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<jats:title>Abstract</jats:title> <jats:p>We report results of the first broadband observation of the transient X-ray pulsar GRO J1008−57 performed in the quiescent state. Observations were conducted quasi-simultaneously with NuSTAR and the Mikhail Pavlinsky ART-XC telescope on board SRG right before the beginning of a Type I outburst. GRO J1008−57 was detected in the state with the lowest observed luminosity around several ×10<jats:sup>34</jats:sup> erg s<jats:sup>−1</jats:sup> and consequently accreting from the cold disk. Timing analysis allowed us to detect pulsations during this state for the first time. The observed pulsed fraction of about 20% is, however, almost three times lower than in brighter states when the accretion proceeds through the standard disk. We traced the evolution of the broadband spectrum of the source on a scale of three orders of magnitude in luminosity and found that at the lowest luminosities the spectrum transforms into the double-hump structure similarly to other X-ray pulsars accreting at low luminosities (X Persei, GX 304–1, and A 0535+262) reinforcing the conclusion that this spectral shape is typical for these objects.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We report the serendipitous detection of an H<jats:sub>2</jats:sub>-bearing damped Ly<jats:italic>α</jats:italic> absorber at <jats:italic>z</jats:italic> = 0.576 in the spectrum of the QSO J0111–0316 in the Cosmic Ultraviolet Baryon Survey. Spectroscopic observations from Hubble Space Telescope-COS in the far-ultraviolet reveal a damped absorber with log[<jats:italic>N</jats:italic>(H <jats:sc>i</jats:sc>)/cm<jats:sup>−2</jats:sup>] = 20.1 ± 0.2 and log[<jats:italic>N</jats:italic>(H<jats:sub>2</jats:sub>)/cm<jats:sup>−2</jats:sup>] <jats:inline-formula> <jats:tex-math> <?CDATA $={18.97}_{-0.06}^{+0.05}$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabf0a0ieqn1.gif" xlink:type="simple" /> </jats:inline-formula>. The diffuse molecular gas is found in two velocity components separated by Δ <jats:italic>ν</jats:italic> ≈ 60 km s<jats:sup>−1</jats:sup>, with &gt;99.9% of the total H<jats:sub>2</jats:sub> column density concentrated in one component. At a metallicity of ≈50% of solar, there is evidence for Fe enhancement and dust depletion, with a dust-to-gas ratio <jats:italic>κ</jats:italic> <jats:sub>O</jats:sub> ≈ 0.4. A galaxy redshift survey conducted with IMACS and LDSS-3C on Magellan reveals an overdensity of nine galaxies at projected distance <jats:italic>d</jats:italic> ≤ 600 proper kpc (pkpc) and line-of-sight velocity offset Δ <jats:italic>ν</jats:italic> <jats:sub> <jats:italic>g</jats:italic> </jats:sub> ≤ 300 km s<jats:sup>−1</jats:sup> from the absorber. The closest is a massive, early-type galaxy at <jats:italic>d</jats:italic> = 41 pkpc that contains ≈70% of the total stellar mass identified at <jats:italic>d</jats:italic> ≤ 310 pkpc of the H<jats:sub>2</jats:sub> absorber. The close proximity of the H<jats:sub>2</jats:sub>-bearing gas to the quiescent galaxy and the Fe-enhanced chemical abundance pattern of the absorber suggest a physical connection, in contrast to a picture in which DLAs are primarily associated with gas-rich dwarfs. This case study illustrates that deep galaxy redshift surveys are needed to gain insight into the diverse environments that host dense and potentially star-forming gas.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Prompted by a recent paper by Dima and Schad, we reconsider the problem of inferring magnetic properties of the corona using polarimetric observations of magnetic dipole (M1) lines. Dima and Schad point to a potential source of degeneracy in a formalism developed by Plowman, which under some circumstances can lead to the solution being under-determined. Here we clarify the nature of the problem. Its resolution lies in solving for the scattering geometry using the elongation of the observed region of the corona. We discuss some conceptual problems that arise when casting the problem for inversion in the observer’s reference frame, and satisfactorily resolve difficulties identified by Plowman, Dima, and Schad.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We report a series of numerical experiments for the propagation of a momentum pulse representing a chromospheric jet, simulated using an idealized magnetohydrodynamic model. The jet in a stratified lower solar atmosphere is subjected to a varied initial driver (amplitude, period) and magnetic field conditions to examine the parameter influence over jet morphology and kinematics. The simulated jet captured key observed spicule characteristics including maximum heights, field-aligned mass motions/trajectories, and cross-sectional width deformations. Next, the jet features also show a prominent bright, bulb-like apex, similar to reported observed chromospheric jets, formed due to the higher density of plasma and/or waves. Furthermore, the simulations highlight the presence of not yet observed internal <jats:italic>crisscross/knots</jats:italic> substructures generated by shock waves reflected within the jet structure. Therefore we suggest verifying these predicted fine-scale structures in highly localized lower solar atmospheric jets, e.g., in spicules or fibrils by high-resolution observations, offered by the Daniel K. Inoyue Solar Telescope or otherwise.