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<jats:title>Abstract</jats:title> <jats:p>The existence of primordial black holes (PBHs), which may form from the collapse of matter overdensities shortly after the Big Bang, is still under debate. Among the potential signatures of PBHs are gravitational waves (GWs) emitted from binary black hole (BBH) mergers at redshifts <jats:italic>z</jats:italic> ≳ 30, where the formation of astrophysical black holes is unlikely. Future ground-based GW detectors, the Cosmic Explorer and Einstein Telescope, will be able to observe equal-mass BBH mergers with total mass of <jats:inline-formula> <jats:tex-math> <?CDATA ${ \mathcal O }(10\mbox{--}100)\,{M}_{\odot }$?> </jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi mathvariant="italic"></mml:mi> <mml:mo stretchy="false">(</mml:mo> <mml:mn>10</mml:mn> <mml:mo>–</mml:mo> <mml:mn>100</mml:mn> <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="apjlac6beaieqn1.gif" xlink:type="simple" /> </jats:inline-formula> at such distances. In this work, we investigate whether the redshift measurement of a single BBH source can be precise enough to establish its primordial origin. We simulate BBHs of different masses, mass ratios and orbital orientations. We show that for BBHs with total masses between 20 <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub> and 40 <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub> merging at <jats:italic>z</jats:italic> ≥ 40, one can infer <jats:italic>z</jats:italic> &gt; 30 at up to 97% credibility, with a network of one Einstein Telescope, one 40 km Cosmic Explorer in the US, and one 20 km Cosmic Explorer in Australia. This number reduces to 94% with a smaller network made of one Einstein Telescope and one 40 km Cosmic Explorer in the US. We also analyze how the measurement depends on the Bayesian priors used in the analysis and verify that priors that strongly favor the wrong model yield smaller Bayesian evidences.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Short-lived radionuclides (SLRs) provide important information about the chronology of the early solar system. Among them, <jats:sup>41</jats:sup>Ca, due to its decay to <jats:sup>41</jats:sup>K with a half-life of only 0.1 Ma, is particularly valuable in constraining the timescales and origins of both SLRs and the formation of the oldest solar system materials, the Ca–Al-rich inclusions (CAIs). The initial abundance of <jats:sup>41</jats:sup>Ca in the solar system, expressed as the (<jats:sup>41</jats:sup>Ca/<jats:sup>40</jats:sup>Ca)<jats:sub>I</jats:sub> ratio, is the key to unveiling the origin of this nuclide. Here, we report a new solar system (<jats:sup>41</jats:sup>Ca/<jats:sup>40</jats:sup>Ca)<jats:sub>I</jats:sub> ratio of 2.0 × 10<jats:sup>−8</jats:sup> derived from the K isotope compositions of two CAIs. This new ratio is about four times higher than the previous value inferred from a mineral isochron. Such a high (<jats:sup>41</jats:sup>Ca/<jats:sup>40</jats:sup>Ca)<jats:sub>I</jats:sub> ratio in the CAIs exceeds that expected for the protosolar molecular cloud by ∼1000×, implying very late injection of the <jats:sup>41</jats:sup>Ca (and possibly other SLRs) into the protosolar molecular cloud. The correlated enrichments of <jats:sup>41</jats:sup>Ca and <jats:sup>26</jats:sup>Al in the bulk CAI samples hint at a common stellar origin of both SLRs. The injection time estimated from our new data depends on the stellar source—it ranges from 0.6 Ma for a Wolf–Rayet wind to 1.0 Ma for a TP-AGB star ejecta.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The recently discovered peculiar gamma-ray burst GRB 200826A poses a dilemma for the collapsar model. Although all other characteristics of the burst are consistent with it being a Type II (i.e., collapse of a massive star) event, the observed duration of the event is only approximately 1 s, which is at odds with the predicted allowable timescale range for a collapsar event. To resolve this dilemma, this Letter proposes that the original burst could be an intrinsically long GRB comprising a precursor and a main emission phase. However, the main emission phase is missed due to either precession of the jet or the obstruction by a companion star, leaving only the precursor observed as a short-duration GRB 200826A. Interestingly, we found that the temporal and spectral properties of GRB 200826A broadly resembled those of the bright precursor observed in GRB 160625B. Furthermore, assuming the prototype burst of GRB 200826A is similar to that of GRB 160625B, we found that the observer may indeed miss its main emission because of geometric effects caused either by jet precession or companion-obstruction models. Our approach provides a natural explanation for the GRB 200826A–like bursts and agrees with the rarity of those events.