmed NS–BH merger detected by the LVC. We show that the relative amount of light and heavy NS–BH mergers can provide clues about the environments in which they developed.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The origin and fate of the most extended extragalactic neutral cloud known in the local universe, the Leo ring, is still debated 38 yr after its discovery. Its existence is alternatively attributed to leftover primordial gas with some low level of metal pollution versus enriched gas stripped during a galaxy−galaxy encounter. Taking advantage of Multi Unit Spectroscopic Explorer operating at the Very Large Telescope, we performed optical integral field spectroscopy of three H <jats:sc>I</jats:sc> clumps in the Leo ring where ultraviolet continuum emission has been found. We detected, for the first time, ionized hydrogen in the ring and identify four nebular regions powered by massive stars. These nebulae show several metal lines ([O <jats:sc>III</jats:sc>], [N <jats:sc>II</jats:sc>], [S <jats:sc>II</jats:sc>]) that allowed reliable measures of metallicities, found to be close to or above the solar value (0.8 ≤ <jats:italic>Z</jats:italic>/<jats:italic>Z</jats:italic> <jats:sub>⊙</jats:sub> ≤ 1.4). Given the faintness of the diffuse stellar counterparts, less than 3% of the observed heavy elements could have been produced locally in the main body of the ring and not much more than 15% in the H <jats:sc>I</jats:sc> clump toward M96. This inference, and the chemical homogeneity among the regions, convincingly demonstrates that the gas in the ring is not primordial, but has been pre-enriched in a galaxy disk, then later removed and shaped by tidal forces and it is forming a sparse population of stars.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We present the first high-resolution abundance analysis of the globular cluster VVV CL001, which resides in a region dominated by high interstellar reddening toward the Galactic bulge. Using <jats:italic>H</jats:italic>-band spectra acquired by the Apache Point Observatory Galactic Evolution Experiment, we identified two potential members of the cluster, and estimated from their Fe <jats:sc>i</jats:sc> lines that the cluster has an average metallicity of [Fe/H] = −2.45 with an uncertainty due to systematics of 0.24 dex. We find that the light-(N), <jats:italic>α</jats:italic>-(O, Mg, Si), and Odd-<jats:italic>Z</jats:italic> (Al) elemental abundances of the stars in VVV CL001 follow the same trend as other Galactic metal-poor globular clusters. This makes VVV CL001 possibly the most metal-poor globular cluster identified so far within the Sun’s galactocentric distance and likely one of the most metal-deficient clusters in the Galaxy after ESO280-SC06. Applying statistical isochrone fitting, we derive self-consistent age, distance, and reddening values, yielding an estimated age of <jats:inline-formula> <jats:tex-math> <?CDATA ${11.9}_{-4.05}^{+3.12}$?> </jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msubsup> <mml:mrow> <mml:mn>11.9</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>4.05</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>3.12</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjlabdf47ieqn1.gif" xlink:type="simple" /> </jats:inline-formula> Gyr at a distance of <jats:inline-formula> <jats:tex-math> <?CDATA ${8.22}_{-1.93}^{+1.84}$?> </jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msubsup> <mml:mrow> <mml:mn>8.22</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>1.93</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>1.84</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjlabdf47ieqn2.gif" xlink:type="simple" /> </jats:inline-formula> kpc, revealing that VVV CL001 is also an old GC in the inner Galaxy. The Galactic orbit of VVV CL001 indicates that this cluster lies on a halo-like orbit that appears to be highly eccentric. Both chemistry and dynamics support the hypothesis that VVV CL001 could be an ancient fossil relic left behind by a massive merger event during the early evolution of the Galaxy, likely associated with either the Sequoia or the Gaia–Enceladus–Sausage structures.