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<jats:title>Abstract</jats:title> <jats:p>We report follow-up observations of the Crab Nebula with the PolarLight X-ray polarimeter, which revealed a possible variation in polarization associated with a pulsar glitch in 2019. The new observations confirm that the polarization has recovered roughly 100 days after the glitch. With the new observations, we find that the polarization angle (PA) measured with PolarLight from the total nebular emission has a difference of 18.°0 ± 4.°6 from that measured 42 yr ago with OSO-8, indicating a secular evolution of polarization with either the Crab Nebula or pulsar. The long-term variation in PA could be a result of multiple glitches in the history, magnetic reconnection, or movement of synchrotron emitting structures in the nebula, or secular evolution of the pulsar magnetic geometry.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Inconsistent conclusions are obtained from recent active galactic nuclei (AGNs) accretion disk inter-band time-lag measurements. While some works show that the measured time lags are significantly larger (by a factor of ∼3) than the theoretical predictions of the Shakura &amp; Sunyaev disk (SSD) model, others find that the time-lag measurements are consistent with (or only slightly larger than) that of the SSD model. These conflicting observational results might be symptoms of our poor understanding of AGN accretion physics. Here we show that sources with larger-than-expected time lags tend to be less luminous AGNs. Such a dependence is unexpected if the inter-band time lags are attributed to the light-travel-time delay of the illuminating variable X-ray photons to the static SSD. If, instead, the measured inter-band lags are related not only to the static SSD but also to the outer broad emission-line regions (BLRs; e.g., the blended broad emission lines and/or diffuse continua), our result indicates that the contribution of the non-disk BLR to the observed ultraviolet/optical continuum decreases with increasing luminosity (<jats:italic>L</jats:italic>), i.e., an anti-correlation resembling the well-known Baldwin effect. Alternatively, we argue that the observed dependence might be a result of coherent disk thermal fluctuations as the relevant thermal timescale, <jats:italic>τ</jats:italic> <jats:sub>TH</jats:sub> ∝ <jats:italic>L</jats:italic> <jats:sup>0.5</jats:sup>. With future accurate measurements of inter-band time lags, the above two scenarios can be distinguished by inspecting the dependence of inter-band time lags upon either the BLR components in the variable spectra or the timescales.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The results from the ESA Gaia astrometric mission and deep photometric surveys have revolutionized our knowledge of the Milky Way. There are many ongoing efforts to search these data for stellar substructure to find evidence of individual accretion events that built up the Milky Way and its halo. One of these newly identified features, called Nyx, was announced as an accreted stellar stream traveling in the plane of the disk. Using a combination of elemental abundances and stellar parameters from the GALAH and Apache Point Observatory Galactic Evolution Experiment (APOGEE) surveys, we find that the abundances of the highest likelihood Nyx members are entirely consistent with membership of the thick disk, and inconsistent with a dwarf galaxy origin. We conclude that the postulated Nyx stream is most probably a high-velocity component of the Milky Way’s thick disk. With the growing availability of large data sets including kinematics, stellar parameters, and detailed abundances, the probability of detecting chance associations increases, and hence new searches for substructure require confirmation across as many data dimensions as possible.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Stellar evolution theory predicts a “gap” in the black hole birth function caused by the pair instability. Many presupernova stars that have a core mass below some limiting value, <jats:italic>M</jats:italic> <jats:sub>low</jats:sub>, after all pulsational activity is finished, collapse to black holes, while more massive ones, up to some limiting value, <jats:italic>M</jats:italic> <jats:sub>high</jats:sub>, explode, promptly and completely, as pair-instability supernovae. Previous work has suggested <jats:italic>M</jats:italic> <jats:sub>low</jats:sub> ≈ 50<jats:italic> M</jats:italic> <jats:sub>⊙</jats:sub> and <jats:italic>M</jats:italic> <jats:sub>high</jats:sub> ≈ 130<jats:italic> M</jats:italic> <jats:sub>⊙</jats:sub>. These calculations have been challenged by recent LIGO observations that show many black holes merging with individual masses <jats:italic>M</jats:italic> <jats:sub>low</jats:sub> ≳ 65 <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub>. Here we explore four factors affecting the theoretical estimates for the boundaries of this mass gap: nuclear reaction rates, evolution in detached binaries, rotation, and hyper-Eddington accretion after black hole birth. Current uncertainties in reaction rates by themselves allow <jats:italic>M</jats:italic> <jats:sub>low</jats:sub> to rise to 64 <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub> and <jats:italic>M</jats:italic> <jats:sub>high</jats:sub> as large as 161 <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub>. Rapid rotation could further increase <jats:italic>M</jats:italic> <jats:sub>low</jats:sub> to ∼70 <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub>, depending on the treatment of magnetic torques. Evolution in detached binaries and super-Eddington accretion can, with great uncertainty, increase <jats:italic>M</jats:italic> <jats:sub>low</jats:sub> still further. Dimensionless Kerr parameters close to unity are allowed for the more massive black holes produced in close binaries, though they are generally smaller.