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The 559 Hz black-widow pulsar PSR J1555−2908, originally discovered in radio, is also a bright gamma-ray pulsar. Timing its pulsations using 12 yr of Fermi-Large Area Telescope gamma-ray data reveals long-term variations in its spin frequency that are much larger than is observed from other millisecond pulsars. While this variability in the pulsar rotation rate could be intrinsic “timing noise,” here we consider an alternative explanation: the variations arise from the presence of a very-low-mass third object in a wide multiyear orbit around the neutron star and its low-mass companion. With current data, this hierarchical-triple-system model describes the pulsar’s rotation slightly more accurately than the best-fitting timing noise model. Future observations will show if this alternative explanation is correct.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Infrared (IR) band strengths are needed to extract accurate molecular abundances from astronomical observations of interstellar and solar system ices. However, laboratory measurements of such intensities often have required multiple assumptions about ice composition and thickness. Here we describe a method that circumvents most of the common assumptions and uncertainties in IR band-strength determinations. We have applied the method to measure IR band strengths of OCS, H<jats:sub>2</jats:sub>S, and SO<jats:sub>2</jats:sub> in the absence and presence of H<jats:sub>2</jats:sub>O ice at 10 K, the first measurements of their type. Our results show for the first time that the presence of H<jats:sub>2</jats:sub>O makes little difference in IR intensities for these three sulfur-containing molecules’ strongest IR features. The immediate application will be to laboratory studies of low-temperature chemistry of interstellar and cometary ices.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We have selected six sources (G209.55–19.68S2, G205.46–14.56S1<jats:sub>-</jats:sub>A, G203.21–11.20W2, G191.90–11.21S, G205.46–14.56S3, and G206.93–16.61W2) from the Atacama Large Millimeter/submillimeter Array Survey of Orion Planck Galactic Cold Clumps (ALMASOP), in which these sources have been mapped in the CO (<jats:italic>J</jats:italic> = 2−1), SiO (<jats:italic>J</jats:italic> = 5−4), and C<jats:sup>18</jats:sup>O (<jats:italic>J</jats:italic> = 2−1) lines. These sources have high-velocity SiO jets surrounded by low-velocity CO outflows. The SiO jets consist of a chain of knots. These knots have been thought to be produced by semiperiodic variations in jet velocity. Therefore, we adopt a shock-forming model, which uses such variations to estimate the inclination angle and velocity of the jets. We also derive the inclination angle of the CO outflows using the wide-angle wind-driven shell model and find it to be broadly consistent with that of the associated SiO jets. In addition, we apply this shock-forming model to another three protostellar sources with SiO jets in the literature—HH 211, HH 212, and L1448C(N)—and find that their inclination angle and jet velocity are consistent with those previously estimated from proper-motion and radial-velocity studies.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>When a star passes too close to a supermassive black hole, it gets disrupted by strong tidal forces. The stellar debris then evolves into an elongated stream of gas that partly falls back toward the black hole. We present an analytical model describing for the first time the full stream evolution during such a tidal disruption event (TDE). Our framework consists of dividing the stream into different sections of elliptical geometry, whose properties are independently evolved in their comoving frame under the tidal, pressure, and self-gravity forces. Through an explicit treatment of the tidal force and the inclusion of the gas angular momentum, we can accurately follow the stream evolution near pericenter. Our model evolves the longitudinal stream stretching and both transverse widths simultaneously. For the latter, we identify two regimes