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We present the results of Atacama Large Millimeter/submillimeter Array observations in <jats:sup>12</jats:sup>CO(1–0) emission at 0.58 × 0.52 pc<jats:sup>2</jats:sup> resolution toward the brightest H <jats:sc>ii</jats:sc> region N66 of the Small Magellanic Cloud (SMC). The <jats:sup>12</jats:sup>CO(1–0) emission toward the north of N66 reveals clumpy filaments with multiple velocity components. Our analysis shows that a blueshifted filament at a velocity range of 154.4–158.6 km s<jats:sup>−1</jats:sup> interacts with a redshifted filament at a velocity of 158.0–161.8 km s<jats:sup>−1</jats:sup>. A third velocity component at a velocity range of 161–165.0 km s<jats:sup>−1</jats:sup> constitutes hub-filaments. An intermediate-mass young stellar object (YSO) and a young pre-main-sequence star cluster have hitherto been reported in the intersection of these filaments. We find a V-shape distribution in the position–velocity diagram at the intersection of two filaments. This indicates the physical association of those filaments due to a cloud–cloud collision. We determine the collision timescale ∼0.2 Myr using the relative velocity (∼5.1 km s<jats:sup>−1</jats:sup>) and displacement (∼1.1 pc) of those interacting filaments. These results suggest that the event occurred about 0.2 Myr ago and triggered the star formation, possibly an intermediate-mass YSO. We report the first observational evidence for a cloud–cloud collision that triggers star formation in N66N of the low metallicity ∼0.2 <jats:italic>Z</jats:italic> <jats:sub>⊙</jats:sub> galaxy, the SMC, with similar kinematics as in N159W-South and N159E of the Large Magellanic Cloud.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The observation of a 266.94 GHz feature in the Venus spectrum has been attributed to phosphine (PH<jats:sub>3</jats:sub>) in the Venus clouds, suggesting unexpected geological, chemical, or even biological processes. Since both PH<jats:sub>3</jats:sub> and sulfur dioxide (SO<jats:sub>2</jats:sub>) are spectrally active near 266.94 GHz, the contribution to this line from SO<jats:sub>2</jats:sub> must be determined before it can be attributed, in whole or part, to PH<jats:sub>3</jats:sub>. An undetected SO<jats:sub>2</jats:sub> reference line, interpreted as an unexpectedly low SO<jats:sub>2</jats:sub> abundance, suggested that the 266.94 GHz feature could be attributed primarily to PH<jats:sub>3</jats:sub>. However, the low SO<jats:sub>2</jats:sub> and the inference that PH<jats:sub>3</jats:sub> was in the cloud deck posed an apparent contradiction. Here we use a radiative transfer model to analyze the PH<jats:sub>3</jats:sub> discovery, and explore the detectability of different vertical distributions of PH<jats:sub>3</jats:sub> and SO<jats:sub>2</jats:sub>. We find that the 266.94 GHz line does not originate in the clouds, but above 80 km in the Venus mesosphere. This level of line formation is inconsistent with chemical modeling that assumes generation of PH<jats:sub>3</jats:sub> in the Venus clouds. Given the extremely short chemical lifetime of PH<jats:sub>3</jats:sub> in the Venus mesosphere, an implausibly high source flux would be needed to maintain the observed value of 20 ± 10 ppb. We find that typical Venus SO<jats:sub>2</jats:sub> vertical distributions and abundances fit the JCMT 266.94 GHz feature, and the resulting SO<jats:sub>2</jats:sub> reference line at 267.54 GHz would have remained undetectable in the ALMA data due to line dilution. We conclude that nominal mesospheric SO<jats:sub>2</jats:sub> is a more plausible explanation for the JCMT and ALMA data than PH<jats:sub>3</jats:sub>.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Since the day of its explosion, SN 1987A (SN87A) was closely monitored with the aim to study its evolution and to detect its central compact relic. The detection of neutrinos from the supernova strongly supports the formation of a neutron star (NS). However, the constant and fruitless search for this object has led to different hypotheses on its nature. To date, the detection in the Atacama Large Millimeter/submillimeter Array data of a feature that is somehow compatible with the emission arising from a proto-pulsar wind nebula (PWN) is the only hint of the existence of such elusive compact object. Here we tackle this 33 yr old issue by analyzing archived observations of SN87A performed by Chandra and NuSTAR in different years. We firmly detect nonthermal emission in the 10–20 kev energy band, due to synchrotron radiation. The possible physical mechanism powering such emission is twofold: diffusive shock acceleration (DSA) or emission arising from an absorbed PWN. By relating a state-of-the-art magnetohydrodynamic simulation of SN87A to the actual data, we reconstruct the absorption pattern of the PWN embedded in the remnant and surrounded by cold ejecta. We found that, even though the DSA scenario cannot be firmly excluded, the most likely scenario that well explains the data is that of PWN emission.</jats:p>