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<jats:title>Abstract</jats:title> <jats:p>We present the discovery of 528.6 Hz pulsations in the new X-ray transient MAXI J1816–195. Using NICER, we observed the first recorded transient outburst from the neutron star low-mass X-ray binary MAXI J1816–195 over a period of 28 days. From a timing analysis of the 528.6 Hz pulsations, we find that the binary system is well described as a circular orbit with an orbital period of 4.8 hr and a projected semimajor axis of 0.26 lt-s for the pulsar, which constrains the mass of the donor star to 0.10–0.55 <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub>. Additionally, we observed 15 thermonuclear X-ray bursts showing a gradual evolution in morphology over time, and a recurrence time as short as 1.4 hr. We did not detect evidence for photospheric radius expansion, placing an upper limit on the source distance of 8.6 kpc.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We present Hubble Space Telescope (HST) imaging of the site of SN 2015bh in the nearby spiral galaxy NGC 2770 taken between 2017 and 2019, nearly four years after the peak of the explosion. In 2017–2018, the transient fades steadily in optical filters before declining more slowly to <jats:italic>F814W</jats:italic> = −7.1 mag in 2019, ≈4 mag below the level of its eruptive luminous blue variable (LBV) progenitor observed with HST in 2008–2009. The source fades at a constant color of <jats:italic>F555W</jats:italic> <jats:italic>−</jats:italic> <jats:italic>F814W</jats:italic> = 0.4 mag until 2018, similar to SN 2009ip and consistent with a spectrum dominated by interaction of the ejecta with circumstellar material (CSM). A deep optical spectrum obtained in 2021 lacks signatures of ongoing interaction (<jats:italic>L</jats:italic> <jats:sub>H<jats:italic>α</jats:italic> </jats:sub> ≲ 10<jats:sup>38</jats:sup> erg s<jats:sup>−1</jats:sup> for broadened emission ≲2000 km s<jats:sup>−1</jats:sup>), but indicates the presence of a nearby H <jats:sc>ii</jats:sc> region (≲300 pc). The color evolution of the fading source makes it unlikely that emission from a scattered-light echo or binary OB companion of the progenitor contributes significantly to the flattening of the late-time light curve. The remaining emission in 2019 may plausibly be attributed an evolved/inflated companion or an unresolved (≲3 pc), young stellar cluster. Importantly, the color evolution of SN 2015bh rules out scenarios in which the surviving progenitor is obscured by nascent dust and does not clearly indicate a transition to a hotter, optically faint state. The simplest explanation is that the massive progenitor did not survive. SN 2015bh likely represents a remarkable example of the terminal explosion of a massive star preceded by decades of end-stage eruptive variability.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We fit the multiband lightcurves of 40 fast blue optical transients (FBOTs) with the magnetar engine model. The mass of the FBOT ejecta, the initial spin period, and the polar magnetic field of the FBOT magnetars are respectively constrained to <jats:inline-formula> <jats:tex-math> <?CDATA ${M}_{\mathrm{ej}}={0.11}_{-0.09}^{+0.22}\,{M}_{\odot }$?> </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>ej</mml:mi> </mml:mrow> </mml:msub> <mml:mo>=</mml:mo> <mml:msubsup> <mml:mrow> <mml:mn>0.11</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>0.22</mml:mn> </mml:mrow> </mml:msubsup> <mml:mspace width="0.25em" /> <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="apjlac86d2ieqn1.gif" xlink:type="simple" /> </jats:inline-formula>, <jats:inline-formula> <jats:tex-math> <?CDATA ${P}_{{\rm{i}}}={9.1}_{-4.4}^{+9.3}\,\mathrm{ms}$?> </jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mrow> <mml:mi>P</mml:mi> </mml:mrow> <mml:mrow> <mml:mi mathvariant="normal">i</mml:mi> </mml:mrow> </mml:msub> <mml:mo>=</mml:mo> <mml:msubsup> <mml:mrow> <mml:mn>9.1</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>4.4</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>9.3</mml:mn> </mml:mrow> </mml:msubsup> <mml:mspace width="0.25em" /> <mml:mi>ms</mml:mi> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjlac86d2ieqn2.gif" xlink:type="simple" /> </jats:inline-formula>, and <jats:inline-formula> <jats:tex-math> <?CDATA ${B}_{{\rm{p}}}={11}_{-7}^{+18}\times {10}^{14}\,{\rm{G}}$?