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The low frequency cutoffs <jats:italic>f</jats:italic> <jats:sub> <jats:italic>lo</jats:italic> </jats:sub> and the observed plasma frequency <jats:italic>f</jats:italic> <jats:sub> <jats:italic>p</jats:italic> </jats:sub> of 176 type III radio bursts are investigated in this paper. These events are observed by the Parker Solar Probe when it is in the encounter phase from the first to the fifth orbit. The result shows that the distribution of cutoffs <jats:italic>f</jats:italic> <jats:sub> <jats:italic>lo</jats:italic> </jats:sub> is widely spread between 200 kHz and 1.6 MHz. While the plasma frequency <jats:italic>f</jats:italic> <jats:sub> <jats:italic>p</jats:italic> </jats:sub> at the spacecraft is between 50 and 250 kHz, which is almost all smaller than <jats:italic>f</jats:italic> <jats:sub> <jats:italic>lo</jats:italic> </jats:sub>. The result also shows that the maximum probability distribution of <jats:italic>f</jats:italic> <jats:sub> <jats:italic>lo</jats:italic> </jats:sub> (∼680 kHz) is remarkably higher than that observed by Ulysses and Wind (∼100 kHz) in previous research. Three possible reasons, i.e., solar activity intensity, event electing criteria, and radiation attenuation effect, are also preliminarily discussed.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The melt productivity of a differentiated planet's mantle is primarily controlled by its iron content, which is itself approximated by the planet's core mass fraction (CMF). Here we show that estimates of an exoplanet's CMF allows robust predictions of the thickness, composition, and mineralogy of the derivative crust. These predicted crustal compositions allow constraints to be placed on volatile cycling between surface and the deep planetary interior, with implications for the evolution of habitable planetary surfaces. Planets with large, terrestrial-like CMFs (≥0.32) will exhibit thin crusts that are inefficient at transporting surface water and other volatiles into the underlying mantle. By contrast, rocky planets with smaller CMFs (≤0.24) and higher, Mars-like, mantle iron contents will develop thick crusts capable of stabilizing hydrous minerals, which can effectively sequester volatiles into planetary interiors and act to remove surface water over timescales relevant to evolution. The extent of core formation has profound consequences for the subsequent planetary surface environment and may provide additional constraints in the hunt for habitable, Earth-like exoplanets.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The Milky Way disk consists of two prominent components—a thick, alpha-rich, low-metallicity component and a thin, metal-rich, low-alpha component. External galaxies have been shown to contain thin- and thick-disk components, but whether distinct components in the [<jats:italic>α</jats:italic>/Fe]–[Z/H] plane exist in other Milky Way-like galaxies is not yet known. We present Very Large Telescope (VLT)—Multi Unit Spectroscopic Explorer (MUSE) observations of UGC 10738, a nearby, edge-on Milky Way-like galaxy. We demonstrate through stellar population synthesis model fitting that UGC 10738 contains alpha-rich and alpha-poor stellar populations with similar spatial distributions to the same components in the Milky Way. We discuss how the finding that external galaxies also contain chemically distinct disk components may act as a significant constraint on the formation of the Milky Way’s own thin and thick disk.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Anomalous X-ray pulsars and soft gamma repeaters are slowly rotating, young, and isolated neutron stars exhibiting sporadic outbursts and high X-ray quiescent luminosities. They are believed to be powered by ultrastrong magnetic fields, <jats:italic>B</jats:italic> ∼ 10<jats:sup>14</jats:sup>−10<jats:sup>15</jats:sup> G, associated with “magnetars.” In the peculiar case of SGR 0418+5729, timing parameters imply a dipolar <jats:italic>B</jats:italic>-field of 6.1 × 10<jats:sup>12</jats:sup> G. This discovery has challenged the traditional picture of magnetars in terms of <jats:italic>B</jats:italic>-field strengths, evolutionary stages, and ages. Here we provide a novel approach to estimate a magnetar’s age by considering the self-consistent time evolution of a plasma-filled oblique pulsar with the state-of-the-art magnetospheric particle acceleration gaps. The rotational period of magnetars increases over time due to angular momentum extraction by gravitational-wave radiations, magnetic dipole radiations, and particle winds. These torques also change the obliquity angle between the magnetic and rotation axes. For SGR 0418+5729, we obtain a dipolar <jats:italic>B</jats:italic>-field of 1.0 × 10<jats:sup>14</jats:sup> G, and a realistic age of ∼18 kyr, consistent within the magnetar paradigm.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Ultrahigh-energy cosmic rays (UHECRs) are atomic nuclei from space with vastly higher energies than any other particles ever observed. Their origin and chemical composition remain a mystery. As we show here, the large and intermediate angular scale anisotropies observed by the Pierre Auger Observatory are a powerful tool for understanding the origin of UHECRs. Without specifying any particular production mechanism but only postulating that the source distribution follows the matter distribution of the local universe, a good accounting of the magnitude, direction, and energy dependence of the dipole anisotropy at energies above 8 × 10<jats:sup>18</jats:sup> eV is obtained after taking into account the impact of energy losses during propagation (the “GZK horizon”), diffusion in the extragalactic magnetic field, and deflections in the Galactic magnetic field (GMF). This is a major step toward the long-standing hope of using UHECR anisotropies to constrain UHECR composition and magnetic fields. The observed dipole anisotropy is incompatible with a pure proton composition in this scenario. With a more accurate treatment of energy losses, it should be possible to further constrain the cosmic-ray composition and properties of the extragalactic magnetic field, self-consistently improve the GMF model, and potentially expose individual UHECR sources.</jats:p>