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<jats:title>Abstract</jats:title> <jats:p>In this paper, we present photometric and spectroscopic observations of the subluminous Type Ia supernova (SN Ia) 2012ij, which has an absolute <jats:italic>B</jats:italic>-band peak magnitude <jats:inline-formula> <jats:tex-math> <?CDATA ${M}_{B,\max }=-17.95\pm 0.15$?> </jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mrow> <mml:mi>M</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>B</mml:mi> <mml:mo>,</mml:mo> <mml:mi>max</mml:mi> </mml:mrow> </mml:msub> <mml:mo>=</mml:mo> <mml:mo>−</mml:mo> <mml:mn>17.95</mml:mn> <mml:mo>±</mml:mo> <mml:mn>0.15</mml:mn> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjac4e17ieqn1.gif" xlink:type="simple" /> </jats:inline-formula> mag. The <jats:italic>B</jats:italic>-band light curve exhibits a fast postpeak decline with Δ<jats:italic>m</jats:italic> <jats:sub>15</jats:sub>(<jats:italic>B</jats:italic>) = 1.86 ± 0.05 mag. All the <jats:italic>R</jats:italic>- and <jats:italic>I</jats:italic>/<jats:italic>i</jats:italic>-band light curves show a weak secondary peak/shoulder feature at about 3 weeks after the peak, like some transitional subclass of SNe Ia, which could result from an incomplete merger of near-infrared (NIR) double peaks. The spectra are characterized by Ti <jats:sc>ii</jats:sc> and strong Si <jats:sc>ii</jats:sc> <jats:italic>λ</jats:italic>5972 absorption features that are usually seen in low-luminosity objects like SN 1999by. The NIR spectrum before maximum light reveals weak carbon absorption features, implying the existence of unburned materials. We compare the observed properties of SN 2012ij with those predicted by the sub-Chandrasekhar-mass and the Chandrasekhar-mass delayed-detonation models and find that both optical and NIR spectral properties can be explained to some extent by these two models. By comparing the secondary maximum features in the <jats:italic>I</jats:italic> and <jats:italic>i</jats:italic> bands, we suggest that SN 2012ij is a transitional object linking normal SNe Ia to typical 91bg-like ones. From the published sample of SNe Ia from the Carnegie Supernova Project II, we estimate that the fraction of SN 2012ij–like SNe Ia is not lower than ∼2%.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Hercules X-1/HZ Hercules (Her X-1/HZ Her) is an X-ray binary monitored by multiple X-ray missions since the last century. With the abundance of long-term observations, we present a complete set of orbital lightcurves of Her X-1/HZ Her during the six states of the 35 day cycle in multiple energy bands. These illustrate in detail the changing lightcurve caused by the rotating twisted-tilted accretion disk surrounding the neutron star. The orbital lightcurves during the main high state are analyzed in 0.05 35 day phase intervals. These show the regular occurrence of pre-eclipse dips that march to earlier orbital phases as the 35 day phases increase. From the multiband lightcurves, we derive the time-average orbital phase dependence of column density for photoelectric absorption and energy-independent transmission as a function of 35 day phase. The X-ray lightcurves during low states are similar in shape to the optical low-state lightcurve, but X-ray leads optical by ≃0.04–0.08 in orbital phase.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We apply a conventional accretion disk model to the FU Ori–type objects HBC 722 and Gaia 17bpi. Our base model is a steady-state, thin Keplerian disk featuring a modified Shakura–Sunyaev temperature profile, with each annulus radiating as an area-weighted spectrum given by a NextGen atmosphere at the appropriate temperature. We explore departures from the standard model by altering the temperature distribution in the innermost region of the disk to account for “boundary region”–like effects. We consider the overall spectral energy distribution as well as medium- and high-resolution spectra in evaluating best-fit models to the data. Parameter degeneracies are studied via a Markov Chain Monte Carlo parameter estimation technique. Allowing all parameters to vary, we find accretion rates for HBC 722 of <jats:inline-formula> <jats:tex-math> <?CDATA $\dot{M}={10}^{-4.90}{M}_{\odot }{\mathrm{yr}}^{-1}\,{}_{-0.40}^{+0.99}\mathrm{dex}$?