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<jats:title>Abstract</jats:title> <jats:p>We study the multimessenger signals from the merger of a black hole with a magnetized neutron star using resistive magnetohydrodynamics simulations coupled to full general relativity. We focus on a case with a 5:1 mass ratio, where only a small amount of the neutron star matter remains post-merger, but we nevertheless find that significant electromagnetic radiation can be powered by the interaction of the neutron star’s magnetosphere with the black hole. In the lead-up to merger, strong twisting of magnetic field lines from the inspiral leads to plasmoid emission and results in a luminosity in excess of that expected from unipolar induction. We find that the strongest emission occurs shortly after merger during a transitory period in which magnetic loops form and escape the central region. The remaining magnetic field collimates around the spin axis of the remnant black hole before dissipating, an indication that, in more favorable scenarios (higher black hole spin/lower mass ratio) with larger accretion disks, a jet would form.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>One of the most important and well-established empirical results in astronomy is the Kennicutt–Schmidt relation between the density of interstellar gas and the rate at which that gas forms stars. A tight correlation between these quantities has long been measured at galactic scales. More recently, using surveys of YSOs, a KS relationship has been found within molecular clouds relating the surface density of star formation to the surface density of gas; however, the scaling of these laws varies significantly from cloud to cloud. In this Letter, we use a recently developed, high-accuracy catalog of young stellar objects from Spitzer combined with high-dynamic-range gas column density maps of 12 nearby (&lt;1.5 kpc) molecular clouds from Herschel to re-examine the KS relation within individual molecular clouds. We find a tight, linear correlation between clouds’ star formation rate per unit area and their gas surface density normalized by the gas freefall time. The measured intracloud KS relation, which relates star formation rate to the volume density, extends over more than two orders of magnitude within each cloud and is nearly identical in each of the 12 clouds, implying a constant star formation efficiency per freefall time <jats:italic>ϵ</jats:italic> <jats:sub>ff</jats:sub> ≈ 0.026. The finding of a universal correlation within individual molecular clouds, including clouds that contain no massive stars or massive stellar feedback, favors models in which star formation is regulated by local processes such as turbulence or stellar feedback such as protostellar outflows, and disfavors models in which star formation is regulated only by galaxy properties or supernova feedback on galactic scales.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The detection and characterization of supermassive black holes (SMBHs) in local low mass galaxies is crucial to our understanding of the origins of SMBHs. This statement assumes that low mass galaxies have had a relatively quiet cosmic history, so that their black holes have not undergone significant growth and therefore can be treated as relics of the original SMBH seeds. While recent studies have found optical signatures of active galactic nuclei (AGNs) in a growing population of dwarf galaxies, these studies are biased against low metallicity and relatively merger-free galaxies, thus missing precisely the demographic in which to search for the relics of SMBH seeds. Here, we report the detection of the [Si <jats:sc>vi</jats:sc>]1.963 <jats:italic>μ</jats:italic>m coronal line (CL), a robust indicator of an AGN in the galaxy SDSS J160135.95+311353.7, a nearby (<jats:italic>z</jats:italic> = 0.031) low metallicity galaxy with a stellar mass approximately an order of magnitude lower than the LMC (<jats:italic>M</jats:italic> <jats:sub>*</jats:sub> ≈ 10<jats:sup>8.56</jats:sup> <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub>) and no optical evidence for an AGN. The AGN bolometric luminosity implied by the CL detection is ≈10<jats:sup>42</jats:sup> erg s<jats:sup>−1</jats:sup>, precisely what is predicted from its near-infrared continuum emission based on well-studied AGNs. Our results are consistent with a black hole of mass ≈10<jats:sup>5</jats:sup> <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub>, in line with expectations based on its stellar mass. This is the first time a near-infrared CL has been detected in a low mass, low metallicity galaxy with no optical evidence for AGN activity, providing confirmation of the utility of infrared CLs in finding AGNs in low mass galaxies when optical diagnostics fail. These observations highlight a powerful avenue of investigation to hunt for low mass black holes in the James Webb Space Telescope era.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Fuzzy dark matter (FDM) is an attractive dark matter candidate motivated by small-scale problems in astrophysics and with a rich phenomenology on those scales. We scrutinize the FDM model, more specifically the mass of the FDM particle, through a dynamical analysis for the Galactic ultrafaint dwarf (UFD) galaxies. We use a sample of 18 UFDs to place the strongest constraints to date on the mass of the FDM particle, updating on previous bounds using a subset of the sample used here. We find that most of the sample UFDs prefer an FDM particle mass heavier than 10<jats:sup>−21</jats:sup> eV. In particular, Segue 1 provides the strongest constraint, with <jats:inline-formula> <jats:tex-math> <?CDATA ${m}_{\psi }={1.1}_{-0.7}^{+8.3}\times {10}^{-19}\,\mathrm{eV}$?