> </jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mrow> <mml:mi>B</mml:mi> </mml:mrow> <mml:mrow> <mml:mi mathvariant="normal">p</mml:mi> </mml:mrow> </mml:msub> <mml:mo>=</mml:mo> <mml:msubsup> <mml:mrow> <mml:mn>11</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>7</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>18</mml:mn> </mml:mrow> </mml:msubsup> <mml:mo>×</mml:mo> <mml:msup> <mml:mrow> <mml:mn>10</mml:mn> </mml:mrow> <mml:mrow> <mml:mn>14</mml:mn> </mml:mrow> </mml:msup> <mml:mspace width="0.25em" /> <mml:mi mathvariant="normal">G</mml:mi> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjlac86d2ieqn3.gif" xlink:type="simple" /> </jats:inline-formula>. The wide distribution of the value of <jats:italic>B</jats:italic> <jats:sub>p</jats:sub> spreads the parameter ranges of the magnetars from superluminous supernovae (SLSNe) to broad-line Type Ic supernovae (SNe Ic-BL; some are observed to be associated with long-duration gamma-ray bursts), which are also suggested to be driven by magnetars. Combining FBOTs with the other transients, we find a strong universal anticorrelation of <jats:inline-formula> <jats:tex-math> <?CDATA ${P}_{{\rm{i}}}\propto {M}_{\mathrm{ej}}^{-0.41}$?> </jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mrow> <mml:mi>P</mml:mi> </mml:mrow> <mml:mrow> <mml:mi mathvariant="normal">i</mml:mi> </mml:mrow> </mml:msub> <mml:mo>∝</mml:mo> <mml:msubsup> <mml:mrow> <mml:mi>M</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>ej</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>0.41</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjlac86d2ieqn4.gif" xlink:type="simple" /> </jats:inline-formula>, indicating they could share a common origin. To be specific, it is suspected that all of these transients originate from the collapse of extremely stripped stars in close binary systems, but with different progenitor masses. As a result, FBOTs distinguish themselves by their small ejecta masses with an upper limit of ∼1 <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub>, which leads to an observational separation in the rise time of the lightcurves of ∼10 days. In addition, FBOTs together with SLSNe can be separated from SNe Ic-BL by an empirical line in the <jats:italic>M</jats:italic> <jats:sub>peak</jats:sub>–<jats:italic>t</jats:italic> <jats:sub>rise</jats:sub> plane corresponding to an energy requirement of the mass of <jats:sup>56</jats:sup>Ni of ∼0.3<jats:italic>M</jats:italic> <jats:sub>ej</jats:sub>, where <jats:italic>M</jats:italic> <jats:sub>peak</jats:sub> is the peak absolute magnitude of the transients and <jats:italic>t</jats:italic> <jats:sub>rise</jats:sub> is the rise time.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We report confirmation of a large, evolved, bipolar planetary nebula and its blue, white dwarf central star as a member of the ∼500 Myr old Galactic open star cluster M37 (NGC 2099). This is only the third known example of a planetary nebula in a Galactic open cluster and was found via our ongoing program of identifying and studying planetary nebulae—open cluster associations. High confidence in the association comes from the consistent radial velocities and proper motions for the confirmed central star and cluster stars from Gaia, reddening agreement, and location of the planetary nebula well within the tidal cluster boundary. Interestingly, all three Galactic examples have bipolar morphology and likely Type-I chemistry, both characteristics of higher mass progenitors. In this case the progenitor star mass is in the midrange of ∼2.8 <jats:italic>M</jats:italic> <jats:sub>☉</jats:sub>. It provides a valuable, additional point on the key stellar initial-to-final mass relation independent of cluster white dwarf estimates and also falls in a gap in the poorly sampled mass region. This planetary nebula also appears to have the largest kinematical age ever determined and implies increased visibility lifetimes when they are located in clusters.