> </jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mover accent="true"> <mml:mrow> <mml:mi>M</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>̇</mml:mo> </mml:mrow> </mml:mover> <mml:mo>=</mml:mo> <mml:msup> <mml:mrow> <mml:mn>10</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>4.90</mml:mn> </mml:mrow> </mml:msup> <mml:msub> <mml:mrow> <mml:mi>M</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>⊙</mml:mo> </mml:mrow> </mml:msub> <mml:msup> <mml:mrow> <mml:mi>yr</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>1</mml:mn> </mml:mrow> </mml:msup> <mml:mspace width="0.25em" /> <mml:msubsup> <mml:mrow /> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>0.40</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>0.99</mml:mn> </mml:mrow> </mml:msubsup> <mml:mi>dex</mml:mi> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjac496bieqn1.gif" xlink:type="simple" /> </jats:inline-formula> and for Gaia 17bpi of <jats:inline-formula> <jats:tex-math> <?CDATA $\dot{M}={10}^{-6.70}{M}_{\odot }{\mathrm{yr}}^{-1}\,{}_{-0.36}^{+0.46}\mathrm{dex};$?> </jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mover accent="true"> <mml:mrow> <mml:mi>M</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>̇</mml:mo> </mml:mrow> </mml:mover> <mml:mo>=</mml:mo> <mml:msup> <mml:mrow> <mml:mn>10</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>6.70</mml:mn> </mml:mrow> </mml:msup> <mml:msub> <mml:mrow> <mml:mi>M</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>⊙</mml:mo> </mml:mrow> </mml:msub> <mml:msup> <mml:mrow> <mml:mi>yr</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>1</mml:mn> </mml:mrow> </mml:msup> <mml:mspace width="0.25em" /> <mml:msubsup> <mml:mrow /> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>0.36</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>0.46</mml:mn> </mml:mrow> </mml:msubsup> <mml:mi>dex</mml:mi> <mml:mo>;</mml:mo> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjac496bieqn2.gif" xlink:type="simple" /> </jats:inline-formula> the corresponding maximum disk temperatures are <jats:inline-formula> <jats:tex-math> <?CDATA ${7100}_{-500}^{+300}$?> </jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msubsup> <mml:mrow> <mml:mn>7100</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>500</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>300</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjac496bieqn3.gif" xlink:type="simple" /> </jats:inline-formula> K and <jats:inline-formula> <jats:tex-math> <?CDATA ${7900}_{-400}^{+900}$?> </jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msubsup> <mml:mrow> <mml:mn>7900</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>400</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>900</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjac496bieqn4.gif" xlink:type="simple" /> </jats:inline-formula> K, respectively. While the accretion rate of HBC 722 is on the same order as other FU Ori–type objects, Gaia 17bpi has a lower rate than previously reported as typical, commensurate with its lower luminosity. Alternate models that fix some disk or stellar parameters are also presented, with tighter confidence intervals on the remaining fitted parameters. In order to improve upon the somewhat large credible intervals for the <jats:inline-formula> <jats:tex-math> <?CDATA $\dot{M}$?> </jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mover accent="true"> <mml:mrow> <mml:mi>M</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>̇</mml:mo> </mml:mrow> </mml:mover> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjac496bieqn5.gif" xlink:type="simple" /> </jats:inline-formula> values, and to make progress on boundary layer characterization, flux-calibrated ultraviolet spectroscopy is needed.