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<jats:title>Abstract</jats:title> <jats:p>It is still a highly debated question as to whether fast radio bursts (FRBs) are classified into one or two populations. To probe this question, we perform a statistical analysis using the first Canadian Hydrogen Intensity Mapping Experiment Fast Radio Burst (CHIME/FRB) catalog and identify a few discriminant properties between repeating and non-repeating FRBs such as the repetition rate, duration, bandwidth, spectral index, peak luminosity, and potential peak frequency. If repeating and non-repeating FRBs belong to one population, their distribution distinctions for the repetition rate and duration can be explained by the selection effect due to the beamed emission as in Connor et al. However, we obtain that the distribution distinctions for the spectral index and potentially the peak frequency cannot be explained by the beamed emission within the framework of either the coherent curvature radiation or synchrotron maser emission. This indicates that there could be two populations. We further discuss three possible scenarios for the required two populations.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>In this paper, we discuss the influence of the gravitational darkening effect on the emergent spectrum of a fast-rotating, flattened neutron star. Model atmosphere codes always calculate spectra of emergent intensities and fluxes emitted from the unit surface on the star in plane-parallel geometry. Here we took a step beyond that and calculated a small sample grid of theoretical spectra integrated over the distorted surface of a sample rotating neutron star seen by a distant observer at various inclination angles. We assumed parameters like two dimensionless angular velocities <jats:inline-formula> <jats:tex-math> <?CDATA ${\bar{{\rm{\Omega }}}}^{2}=0.30$?> </jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msup> <mml:mrow> <mml:mover accent="true"> <mml:mrow> <mml:mi mathvariant="normal">Ω</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>¯</mml:mo> </mml:mrow> </mml:mover> </mml:mrow> <mml:mrow> <mml:mn>2</mml:mn> </mml:mrow> </mml:msup> <mml:mo>=</mml:mo> <mml:mn>0.30</mml:mn> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjac426cieqn1.gif" xlink:type="simple" /> </jats:inline-formula> and 0.60, the effective temperature of a nonrotating star <jats:italic>T</jats:italic> <jats:sub>eff</jats:sub> = 2.20 × 10<jats:sup>7</jats:sup> K, the logarithm of the surface gravity of a spherical star <jats:inline-formula> <jats:tex-math> <?CDATA $\mathrm{log}(g)=14.40$?> </jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>log</mml:mi> <mml:mo stretchy="false">(</mml:mo> <mml:mi>g</mml:mi> <mml:mo stretchy="false">)</mml:mo> <mml:mo>=</mml:mo> <mml:mn>14.40</mml:mn> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjac426cieqn2.gif" xlink:type="simple" /> </jats:inline-formula> (cgs), and inclination angles from <jats:italic>i</jats:italic> = 0° to <jats:italic>i</jats:italic> = 90° with step Δ<jats:italic>i</jats:italic> = 10°. We assumed that the atmosphere consists of a mixture of hydrogen and helium with <jats:italic>M</jats:italic> <jats:sub>H</jats:sub> = 0.70 and <jats:italic>M</jats:italic> <jats:sub>He</jats:sub> = 0.30. At each point on the neutron star surface, we calculated true intensities for local values of parameters (<jats:italic>T</jats:italic> <jats:sub>eff</jats:sub> and <jats:inline-formula> <jats:tex-math> <?CDATA $\mathrm{log}(g)$?> </jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>log</mml:mi> <mml:mo stretchy="false">(</mml:mo> <mml:mi>g</mml:mi> <mml:mo stretchy="false">)</mml:mo> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjac426cieqn3.gif" xlink:type="simple" /> </jats:inline-formula>), and these monochromatic intensities are next integrated over the whole surface to obtain the emergent spectrum. In this paper, we compute for the first time theoretical spectra of the fast-rotating neutron star. Our work clearly shows that the gravitational darkening effect strongly influences the spectrum and should be included in realistic models of the atmospheres of rotating neutron stars.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We present results from deep Chandra observations of the young Type Ia supernova remnant (SNR) 0509–68.7, also known as N103B, located in the Large Magellanic Cloud (LMC). The remnant displays an asymmetry in brightness, with the western hemisphere appearing significantly brighter than the eastern one. Previous multiwavelength observations have attributed the difference to a density gradient and suggested origins in circumstellar material, drawing similarities to Kepler’s SNR. We apply a clustering technique combined with traditional imaging analysis to spatially locate various emission components within the remnant. We find that O and Mg emission is strongest along the blast wave, and coincides with Spitzer observations of dust emission and optical emission from the nonradiative shocks. The abundances of O and Mg in these regions are enhanced relative to the average LMC abundances and appear as a distinct spatial distribution compared to the ejecta products, supporting the interpretation based on a circumstellar medium. We also find that the spatial distribution of Cr is identical to that of Fe in the interior of the remnant, and does not coincide at all with the O and Mg emission.