> </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>ψ</mml:mi> </mml:mrow> </mml:msub> <mml:mo>=</mml:mo> <mml:msubsup> <mml:mrow> <mml:mn>1.1</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>0.7</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>8.3</mml:mn> </mml:mrow> </mml:msubsup> <mml:mo>×</mml:mo> <mml:msup> <mml:mrow> <mml:mn>10</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>19</mml:mn> </mml:mrow> </mml:msup> <mml:mspace width="0.25em" /> <mml:mi>eV</mml:mi> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjlabf501ieqn1.gif" xlink:type="simple" /> </jats:inline-formula>. The constraints found here are the first that are compatible with various other independent cosmological and astrophysical bounds found in the literature, in particular with the latest bounds using the Ly<jats:italic>α</jats:italic> forest. We also find that the constraints obtained in this work are not compatible with the bounds from luminous dwarf galaxies, as already pointed out in the previous work using UFDs. This could indicate that although a viable dark matter model, it might be challenging for the FDM model to solve the small-scale problems.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Microquasars with high-mass companion stars are promising very high energy (VHE; 0.1–100 TeV) gamma-ray emitters, but their behaviors above 10 TeV are poorly known. Using the High Altitude Water Cerenkov (HAWC) observatory, we search for excess gamma-ray emission coincident with the positions of known high-mass microquasars (HMMQs). No significant emission is observed for LS 5039, Cyg X-1, Cyg X-3, and SS 433 with 1523 days of HAWC data. We set the most stringent limit above 10 TeV obtained to date on each individual source. Under the assumption that HMMQs produce gamma rays via a common mechanism, we have performed source-stacking searches, considering two different scenarios: (I) gamma-ray luminosity is a fraction <jats:italic>ϵ</jats:italic> <jats:sub> <jats:italic>γ</jats:italic> </jats:sub> of the microquasar jet luminosity, and (II) VHE gamma rays are produced by relativistic electrons upscattering the radiation field of the companion star in a magnetic field <jats:italic>B</jats:italic>. We obtain <jats:italic>ϵ</jats:italic> <jats:sub> <jats:italic>γ</jats:italic> </jats:sub> &lt; 5.4 × 10<jats:sup>−6</jats:sup> for scenario I, which tightly constrains models that suggest observable high-energy neutrino emission by HMMQs. In the case of scenario II, the nondetection of VHE gamma rays yields a strong magnetic field, which challenges synchrotron radiation as the dominant mechanism of the microquasar emission between 10 keV and 10 MeV.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The radio spectra of main-sequence stars remain largely unconstrained due to the lack of observational data to inform stellar atmosphere models. As such, the dominant emission mechanisms at long wavelengths, how they vary with spectral type, and how much they contribute to the expected brightness at a given radio wavelength are still relatively unknown for most spectral types. We present radio continuum observations of Altair, a rapidly rotating A-type star. We observed Altair with NOEMA in 2018 and 2019 at 1.34, 2.09, and 3.22 mm and with the Very Large Array in 2019 at 6.7 and 9.1 mm. In the radio spectra, we see a brightness temperature minimum at millimeter wavelengths followed by a steep rise to temperatures larger than the optical photosphere, behavior that is unexpected for A-type stars. We use these data to produce the first submillimeter to centimeter spectrum of a rapidly rotating A-type star informed by observations. We generated both PHOENIX and KINICH-PAKAL model atmospheres and determine the KINICH-PAKAL model better reproduces Altair’s radio spectrum. The synthetic spectrum shows a millimeter brightness temperature minimum followed by significant emission over that of the photosphere at centimeter wavelengths. Together, these data and models show how the radio spectrum of an A-type star can reveal the presence of a chromosphere, likely induced by rapid rotation, and that a Rayleigh Jean’s extrapolation of the stellar photosphere is not an adequate representation of a star’s radio spectrum.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Recent astronomical observations of both isomers E and Z of imines such as cyanomethanimine, ethanimine, and 2-propyn-1-imine have revealed that the abundances in the interstellar medium (ISM) of these isomers differ by factors of ∼3–10. Several theories have been proposed to explain the observed behavior, but none of them successfully explains the [E]/[Z] ratios. In this work we present a detailed study of the kinetics of the one-step E-Z isomerization reactions of cyanomethanimine, ethanimine, and 2-propyn-1-imine under interstellar conditions (in the 10–400 K temperature range). This reaction was previously thought to be nonviable in the ISM due to its associated high-energy barrier (about 13,000 K). In this Letter, we show that considering the multidimensional small curvature tunneling approximation, the tunneling effect enables the isomerization even at low temperatures. This is due to the fact that the representative tunneling energy lies in the vibrational ground state of the least stable isomer up to approximately 150 K, making the reaction constants of the isomerization from the least stable to the most stable isomer basically constant. The predicted [E]/[Z] ratios are almost the same as those reported from the astronomical observations for all imines observed. This study demonstrates that the [E]/[Z] ratio of imines in the ISM strongly depends on their relative stability.