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We analyze the global structure of the Milky Way (MW)'s stellar halo, including its dominant subcomponent, Gaia-Sausage-Enceladus (GSE). The method for reconstructing the global distribution of this old stellar component is to employ the superposition of the orbits covering the large MW’s space, where each of the orbit-weighting factors is assigned following the probability that the star is located at its currently observed position. The selected local, metal-poor sample with [Fe/H] &lt;−1, using Gaia Early Data Release 3 and Sloan Digital Sky Survey Data Release 16, shows that the global shape of the halo is systematically rounder at all radii in more metal-poor ranges, such that an axial ratio, <jats:italic>q</jats:italic>, is nearly 1 for [Fe/H] &lt;−2.2 and ∼0.7 for −1.4 &lt; [Fe/H] &lt; −1.0. It is also found that a halo in the relatively metal-rich range of [Fe/H] &gt;−1.8 actually shows a boxy/peanut-like shape, suggesting a major merger event. The distribution of azimuthal velocities shows a disk-like flattened structure at −1.4 &lt; [Fe/H] &lt; −1.0, which is thought to be the metal-weak thick disk. For the subsample of stars showing GSE-like kinematics, at [Fe/H] &gt;−1.8, its global density distribution has an axis ratio of 0.9, which is more spherical than the general halo sample, and an outer ridge at <jats:italic>r</jats:italic> ~ 20 kpc. This spherical shape is consistent with the features of accreted halo components, and the ridge suggests that the orbit of GSE’s progenitor had an apocenter of ∼20 kpc. Implications for the formation of the stellar halo are also presented.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The electromagnetic energy flux in the lower atmosphere of the Sun is a key tool to describe the energy balance of the solar atmosphere. Current investigations on energy flux in the solar atmosphere focus primarily on the vertical electromagnetic flux through the photosphere, ignoring the Poynting flux in other directions and its possible contributions to local heating. Based on a realistic Bifrost simulation of a quiet-Sun (coronal hole) atmosphere, we find that the total electromagnetic energy flux in the photosphere occurs mainly parallel to the photosphere, concentrating in small regions along intergranular lanes. Thereby, it was possible to define a proxy for this energy flux based on only variables that can be promptly retrieved from observations, namely, horizontal velocities of the small-scale magnetic elements and their longitudinal magnetic flux. Our proxy accurately describes the actual Poynting flux distribution in the simulations, with the electromagnetic energy flux reaching 10<jats:sup>10</jats:sup> erg cm<jats:sup>−2</jats:sup> s<jats:sup>−1</jats:sup>. To validate our findings, we extended the analysis to <jats:sc>Sunrise</jats:sc>/IMaX data. First, we show that Bifrost realistically describes photospheric quiet-Sun regions, as the simulation presents similar distributions for line-of-sight magnetic flux and horizontal velocity field. Second, we found very similar horizontal Poynting flux proxy distributions for the simulated photosphere and observational data. Our results also indicate that the horizontal Poynting flux in the observations is considerably larger than the vertical electromagnetic flux from previous observational estimates. Therefore, our analysis confirms that the electromagnetic energy flux in the photosphere is mainly horizontal and is most intense in localized regions along intergranular lanes.