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Motivated by the recent discovery of the pulsar J1835−3259B with a spin period 1.83 ms in the globular cluster (GC) NGC 6652, we analyze the <jats:italic>γ</jats:italic>-ray data obtained with the Large Area Telescope on board the Fermi Gamma-ray Space Telescope (Fermi) for the GC and detect the pulsations of this millisecond pulsar (MSP) at a 5.4<jats:italic>σ</jats:italic> confidence level (the weighted H-test value is ∼41). From timing analysis of the data, a pulse profile that is similar to the radio one is established. We thus consider that we have detected the <jats:italic>γ</jats:italic>-ray emission of the MSP, and discuss the implications. Based on the results of our analysis and different studies of the sources in the GC, the observed <jats:italic>γ</jats:italic>-ray emission from the GC could mainly arise from this MSP, like the previous two cases in the GCs NGC 6624 and NGC 6626. Assuming this is the case, the pulsar, at the GC’s distance of 9.46 kpc and having a spin-down luminosity of ≤4.3 × 10<jats:sup>35</jats:sup> erg s<jats:sup>−1</jats:sup>, would have a <jats:italic>γ</jats:italic>-ray luminosity of ≃(5.04 ± 0.44) × 10<jats:sup>34</jats:sup> erg s<jats:sup>−1</jats:sup> and a <jats:italic>γ</jats:italic>-ray efficiency of ≳0.12.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Local thermal instability can plausibly explain the formation of multiphase gas in many different astrophysical environments, but the theory of local TI is only well-understood in the optically thin limit of the equations of radiation hydrodynamics (RHD). Here, we lay groundwork for transitioning from this limit to a full RHD treatment assuming a gray opacity formalism. We consider a situation where the gas becomes thermally unstable due to the hardening of the radiation field when the main radiative processes are free–free cooling and Compton heating. We identify two ways in which this can happen: (i) when the Compton temperature increases with time, through a rise in either the intensity or energy of a hard X-ray component; and (ii) when attenuation reduces the flux of the thermal component such that the Compton temperature increases with depth through the slab. Both ways likely occur in the broad-line region of active galactic nuclei where columns of gas can be ionization-bounded. In such instances where attenuation is significant, thermal equilibrium solution curves become position-dependent and it no longer suffices to assess the stability of an irradiated column of gas at all depths using a single equilibrium curve. We demonstrate how to analyze a new equilibrium curve—the attenuation curve—for this purpose, and we show that, by Field’s instability criterion, a negative slope along this curve indicates that constant-density slabs are thermally unstable whenever the gas temperature increases with depth.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We report the discovery of a cold stream near the southern Galactic pole (dubbed as SGP-S) detected in Gaia Early Data Release 3. The stream is at a heliocentric distance of ∼9.5 kpc and spans nearly 58° by 0.°6 on sky. The color–magnitude diagram of SGP-S indicates an old and metal-poor (age ∼12 Gyr, [M/H] ∼ −2.0 dex) stellar population. The stream’s surface brightness reaches an exceedingly low level of Σ<jats:sub> <jats:italic>G</jats:italic> </jats:sub> ≃ 36.2 mag arcsec<jats:sup>−2</jats:sup>. Neither extant globular clusters nor other known streams are associated with SGP-S.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We describe Keck-telescope spectrophotometry and imaging of the companion of the “black widow” pulsar PSR J0952−0607, the fastest known spinning neutron star (NS) in the disk of the Milky Way. The companion is very faint at minimum brightness, presenting observational challenges, but we have measured multicolor light curves and obtained radial velocities over the illuminated “day” half of the orbit. The model fits indicate system inclination <jats:italic>i</jats:italic> = 59.°8 ± 1.°9 and a pulsar mass <jats:italic>M</jats:italic> <jats:sub>NS</jats:sub> = 2.35 ± 0.17 <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub>, the largest well-measured mass found to date. Modeling uncertainties are small, since the heating is not extreme; the companion lies well within its Roche lobe and a simple direct-heating model provides the best fit. If the NS started at a typical pulsar birth mass, nearly 1 <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub> has been accreted; this may be connected with the especially low intrinsic dipole surface field, estimated at 6 × 10<jats:sup>7</jats:sup> G. Joined with reanalysis of other black widow and redback pulsars, we find that the minimum value for the maximum NS mass is <jats:inline-formula> <jats:tex-math> <?CDATA ${M}_{\max }\gt 2.19\,{M}_{\odot }$?