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>HM Cancri is a double degenerate binary with the shortest orbital period presently known. The 5.36 minute period is seen as a large amplitude, soft X-ray modulation, likely resulting from a hot spot produced by direct impact accretion. With such a short orbital period it is expected to have a gravitational wave luminosity comparable to or larger than that in the X-ray, and its orbital frequency is known to be increasing at a rate consistent with the expected loss of angular momentum due to gravitational radiation. We use recent Neutron Star Interior Composition Explorer observations to extend its long-term X-ray timing baseline to almost 20 yr. Phase coherent timing of these new data combined with existing Chandra data demonstrates conclusively that the rate of orbital frequency increase is slowing, and we measure a nonzero <jats:inline-formula> <jats:tex-math> <?CDATA ${\ddot{f}}_{0}=-8.95\pm 1.4\times {10}^{-27}$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjlabf3ccieqn1.gif" xlink:type="simple" /> </jats:inline-formula> Hz s<jats:sup>−2</jats:sup>, which is to our knowledge the first such measurement of its kind for any compact astrophysical binary. With the simultaneous high precision measurement of <jats:inline-formula> <jats:tex-math> <?CDATA ${\dot{f}}_{0}=3.557\pm 0.005\times {10}^{-16}$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjlabf3ccieqn2.gif" xlink:type="simple" /> </jats:inline-formula> Hz s<jats:sup>−1</jats:sup>, we estimate that the system will reach its maximum orbital frequency of <jats:inline-formula> <jats:tex-math> <?CDATA ${f}_{\max }\approx 3.1172091$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjlabf3ccieqn3.gif" xlink:type="simple" /> </jats:inline-formula> mHz in 1260 ± 200 yr, indicating that the system is close to its epoch of maximum orbital frequency. Assuming mass transfer is conservative, the measurement of <jats:inline-formula> <jats:tex-math> <?CDATA $\ddot{f}\lt 0$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjlabf3ccieqn4.gif" xlink:type="simple" /> </jats:inline-formula> implies that the accretion rate from the donor is growing, with <jats:inline-formula> <jats:tex-math> <?CDATA $-5.4\times {10}^{-10}\lt {\ddot{M}}_{2}\lt -4.0\times {10}^{-10}$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjlabf3ccieqn5.gif" xlink:type="simple" /> </jats:inline-formula> <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub> yr<jats:sup>−2</jats:sup>. Further quantitative comparisons with theoretical models should enable more precise inferences regarding its current evolutionary state.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We present low-fRequency radio observations of a fast-rising blue optical transient (FBOT), AT 2018cow, with the upgraded Giant Metrewave Radio Telescope (uGMRT). Our observations span <jats:italic>t</jats:italic> = 11–570 days post-explosion and a frequency range of 250–1450 MHz. The uGMRT light curves are best modeled as synchrotron emission from an inhomogeneous radio-emitting region expanding into an ionized medium. However, due to the lack of information on the source covering factor, which is a measure of the degree of inhomogeneity, we derive various parameters assuming the source covering factor to be unity. These parameters, hence, indicate limits on the actual values in an inhomogeneous model. We derive the lower limit of the shock radius to be <jats:italic>R</jats:italic> ∼ (6.1−14.4) × 10<jats:sup>16</jats:sup> cm at <jats:italic>t</jats:italic> = 138−257 days post-explosion. We find that the fast-moving ejecta from the explosion are moving with velocity <jats:italic>v</jats:italic> &gt; 0.2<jats:italic>c</jats:italic> up to <jats:italic>t</jats:italic> = 257 days post-explosion. The upper limits of the mass-loss rate of the progenitor are <jats:inline-formula> <jats:tex-math> <?CDATA $\dot{M}$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjlabed55ieqn1.gif" xlink:type="simple" /> </jats:inline-formula> ∼ (4.1−1.7) × 10<jats:sup>−6</jats:sup> <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub> yr<jats:sup>−1</jats:sup> at (19.3−45.7) years before the explosion for a wind velocity <jats:italic>v</jats:italic> <jats:sub>w</jats:sub> = 1000 km s<jats:sup>−1</jats:sup>. These <jats:inline-formula> <jats:tex-math> <?CDATA $\dot{M}$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjlabed55ieqn2.gif" xlink:type="simple" /> </jats:inline-formula> values are ∼ 100 times smaller than the previously reported mass-loss rate 2.2 years before the explosion, indicating an enhanced phase of the mass-loss event close to the end-of-life of the progenitor. Our results are in line with the speculation of the presence of a dense circumstellar shell in the vicinity of AT 2018cow from previous radio, ultra-violet, and optical observations.</jats:p>