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We combine 126 new galaxy-O <jats:sc>vi</jats:sc> absorber pairs from the CGM<jats:sup>2</jats:sup> survey with 123 pairs drawn from the literature to examine the simultaneous dependence of the column density of O <jats:sc>vi</jats:sc> absorbers (<jats:italic>N</jats:italic> <jats:sub> <jats:italic>O</jats:italic> VI</jats:sub>) on galaxy stellar mass, star-formation rate, and impact parameter. The combined sample consists of 249 galaxy-O <jats:sc>vi</jats:sc> absorber pairs covering <jats:italic>z</jats:italic> = 0–0.6, with host galaxy stellar masses <jats:italic>M</jats:italic> <jats:sub>*</jats:sub> = 10<jats:sup>7.8</jats:sup>–10<jats:sup>11.2</jats:sup> <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub> and galaxy-absorber impact parameters <jats:italic>R</jats:italic> <jats:sub>⊥</jats:sub> = 0–400 proper kiloparsecs. In this work, we focus on the variation of <jats:italic>N</jats:italic> <jats:sub> <jats:italic>O</jats:italic> VI</jats:sub> with galaxy mass and impact parameter among the star-forming galaxies in the sample. We find that the average <jats:italic>N</jats:italic> <jats:sub> <jats:italic>O</jats:italic> VI</jats:sub> within one virial radius of a star-forming galaxy is greatest for star-forming galaxies with <jats:italic>M</jats:italic> <jats:sub>*</jats:sub> = 10<jats:sup>9.2</jats:sup>–10<jats:sup>10</jats:sup> <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub>. Star-forming galaxies with <jats:italic>M</jats:italic> <jats:sub>*</jats:sub> between 10<jats:sup>8</jats:sup> and 10<jats:sup>11.2</jats:sup> <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub> can explain most O <jats:sc>vi</jats:sc> systems with column densities greater than 10<jats:sup>13.5</jats:sup> cm<jats:sup>−2</jats:sup>. Sixty percent of the O <jats:sc>vi</jats:sc> mass associated with a star-forming galaxy is found within one virial radius, and 35% is found between one and two virial radii. In general, we find that some departure from hydrostatic equilibrium in the CGM is necessary to reproduce the observed O <jats:sc>vi</jats:sc> amount, galaxy mass dependence, and extent. Our measurements serve as a test set for CGM models over a broad range of host galaxy masses.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Core-collapse supernovae can display evidence of interaction with preexisting, circumstellar shells of material by rebrightening and forming spectral lines, and can even change types as hydrogen appears in previously hydrogen-poor spectra. However, a recently observed core-collapse supernova—SN 2019tsf—was found to brighten after roughly 100 days after it was first observed, suggesting that the supernova ejecta was interacting with surrounding material, but it lacked any observable emission lines and thereby challenged the standard supernova-interaction picture. We show through linear perturbation theory that delayed rebrightenings without the formation of spectral lines are generated as a consequence of the finite sound-crossing time of the postshock gas left in the wake of a supernova explosion. In particular, we demonstrate that sound waves—generated in the postshock flow as a consequence of the interaction between a shock and a density enhancement—traverse the shocked ejecta and impinge upon the shock from behind in a finite time, generating sudden changes in the shock properties in the absence of ambient density enhancements. We also show that a blast wave dominated by gas pressure and propagating in a wind-fed medium is unstable from the standpoint that small perturbations lead to the formation of reverse shocks within the postshock flow, implying that the gas within the inner regions of these blast waves should be highly turbulent.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We measure the low-<jats:italic>J</jats:italic> CO line ratios <jats:italic>R</jats:italic> <jats:sub>21</jats:sub> ≡ CO (2–1)/CO (1–0), <jats:italic>R</jats:italic> <jats:sub>32</jats:sub> ≡ CO (3–2)/CO (2–1), and <jats:italic>R</jats:italic> <jats:sub>31</jats:sub> ≡CO (3–2)/CO (1–0) using whole-disk CO maps of nearby galaxies. We draw CO (2–1) from PHANGS-ALMA, HERACLES, and follow-up IRAM surveys; CO (1–0) from COMING and the Nobeyama CO Atlas of Nearby Spiral Galaxies; and CO (3–2) from the James Clerk Maxwell Telescope Nearby Galaxy Legacy Survey and Atacama Pathfinder Experiment Large APEX Sub-Millimetre Array mapping. All together, this yields 76, 47, and 29 maps of <jats:italic>R</jats:italic> <jats:sub>21</jats:sub>, <jats:italic>R</jats:italic> <jats:sub>32</jats:sub>, and <jats:italic>R</jats:italic> <jats:sub>31</jats:sub> at 20″ ∼ 1.3 kpc resolution, covering 43, 34, and 20 galaxies. Disk galaxies with high stellar mass, <jats:inline-formula> <jats:tex-math> <?CDATA $\mathrm{log}({M}_{\star }/{M}_{\odot })=10.25\mbox{--}11$?