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The nova rate in the Milky Way remains largely uncertain, despite its vital importance in constraining models of Galactic chemical evolution as well as understanding progenitor channels for Type Ia supernovae. The rate has been previously estimated to be in the range of ≈10–300 yr<jats:sup>−1</jats:sup>, either based on extrapolations from a handful of very bright optical novae or the nova rates in nearby galaxies; both methods are subject to debatable assumptions. The total discovery rate of optical novae remains much smaller (≈5–10 yr<jats:sup>−1</jats:sup>) than these estimates, even with the advent of all-sky optical time-domain surveys. Here, we present a systematic sample of 12 spectroscopically confirmed Galactic novae detected in the first 17 months of Palomar Gattini-IR (PGIR), a wide-field near-infrared time-domain survey. Operating in the <jats:italic>J</jats:italic> band (≈1.2 <jats:italic>μ</jats:italic>m), which is significantly less affected by dust extinction compared to optical bands, the extinction distribution of the PGIR sample is highly skewed to a large extinction values (&gt;50% of events obscured by <jats:italic>A</jats:italic> <jats:sub> <jats:italic>V</jats:italic> </jats:sub> ≳ 5 mag). Using recent estimates for the distribution of Galactic mass and dust, we show that the extinction distribution of the PGIR sample is commensurate with dust models. The PGIR extinction distribution is inconsistent with that reported in previous optical searches (null-hypothesis probability &lt;0.01%), suggesting that a large population of highly obscured novae have been systematically missed in previous optical searches. We perform the first quantitative simulation of a 3<jats:italic>π</jats:italic> time-domain survey to estimate the Galactic nova rate using PGIR, and derive a rate of <jats:inline-formula> <jats:tex-math> <?CDATA $\approx {43.7}_{-8.7}^{+19.5}$?> </jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mo>≈</mml:mo> <mml:msubsup> <mml:mrow> <mml:mn>43.7</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>8.7</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>19.5</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabeb75ieqn1.gif" xlink:type="simple" /> </jats:inline-formula> yr<jats:sup>−1</jats:sup>. Our results suggest that all-sky near-infrared time-domain surveys are well poised to uncover the Galactic nova population.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Observations by the spacecraft Extreme Ultraviolet Explorer (EUVE), Far Ultraviolet Spectroscopic Explorer (FUSE), Chandra, and XMM-Newton of Capella (<jats:italic>α</jats:italic> Aurigae) have encountered problems with the relative intensities of the Fe <jats:sc>xviii</jats:sc> and Fe <jats:sc>xix</jats:sc> line emission in the soft-X-ray (XUV) and extreme-ultraviolet spectral ranges versus various model predictions based on theoretical atomic data. The reason may either lie in astrophysical phenomena, e.g., at emission or because of absorption by the interstellar medium, in the theoretical atomic data, in one or more spectrometer calibrations, or in the spectral modeling. By measurements using an electron beam ion trap we provide laboratory data obtained under conditions that are reasonably close to stellar emission regions. The laboratory data on Fe <jats:sc>xviii</jats:sc> and Fe <jats:sc>xix</jats:sc> show line ratios that are rather similar to the observations of Capella. The measurements, therefore, rule out astrophysical phenomena and calibration errors and point to issues with the modeling of the observed Capella emission.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>“Changing-look” quasars are a new class of highly variable active galactic nuclei that have changed their spectral type over surprisingly short timescales of just a few years. The origin of this phenomenon is debated, but is likely to reflect some change in the accretion flow. To investigate the disk–corona systems in these objects, we measure optical/UV–X-ray spectral indices (<jats:inline-formula> <jats:tex-math> <?CDATA ${\alpha }_{\mathrm{OX}}$?> </jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mrow> <mml:mi>α</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>OX</mml:mi> </mml:mrow> </mml:msub> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabeb17ieqn1.gif" xlink:type="simple" /> </jats:inline-formula>) and Eddington ratios (<jats:inline-formula> <jats:tex-math> <?CDATA ${\lambda }_{\mathrm{Edd}}$?> </jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mrow> <mml:mi>λ</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>Edd</mml:mi> </mml:mrow> </mml:msub> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabeb17ieqn2.gif" xlink:type="simple" /> </jats:inline-formula>) of 10 previously discovered changing-look quasars at two or more epochs. By comparing these data with simulated results based on the behavior of X-ray binaries, we find possible similarities in spectral indices below the 1% Eddington ratio. We further investigate the Eddington ratios of changing-look quasars before and after their spectral type changes, and find that changing-look quasars cross the 1% Eddington ratio boundary when their broad emission lines disappear/emerge. This is consistent with the disk-wind model as the origin of broad emission lines.