> </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>max</mml:mi> </mml:mrow> </mml:msub> <mml:mo>&gt;</mml:mo> <mml:mn>2.19</mml:mn> <mml:mspace width="0.25em" /> <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="apjlac8007ieqn1.gif" xlink:type="simple" /> </jats:inline-formula> (2.09 <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub>) at 1<jats:italic>σ</jats:italic> (3<jats:italic>σ</jats:italic>) confidence. This is ∼ 0.15 <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub> heavier than the lower limit on <jats:inline-formula> <jats:tex-math> <?CDATA ${M}_{\max }$?> </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>max</mml:mi> </mml:mrow> </mml:msub> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjlac8007ieqn2.gif" xlink:type="simple" /> </jats:inline-formula> implied by the white dwarf–pulsar binaries measured via radio Shapiro-delay techniques.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We present the direct-imaging discovery of a substellar companion in orbit around a Sun-like star member of the Hyades open cluster. So far, no other substellar companions have been unambiguously confirmed via direct imaging around main-sequence stars in Hyades. The star HIP 21152 is an accelerating star as identified by the astrometry from the Gaia and Hipparcos satellites. We detected the companion, HIP 21152 B, in multiple epochs using the high-contrast imaging from SCExAO/CHARIS and Keck/NIRC2. We also obtained the stellar radial-velocity data from the Okayama 188 cm telescope. The CHARIS spectroscopy reveals that HIP 21152 B’s spectrum is consistent with the L/T transition, best fit by an early T dwarf. Our orbit modeling determines the semimajor axis and the dynamical mass of HIP 21152 B to be 17.5<jats:inline-formula> <jats:tex-math> <?CDATA ${}_{-3.8}^{+7.2}$?> </jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msubsup> <mml:mrow /> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>3.8</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>7.2</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjlac772fieqn1.gif" xlink:type="simple" /> </jats:inline-formula> au and 27.8<jats:inline-formula> <jats:tex-math> <?CDATA ${}_{-5.4}^{+8.4}$?> </jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msubsup> <mml:mrow /> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>5.4</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>8.4</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjlac772fieqn2.gif" xlink:type="simple" /> </jats:inline-formula> <jats:italic>M</jats:italic> <jats:sub>Jup</jats:sub>, respectively. The mass ratio of HIP 21152 B relative to its host is ≈2%, near the planet/brown dwarf boundary suggested by recent surveys. Mass estimates inferred from luminosity-evolution models are slightly higher (33–42 <jats:italic>M</jats:italic> <jats:sub>Jup</jats:sub>). With a dynamical mass and a well-constrained age due to the system’s Hyades membership, HIP 21152 B will become a critical benchmark in understanding the formation, evolution, and atmosphere of a substellar object as a function of mass and age. Our discovery is yet another key proof of concept for using precision astrometry to select direct-imaging targets.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Astrophysical disks that are sufficiently cold and dense are linearly unstable to the formation of axisymmetric rings as a result of the disk’s gravity. In practice, spiral structures are formed, which may in turn produce bound fragments. We study a nonlinear dynamical path that can explain the development of spirals in a local model of a gaseous disk on the subcritical side of the gravitational instability bifurcation. Axisymmetric equilibria can be radially periodic or localized, in the form of standing solitary waves. The solitary solutions have energy slightly larger than a smooth disk. They are further unstable to nonaxisymmetric perturbations with a wide range of azimuthal wavenumbers. The solitary waves may act as a pathway to spirals and fragmentation.</jats:p>