> </jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>log</mml:mi> <mml:mo stretchy="false">(</mml:mo> <mml:msub> <mml:mrow> <mml:mi>M</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>⋆</mml:mo> </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:mo>⊙</mml:mo> </mml:mrow> </mml:msub> <mml:mo stretchy="false">)</mml:mo> <mml:mo>=</mml:mo> <mml:mn>10.25</mml:mn> <mml:mo>–</mml:mo> <mml:mn>11</mml:mn> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjac3490ieqn1.gif" xlink:type="simple" /> </jats:inline-formula>, and star formation rate (SFR) = 1–5 <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub> yr<jats:sup>−1</jats:sup>, dominate the sample. We find galaxy-integrated mean values and a 16%–84% range of <jats:italic>R</jats:italic> <jats:sub>21</jats:sub> = 0.65 (0.50–0.83), <jats:italic>R</jats:italic> <jats:sub>32</jats:sub> = 0.50 (0.23–0.59), and <jats:italic>R</jats:italic> <jats:sub>31</jats:sub> = 0.31 (0.20–0.42). We identify weak trends relating galaxy-integrated line ratios to properties expected to correlate with excitation, including SFR/<jats:italic>M</jats:italic> <jats:sub>⋆</jats:sub> and SFR/<jats:italic>L</jats:italic> <jats:sub>CO</jats:sub>. Within galaxies, we measure central enhancements with respect to the galaxy-averaged value of ∼<jats:inline-formula> <jats:tex-math> <?CDATA ${0.18}_{-0.14}^{+0.09}$?> </jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msubsup> <mml:mrow> <mml:mn>0.18</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>0.14</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>0.09</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjac3490ieqn2.gif" xlink:type="simple" /> </jats:inline-formula> dex for <jats:italic>R</jats:italic> <jats:sub>21</jats:sub>, <jats:inline-formula> <jats:tex-math> <?CDATA ${0.27}_{-0.15}^{+0.13}$?> </jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msubsup> <mml:mrow> <mml:mn>0.27</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>0.15</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>0.13</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjac3490ieqn3.gif" xlink:type="simple" /> </jats:inline-formula> dex for <jats:italic>R</jats:italic> <jats:sub>31</jats:sub>, and <jats:inline-formula> <jats:tex-math> <?CDATA ${0.08}_{-0.09}^{+0.11}$?> </jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msubsup> <mml:mrow> <mml:mn>0.08</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>0.09</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>0.11</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjac3490ieqn4.gif" xlink:type="simple" /> </jats:inline-formula> dex for <jats:italic>R</jats:italic> <jats:sub>32</jats:sub>. All three line ratios anticorrelate with galactocentric radius and positively correlate with the local SFR surface density and specific SFR, and we provide approximate fits to these relations. The observed ratios can be reasonably reproduced by models with low temperature, moderate opacity, and moderate densities, in good agreement with expectations for the cold interstellar medium. Because the line ratios are expected to anticorrelate with the CO (1–0)-to-H<jats:sub>2</jats:sub> conversion factor, <jats:inline-formula> <jats:tex-math> <?CDATA ${\alpha }_{\mathrm{CO}}^{1-0}$?> </jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msubsup> <mml:mrow> <mml:mi>α</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>CO</mml:mi> </mml:mrow> <mml:mrow> <mml:mn>1</mml:mn> <mml:mo>−</mml:mo> <mml:mn>0</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjac3490ieqn5.gif" xlink:type="simple" /> </jats:inline-formula>, these results have general implications for the interpretation of CO emission from galaxies.