depending on whether the dynamics is entirely dominated by the tidal force (ballistic regime) or additionally influenced by pressure and self-gravity (hydrostatic regime). We find that the stream undergoes transverse collapses both shortly after the stellar disruption and upon its return near the black hole, at specific locations determined by the regime of evolution considered. The stream evolution predicted by our model can be used to determine the subsequent interactions experienced by this gas that are at the origin of most of the electromagnetic emission from TDEs. Our results suggest that the accretion disk may be fed at a rate that differs from the standard fallback rate, which would provide novel observational signatures dependent on black hole spin.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>In the local universe, OH megamasers (OHMs) are detected almost exclusively in infrared-luminous galaxies, with a prevalence that increases with IR luminosity, suggesting that they trace gas-rich galaxy mergers. Given the proximity of the rest frequencies of OH and the hyperfine transition of neutral atomic hydrogen (H <jats:sc>i</jats:sc>), radio surveys to probe the cosmic evolution of H <jats:sc>i</jats:sc> in galaxies also offer exciting prospects for exploiting OHMs to probe the cosmic history of gas-rich mergers. Using observations for the Looking At the Distant Universe with the MeerKAT Array (LADUMA) deep H <jats:sc>i</jats:sc> survey, we report the first untargeted detection of an OHM at <jats:italic>z</jats:italic> &gt; 0.5, LADUMA J033046.20−275518.1 (nicknamed “Nkalakatha”). The host system, WISEA J033046.26−275518.3, is an infrared-luminous radio galaxy whose optical redshift <jats:italic>z</jats:italic> ≈ 0.52 confirms the MeerKAT emission-line detection as OH at a redshift <jats:italic>z</jats:italic> <jats:sub>OH</jats:sub> = 0.5225 ± 0.0001 rather than H <jats:sc>i</jats:sc> at lower redshift. The detected spectral line has 18.4<jats:italic>σ</jats:italic> peak significance, a width of 459 ± 59 km s<jats:sup>−1</jats:sup>, and an integrated luminosity of (6.31 ± 0.18 [statistical] ± 0.31 [systematic]) × 10<jats:sup>3</jats:sup> <jats:italic>L</jats:italic> <jats:sub>⊙</jats:sub>, placing it among the most luminous OHMs known. The galaxy’s far-infrared luminosity <jats:italic>L</jats:italic> <jats:sub>FIR</jats:sub> = (1.576 ±0.013) × 10<jats:sup>12</jats:sup> <jats:italic>L</jats:italic> <jats:sub>⊙</jats:sub> marks it as an ultraluminous infrared galaxy; its ratio of OH and infrared luminosities is similar to those for lower-redshift OHMs. A comparison between optical and OH redshifts offers a slight indication of an OH outflow. This detection represents the first step toward a systematic exploitation of OHMs as a tracer of galaxy growth at high redshifts.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Coronal mass ejections (CMEs) are large clouds of magnetized plasma ejected from the Sun and are often associated with the acceleration of electrons that can result in radio emission via various mechanisms. However, the underlying mechanism relating the CMEs and particle acceleration still remains a subject of heated debate. Here, we report multi-instrument radio and extreme ultraviolet (EUV) imaging of a solar eruption event on 2011 September 24. We determine the emission mechanism of a moving radio burst, identify its three-dimensional location with respect to a rapidly expanding EUV wave, and find evidence for CME shocks that produce quasiperiodic acceleration of electron beams.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We present a detailed time-resolved photometric study of the ultracompact X-ray binary candidate 4U 1812–12. The multicolor light curves obtained with HiPERCAM on the 10.4 m Gran Telescopio Canarias show a ≃114 minute modulation similar to a superhump. Under this interpretation, this period should lie very close to the orbital period of the system. Contrary to what its other observational properties suggest (namely, persistent dim luminosity, low optical-to-X-ray flux ratio, and lack of hydrogen features in the optical spectrum), this implies that 4U 1812–12 is most likely not an ultracompact X-ray binary, which is usually defined as a system with an orbital period lower than 80 minutes. We discuss the nature of the system, showing that a scenario in which 4U 1812–12 is the progenitor of an ultracompact X-ray binary may reconcile all the observables.</jats:p>