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The nearby Seyfert type galaxy NGC 1275 contains a bright radio nucleus at its center, revealed through high-spatial resolution imaging to be the source of the jets emanating from the galaxy. Coincident with the emergence of a new component C3 in the nucleus since 2005, flux densities from NGC 1275, at least at radio, millimeter, and <jats:italic>γ</jats:italic>-ray frequencies, had been increasing up through 2017 and leveled off afterwards. We analyze the long-term light curves of the nucleus that span the rising trend to 2015 July, and find a pair of approximately year-long quasi-periodic oscillations, with periods of <jats:italic>P</jats:italic> <jats:sub> <jats:italic>l</jats:italic> </jats:sub> ≃ 345 days and <jats:italic>P</jats:italic> <jats:sub> <jats:italic>h</jats:italic> </jats:sub> ≃ 386 days, respectively, in emission at 1.3 mm wavelength. We discuss the case that there would be a long precession period, <jats:italic>P</jats:italic> <jats:sub>prec</jats:sub> ≃ 9 yr, causing the appearance of <jats:italic>P</jats:italic> <jats:sub> <jats:italic>h</jats:italic> </jats:sub> that is slightly higher than <jats:italic>P</jats:italic> <jats:sub> <jats:italic>l</jats:italic> </jats:sub>. The accretion disk around the central supermassive black hole (SMBH) would be precessing at <jats:italic>P</jats:italic> <jats:sub>prec</jats:sub>, induced by either the Lense–Thirring effect or the existence of a companion SMBH. In the two scenarios, <jats:italic>P</jats:italic> <jats:sub> <jats:italic>l</jats:italic> </jats:sub> would be the jet wobbling timescale or the SMBH binary period, respectively. The finding, which could be verified through high-spatial resolution millimeter imaging, would not only identify the nature of the jet variation but also help reveal the full features of the galaxy.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Magnetic flux ropes with helical field lines and a strong core field are ubiquitous structures in space plasmas. Recently, kinetic-scale flux ropes have been identified by high-resolution observations from the Magnetospheric Multiscale (MMS) spacecraft in the magnetosheath, which have drawn a lot of attention because of their nonideal behavior and internal structures. Detailed investigation of flux rope structure and dynamics requires the development of realistic kinetic models. In this paper, we generalize an equilibrium model to reconstruct a kinetic-scale flux rope previously reported via MMS observations. The key features in the magnetic field and electron pitch-angle distribution measurements of all four satellites are simultaneously reproduced in this reconstruction. Besides validating the model, our results also indicate that the anisotropic features previously attributed to asymmetric magnetic topologies in the magnetosheath can be alternatively explained by the spacecraft motion in the flux rope rest frame.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The recent theoretical advance known as the minimal geometric deformation (MGD) method has initiated renewed interest in investigating higher-curvature gravitational effects in relativistic astrophysics. In this work, we model a strange star within the context of Einstein–Gauss–Bonnet gravity with the help of the MGD technique. Starting off with the Tolman metric ansatz, together with the MIT bag model equation of state applicable to hadronic matter, anisotropy is introduced via the superposition of the seed source and the decoupled energy-momentum tensor. The solution of the governing systems of equations bifurcates into two distinct models, namely, the mimicking of the <jats:italic>θ</jats:italic> sector to the seed radial pressure and energy density and a regular fluid model. Each of these models can be interpreted as self-gravitating static, compact objects with the exterior described by the vacuum Boulware–Deser solution. Utilizing observational data for three stellar candidates, namely PSR J1614–2230, PSR J1903+317, and LMC X-4, we subject our solutions to rigorous viability tests based on regularity and stability. We find that the Einstein–Gauss–Bonnet parameter and the decoupling constant compete against each other for ensuring physically realizable stellar structures. The novel feature of the work is the demonstration of stable compact objects with stellar masses in excess of <jats:italic>M</jats:italic> = 2 <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub> without appealing to exotic matter. The analysis contributes new insights and physical consequences concerning the development of ultracompact astrophysical entities.