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Microlensing is a powerful technique to study the Galactic population of “dark” objects such as exoplanets both bound and unbound, brown dwarfs, low-luminosity stars, old white dwarfs, and neutron stars, and it is almost the only way to study isolated stellar-mass black holes. The majority of previous efforts to search for gravitational microlensing events have concentrated toward high-density fields such as the Galactic bulge. Microlensing events in the Galactic plane have the advantage of closer proximity and better constrained relative proper motions, leading to better constrained estimates of lens mass at the expense of a lower optical depth, than events toward the Galactic bulge. We use the Zwicky Transient Facility Data Release 5 compiled from 2018–2021 to survey the Galactic plane in the region of ∣<jats:italic>b</jats:italic>∣ &lt; 20°. We find a total of 60 candidate microlensing events including three that show a strong microlensing parallax effect. The rate of events traces Galactic structure, decreasing exponentially as a function Galactic longitude with scale length <jats:italic>ℓ</jats:italic> <jats:sub>0</jats:sub> ∼ 37°. On average, we find Einstein timescales of our microlensing events to be about three times as long (∼60 days) as those toward the Galactic bulge (∼20 days). This pilot project demonstrates that microlensing toward the Galactic plane shows strong promise for characterization of dark objects within the Galactic disk.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We report the results from the broadband X-ray monitoring of the new Galactic black hole candidate MAXI J1803−298 with MAXI/GSC and Swift/BAT during its outburst. After the discovery on 2021 May 1, the soft X-ray flux below 10 keV rapidly increased for ∼10 days, then gradually decreased over five months. In the brightest phase, the source exhibited the state transition from the low/hard state to the high/soft state via the intermediate state. The broadband X-ray spectrum during the outburst is well described with a disk blackbody plus its thermal or nonthermal Comptonization. Before the transition, the source spectrum is described by a thermal Comptonization component with a photon index of ∼1.7 and an electron temperature of ∼30 keV, while a strong disk blackbody component is observed after the transition. The spectral properties in these periods are consistent with the low/hard state and the high/soft state, respectively. A sudden flux drop with a duration of a few days, unassociated with a significant change in the hardness ratio, was found in the intermediate state. A possible cause of this variation is that the mass accretion rate rapidly increased at the disk transition, which induced a strong Compton-thick outflow and scattered out the X-ray flux. Assuming a nonspinning black hole, we estimate the black hole mass of MAXI J1803−298 to be<jats:inline-formula> <jats:tex-math> <?CDATA $5.8\pm 0.4\,{(\cos i/\cos 70^\circ )}^{-1/2}(D/8\,\mathrm{kpc})\,{M}_{\odot }$?> </jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mn>5.8</mml:mn> <mml:mo>±</mml:mo> <mml:mn>0.4</mml:mn> <mml:mspace width="0.25em" /> <mml:msup> <mml:mrow> <mml:mo stretchy="false">(</mml:mo> <mml:mi>cos</mml:mi> <mml:mi>i</mml:mi> <mml:mrow> <mml:mo stretchy="true">/</mml:mo> </mml:mrow> <mml:mi>cos</mml:mi> <mml:mn>70</mml:mn> <mml:mo>°</mml:mo> <mml:mo stretchy="false">)</mml:mo> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>1</mml:mn> <mml:mrow> <mml:mo stretchy="true">/</mml:mo> </mml:mrow> <mml:mn>2</mml:mn> </mml:mrow> </mml:msup> <mml:mo stretchy="false">(</mml:mo> <mml:mi>D</mml:mi> <mml:mrow> <mml:mo stretchy="true">/</mml:mo> </mml:mrow> <mml:mn>8</mml:mn> <mml:mspace width="0.25em" /> <mml:mi>kpc</mml:mi> <mml:mo stretchy="false">)</mml:mo> <mml:mspace width="0.50em" /> <mml:msub> <mml:mrow> <mml:mi>M</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>⊙</mml:mo> </mml:mrow> </mml:msub> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjac517bieqn1.gif" xlink:type="simple" /> </jats:inline-formula> (where <jats:italic>i</jats:italic> and <jats:italic>D</jats:italic> are the inclination angle and the distance, respectively) from the inner disk radius obtained in the high/soft state.</jats:p>