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Gas phase formation processes feasible at low temperatures are determined theoretically for 38 isomers obeying the C<jats:sub>3</jats:sub>O<jats:sub>3</jats:sub>H<jats:sub>6</jats:sub> empirical formula, one of them, the simplest ketose dihydroxyacetone, has been observed in gas phase sources. A preliminary search for isomeric forms first targets ethoxy formic acid (C<jats:sub>2</jats:sub>H<jats:sub>5</jats:sub>–O–COOH) as the most stable isomer followed by lactic acid. Profiles corresponding to the minimum energy pathways reveal that the favored conformers of 14 of these isomers can be formed in the gas phase through 29 barrierless processes involving the OH*, CH<jats:sub>3</jats:sub>O*, HCO*, CH<jats:sub>3</jats:sub>*, CH<jats:sub>2</jats:sub>OH, HCOO*, and OHCO* radicals, all of them observed in the interstellar medium. Kinetic rates are provided at 200, 298, and 500 K, confirming the suitability of 16 processes at low temperatures. Faster processes involve the OH hydroxyl radical whereas, to a lesser degree, the processes involving the HOCO radical and the methoxy methyl radical CH<jats:sub>3</jats:sub>O*, are quite significant. Spectroscopic parameters (rovibrational and torsional) are obtained for methoxy acetic acid (CH<jats:sub>3</jats:sub>–O–CH<jats:sub>2</jats:sub>COOH) for which two low-lying isoenergetic conformers can be produced from the CH<jats:sub>3</jats:sub>OCH<jats:sub>2</jats:sub>* radical predicted to be a precursor of abundant observed molecules. Profiles and spectroscopic properties make methoxy acetic acid a good candidate to be detected in the gas phase of extraterrestrial sources.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We present the largest and deepest late-time radio and millimeter survey to date of superluminous supernovae (SLSNe) and long-duration gamma-ray bursts (LGRBs) to search for associated nonthermal synchrotron emission. Using the Karl G. Jansky Very Large Array (VLA) and the Atacama Large Millimeter/submillimeter Array (ALMA), we observed 43 sources at 6 and 100 GHz on a timescale of ∼ 1–19 yr post-explosion. We do not detect radio/millimeter emission from any of the sources, with the exception of a 6 GHz detection of PTF10hgi, as well as the detection of 6 GHz emission near the location of the SLSN PTF12dam, which we associate with its host galaxy. We use our data to place constraints on central engine emission due to magnetar wind nebulae and off-axis relativistic jets. We also explore nonrelativistic emission from the SN ejecta, and place constraints on obscured star formation in the host galaxies. In addition, we conduct a search for fast radio bursts (FRBs) from some of the sources using VLA phased-array observations; no FRBs are detected to a limit of 16 mJy (7<jats:italic>σ</jats:italic>; 10 ms duration) in about 40 minutes on source per event. A comparison to theoretical models suggests that continued radio monitoring may lead to detections of persistent radio emission on timescales of ≳ a decade.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We present element abundance ratios and ionizing radiation of local young low-mass (∼10<jats:sup>6</jats:sup> <jats:italic> M</jats:italic> <jats:sub>⊙</jats:sub>) extremely metal-poor galaxies (EMPGs) with a 2% solar oxygen abundance (O/H)<jats:sub>⊙</jats:sub> and a high specific star formation rate (sSFR ∼ 300 Gyr<jats:sup>−1</jats:sup>) and other (extremely) metal-poor galaxies, which are compiled from Extremely Metal-Poor Representatives Explored by the Subaru Survey (EMPRESS) and the literature. Weak emission lines such as [Fe <jats:sc>iii</jats:sc>] <jats:italic>λ</jats:italic>4658 and He <jats:sc>ii</jats:sc> <jats:italic>λ</jats:italic>4686 are detected in very deep optical spectra of the EMPGs taken with 8 m class telescopes, including Keck and Subaru, enabling us to derive element abundance ratios with photoionization models. We find that neon-to-oxygen and argon-to-oxygen ratios are comparable to those of known local dwarf galaxies and that the nitrogen-to-oxygen abundance ratios (N/O) are lower than 20% (N/O)<jats:sub>⊙</jats:sub>, consistent with the low oxygen abundance. However, the iron-to-oxygen abundance ratios (Fe/O) of the EMPGs are generally high; the EMPGs with the 2%-solar oxygen abundance show high Fe/O ratios of ∼90%–140% (Fe/O)<jats:sub>⊙</jats:sub>, which are unlikely to be explained by suggested scenarios of Type Ia supernova iron productions, iron’s dust depletion, and metal-poor gas inflow onto previously metal-riched galaxies with solar abundances. Moreover, the EMPGs with the 2%-solar oxygen abundance have very high He <jats:sc>ii</jats:sc> <jats:italic>λ</jats:italic>4686/H<jats:italic>β</jats:italic> ratios of ∼1/40, which are not reproduced by existing models of high-mass X-ray binaries with progenitor stellar masses &lt;120<jats:italic> M</jats:italic> <jats:sub>⊙</jats:sub>. Comparing stellar-nucleosynthesis and photoionization models with a comprehensive sample of EMPGs identified by this and previous EMPG studies, we propose that both the high Fe/O ratios and the high He <jats:sc>ii</jats:sc> <jats:italic>λ</jats:italic>4686/H<jats:italic>β</jats:italic> ratios are explained by the past existence of supermassive (&gt;300<jats:italic> M</jats:italic> <jats:sub>⊙</jats:sub>) stars, which may evolve into intermediate-mass black holes (≳100<jats:italic> M</jats:italic> <jats:sub>⊙</jats:sub>).</jats:p>