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Supermassive black holes (SMBHs) spend most of their lifetime accreting at a rate well below the Eddington limit, manifesting themselves as low-luminosity active galactic nuclei (LLAGNs). The prevalence of a hot wind from LLAGNs is a generic prediction by theories and numerical simulations of black hole accretion and has recently become a crucial ingredient of AGN kinetic feedback in cosmological simulations of galaxy evolution. However, direct observational evidence for this hot wind is still scarce. In this work, we identify significant Fe <jats:sc>xxvi</jats:sc> Ly<jats:italic>α</jats:italic> and Fe <jats:sc>xxv</jats:sc> K<jats:italic>α</jats:italic> emission lines from high-resolution Chandra grating spectra of the LLAGN in NGC 7213, a nearby Sa galaxy hosting a ∼10<jats:sup>8</jats:sup> <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub> SMBH, confirming previous work. We find that these lines exhibit a blueshifted line-of-sight velocity of ∼1100 km s<jats:sup>−1</jats:sup> and a high XXVI Ly<jats:italic>α</jats:italic> to XXV K<jats:italic>α</jats:italic> flux ratio, implying for a ∼16 keV hot plasma. By confronting these spectral features with synthetic X-ray spectra based on our custom magnetohydrodynamical simulations, we find that the high-velocity, hot plasma can be naturally explained by the putative hot wind driven by the hot accretion flow powering this LLAGN. Alternative plausible origins of this hot plasma, including stellar activities, AGN photoionization, and the hot accretion flow itself, are quantitatively disfavored. The inferred kinetic energy and momentum carried by the wind can serve as strong feedback to the environment. We compare NGC 7213 to M81*, in which strong evidence for a hot wind was recently presented, and discuss implications on the universality and detectability of hot winds from LLAGNs.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Cosmic-ray exposure records of 13 lunar meteorites, Dhofar 081, Dhofar 910, Dhofar 911, Northwest Africa (NWA) 482, NWA 2995, NWA 2996, NWA 3136, NWA 3163, NWA 4472, NWA 4734, NWA 4884, NWA 4932, and NWA 4936, were characterized from the abundances of spallogenic (<jats:sup>10</jats:sup>Be and <jats:sup>26</jats:sup>Al) and neutron-captured (<jats:sup>36</jats:sup>Cl, <jats:sup>41</jats:sup>Ca,<jats:sup>150</jats:sup>Sm, and <jats:sup>168</jats:sup>Er) nuclides produced by cosmic-ray irradiation. Assuming a single-stage irradiation model for individual meteorites, 11 of the 13 meteorites had resided at shallow depths in the range of 55 to 330 g cm<jats:sup>−2</jats:sup> from the lunar surface and experienced cosmic-ray irradiations for 140–870 Ma on the Moon. In contrast, 2 of the 13 meteorites, Dhofar 911 and NWA 4932, cannot be simply explained by a single-stage irradiation, but need at least two-stage irradiation on the Moon. Furthermore, the neutron fluences of thermal and epithermal energy regions for individual meteorites were quantified from a combination of the isotopic shifts of <jats:sup>149</jats:sup>Sm–<jats:sup>150</jats:sup>Sm and <jats:sup>167</jats:sup>Er–<jats:sup>168</jats:sup>Er, respectively. Our estimates gave 8–11 times higher epithermal neutron fluences (1.7–13.7 × 10<jats:sup>1</jats:sup> <jats:sup>7</jats:sup> neutrons cm<jats:sup>−2</jats:sup>) than the thermal neutron fluences (0.65–13.8 × 10<jats:sup>16</jats:sup> neutrons cm<jats:sup>−2</jats:sup>) for 9 of the 13 meteorites, which are consistent with those from the lunar regolith materials in our previous study. This result also supports the long cosmic-ray irradiation of most lunar meteorites on the surface of the Moon.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>As main-sequence stars with C &gt; O, dwarf carbon (dC) stars are never born alone but inherit carbon-enriched material from a former asymptotic giant branch (AGB) companion. In contrast to M dwarfs in post-mass-transfer binaries, C<jats:sub>2</jats:sub> and/or CN molecular bands allow dCs to be identified with modest-resolution optical spectroscopy, even after the AGB remnant has cooled beyond detectability. Accretion of substantial material from the AGB stars should spin up the dCs, potentially causing a rejuvenation of activity detectable in X-rays. Indeed, a few dozen dCs have recently been found to have photometric variability with periods under a day. However, most of those are likely post-common-envelope binaries, spin–orbit locked by tidal forces, rather than solely spun-up by accretion. Here, we study the X-ray properties of a sample of the five nearest-known dCs with Chandra. Two are detected in X-rays, the only two for which we also detected short-period photometric variability. We suggest that the coronal activity detected so far in dCs is attributable to rapid rotation due to tidal locking in short binary orbits after a common-envelope phase, late in the thermally pulsing (TP) phase of the former C-AGB primary (TP-AGB).</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We investigate the luminosity of the accretion disk of a static black hole surrounded by dark matter with anisotropic pressure. We calculate all basic orbital parameters of test particles in the accretion disk, such as angular velocity, angular momentum, energy, and radius of the innermost circular stable orbit as functions of the dark matter density, radial pressure, and anisotropic parameter, which establishes the relationship between the radial and tangential pressures. We show that the presence of dark matter with anisotropic pressure makes a noticeable difference in the geometry around a Schwarzschild black hole, affecting the radiative flux, differential luminosity, and spectral luminosity of the accretion disk.</jats:p>