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<jats:title>Abstract</jats:title> <jats:p>Study of the double-detonation Type Ia supernova scenario, in which a helium-shell detonation triggers a carbon-core detonation in a sub-Chandrasekhar-mass white dwarf (WD), has experienced a resurgence in the past decade. New evolutionary scenarios and a better understanding of which nuclear reactions are essential have allowed for successful explosions in WDs with much thinner helium shells than in the original, decades-old incarnation of the double-detonation scenario. In this paper, we present the first suite of light curves and spectra from multidimensional radiative transfer calculations of thin-shell double-detonation models, exploring a range of WD and helium-shell masses. We find broad agreement with the observed light curves and spectra of nonpeculiar Type Ia supernovae, from subluminous to overluminous subtypes, providing evidence that double detonations of sub-Chandrasekhar-mass WDs produce the bulk of observed Type Ia supernovae. Some discrepancies in spectral velocities and colors persist, but these may be brought into agreement by future calculations that include more accurate initial conditions and radiation transport physics.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We report the discovery of two kinematically anomalous atomic hydrogen (H <jats:sc>i</jats:sc>) clouds in M 100 (NGC 4321), which was observed as part of the Deciphering the Interplay between the Interstellar medium, Stars, and the Circumgalactic medium (DIISC) survey in H <jats:sc>i</jats:sc> 21 cm at 3.3 km s<jats:sup>−1</jats:sup> spectroscopic and 44″ × 30″ spatial resolution using the Karl G. Jansky Very Large Array.<jats:xref ref-type="fn" rid="apjac2303fn1"> <jats:sup>15</jats:sup> </jats:xref> <jats:fn id="apjac2303fn1"> <jats:label> <jats:sup>15</jats:sup> </jats:label> <jats:p>The National Radio Astronomy Observatory is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc.</jats:p> </jats:fn> These clouds were identified as structures that show significant kinematic offsets from the rotating disk of M 100. The velocity offsets of 40 km s<jats:sup>−1</jats:sup> observed in these clouds are comparable to the offsets seen in intermediate-velocity clouds (IVCs) in the circumgalactic medium (CGM) of the Milky Way and nearby galaxies. We find that one anomalous cloud in M 100 is associated with star-forming regions detected in H<jats:italic>α</jats:italic> and far-ultraviolet imaging. Our investigation shows that anomalous clouds in M 100 may originate from multiple mechanisms, such as star formation feedback-driven outflows, ram pressure stripping, and tidal interactions with satellite galaxies. Moreover, we do not detect any cool CGM at 38.8 kpc from the center of M 100, giving an upper limit of N(H <jats:sc>i</jats:sc>) ≤1.7 × 10<jats:sup>13</jats:sup> cm<jats:sup>−2</jats:sup> (3<jats:italic>σ</jats:italic>). Since M 100 is in the Virgo cluster, the nonexistence of neutral/cool CGM is a likely pathway for turning it into a red galaxy.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Although microlensing of macrolensed quasars and supernovae provides unique opportunities for several kinds of investigations, it can add unwanted and sometimes substantial noise. While microlensing flux anomalies may be safely ignored for some observations, they severely limit others. “Worst-case” estimates can inform the decision whether or not to undertake an extensive examination of microlensing scenarios. Here, we report “worst-case” microlensing uncertainties for point sources lensed by singular isothermal potentials, parameterized by a convergence equal to the shear and by the stellar fraction. The results can be straightforwardly applied to nonisothermal potentials by utilizing the mass sheet degeneracy. We use microlensing maps to compute the fluctuations in image micromagnifications and estimate the stellar fraction at which the fluctuations are greatest for a given convergence. We find that the worst-case fluctuations happen at the stellar fraction <jats:inline-formula> <jats:tex-math> <?CDATA ${\kappa }_{\star }=\tfrac{1}{| {\mu }_{\mathrm{macro}}| }$?> </jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mrow> <mml:mi>κ</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>⋆</mml:mo> </mml:mrow> </mml:msub> <mml:mo>=</mml:mo> <mml:mstyle displaystyle="false"> <mml:mfrac> <mml:mrow> <mml:mn>1</mml:mn> </mml:mrow> <mml:mrow> <mml:mo stretchy="false">∣</mml:mo> <mml:msub> <mml:mrow> <mml:mi>μ</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>macro</mml:mi> </mml:mrow> </mml:msub> <mml:mo stretchy="false">∣</mml:mo> </mml:mrow> </mml:mfrac> </mml:mstyle> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjac2228ieqn1.gif" xlink:type="simple" /> </jats:inline-formula>. For macrominima, the fluctuations in both magnification and demagnification appear to be bounded (1.5 &gt; Δ<jats:italic>m</jats:italic> &gt; −1.3, where Δ<jats:italic>m</jats:italic> is the magnitude relative to the average macromagnification). Magnifications for macrosaddles are bounded as well (Δ<jats:italic>m</jats:italic> &gt; −1.7). In contrast, demagnifications for macrosaddles appear to have unbounded fluctuations as 1/<jats:italic>μ</jats:italic> <jats:sub>macro</jats:sub> → 0 and <jats:italic>κ</jats:italic> <jats:sub>⋆</jats:sub> → 0.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We present a search for continuous gravitational-wave emission due to r-modes in the pulsar PSR J0537–6910 using data from the LIGO–Virgo Collaboration observing run O3. PSR J0537–6910 is a young energetic X-ray pulsar and is the most frequent glitcher known. The inter-glitch braking index of the pulsar suggests that gravitational-wave emission due to r-mode oscillations may play an important role in the spin evolution of this pulsar. Theoretical models confirm this possibility and predict emission at a level that can be probed by ground-based detectors. In order to explore this scenario, we search for r-mode emission in the epochs between glitches by using a contemporaneous timing ephemeris obtained from NICER data. We do not detect any signals in the theoretically expected band of 86–97 Hz, and report upper limits on the amplitude of the gravitational waves. Our results improve on previous amplitude upper limits from r-modes in J0537-6910 by a factor of up to 3 and place stringent constraints on theoretical models for r-mode-driven spin-down in PSR J0537–6910, especially for higher frequencies at which our results reach below the spin-down limit defined by energy conservation.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We study the gravitational lensing properties of a massive object in a dark matter halo, concentrating on the critical curves and caustics of the combined lens. We model the system in the simplest approximation by a point mass embedded in a spherical Navarro–Frenk–White density profile. The low number of parameters of such a model permits a systematic exploration of its parameter space. We present galleries of critical curves and caustics for different masses and positions of the point in the halo. We demonstrate the existence of a critical mass, above which the gravitational influence of the centrally positioned point is strong enough to eliminate the radial critical curve and caustic of the halo. In the point-mass parameter space we identify the boundaries at which critical-curve transitions and corresponding caustic metamorphoses occur. The number of transitions as a function of the position of the point is surprisingly high, ranging from three for higher masses to as many as eight for lower masses. On the caustics we identify the occurrence of six different types of caustic metamorphoses. We illustrate the peculiar properties of the single radial critical curve and caustic appearing in an additional unusual nonlocal metamorphosis for a critical mass positioned at the halo center. Although we construct the model primarily to study the lensing influence of individual galaxies in a galaxy cluster, it can also be used to study the lensing by dwarf satellite galaxies in the halo of a host galaxy, as well as (super)massive black holes at a general position in a galactic halo.</jats:p>