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<jats:title>Abstract</jats:title> <jats:p>This paper explores small-scale departures from force-free electrodynamics around a rotating neutron star, extending our treatment of resistive instability in a quantizing magnetic field. A secondary, Cerenkov instability is identified: relativistic particles flowing through thin current sheets excite propagating charge perturbations that are localized near the sheets. Growth is rapid at wavenumbers below the inverse ambient skin depth <jats:italic>k</jats:italic> <jats:sub> <jats:italic>p</jats:italic>,ex</jats:sub>. Small-scale Alfvénic wavepackets are promising sources of coherent curvature radiation. When the group Lorentz factor <jats:inline-formula> <jats:tex-math> <?CDATA ${\gamma }_{\mathrm{gr}}\lesssim {({k}_{p,\mathrm{ex}}{R}_{c})}^{1/3}\sim 100$?> </jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mrow> <mml:mi>γ</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>gr</mml:mi> </mml:mrow> </mml:msub> <mml:mo>≲</mml:mo> <mml:msup> <mml:mrow> <mml:mo stretchy="false">(</mml:mo> <mml:msub> <mml:mrow> <mml:mi>k</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>p</mml:mi> <mml:mo>,</mml:mo> <mml:mi>ex</mml:mi> </mml:mrow> </mml:msub> <mml:msub> <mml:mrow> <mml:mi>R</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>c</mml:mi> </mml:mrow> </mml:msub> <mml:mo stretchy="false">)</mml:mo> </mml:mrow> <mml:mrow> <mml:mn>1</mml:mn> <mml:mrow> <mml:mo stretchy="true">/</mml:mo> </mml:mrow> <mml:mn>3</mml:mn> </mml:mrow> </mml:msup> <mml:mo>∼</mml:mo> <mml:mn>100</mml:mn> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjac51d4ieqn1.gif" xlink:type="simple" /> </jats:inline-formula>, where <jats:italic>R</jats:italic> <jats:sub> <jats:italic>c</jats:italic> </jats:sub> is the magnetic curvature radius, a fraction ∼10<jats:sup>−3</jats:sup>–10<jats:sup>−2</jats:sup> of the particle kinetic energy is radiated into the extraordinary mode at a peak frequency ∼10<jats:sup>−2</jats:sup> <jats:italic>ck</jats:italic> <jats:sub> <jats:italic>p</jats:italic>,ex</jats:sub>. Consistency with observations requires a high pair multiplicity (∼10<jats:sup>3–5</jats:sup>) in the pulsar magnetosphere. Neither the primary, slow resistive instability nor the secondary, Alfvénic instability depend directly on the presence of magnetospheric gaps, and may activate where the mean current is fully supplied by outward drift of the corotation charge. The resistive mode is overstable and grows at a rate comparable to the stellar spin frequency; the model directly accommodates strong pulse-to-pulse radio flux variations and coordinated subpulse drift. Alfvén mode growth can track the local plasma conditions, allowing for lower-frequency emission from the outer magnetosphere. Beamed radio emission from charged packets with <jats:italic>γ</jats:italic> <jats:sub>gr</jats:sub> ∼ 50–100 also varies on submillisecond timescales. The modes identified here will be excited inside the magnetosphere of a magnetar, and may mediate Taylor relaxation of the magnetic twist.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The formation and evolutionary history of M31 are closely related to its dynamical structures, which remain unclear due to its high inclination. Gas kinematics could provide crucial evidence for the existence of a rotating bar in M31. Using the position–velocity diagram of [O <jats:sc>III</jats:sc>] and H <jats:sc>i</jats:sc>, we are able to identify clear sharp velocity jump (shock) features with a typical amplitude over 100 km s<jats:sup>−1</jats:sup> in the central region of M31 (4.6 kpc × 2.3 kpc, or <jats:inline-formula> <jats:tex-math> <?CDATA $20^{\prime} \times 10^{\prime} $?> </jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mn>20</mml:mn> <mml:mo accent="false">′</mml:mo> <mml:mo>×</mml:mo> <mml:mn>10</mml:mn> <mml:mo accent="false">′</mml:mo> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjac7964ieqn1.gif" xlink:type="simple" /> </jats:inline-formula>). We also simulate gas morphology and kinematics in barred M31 potentials and find that the bar-induced shocks can produce velocity jumps similar to those in [O <jats:sc>III</jats:sc>]. The identified shock features in both [O <jats:sc>III</jats:sc>] and H <jats:sc>i</jats:sc> are broadly consistent, and they are found mainly on the leading sides of the bar/bulge, following a hallmark pattern expected from the bar-driven gas inflow. Shock features on the far side of the disk are clearer than those on the near side, possibly due to limited data coverage on the near side, as well as to obscuration by the warped gas and dust layers. Further hydrodynamical simulations with more sophisticated physics are desired to fully understand the observed gas features and to better constrain the parameters of the bar in M31.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The period and the period derivative of a pulsar are critical magnitudes for defining the properties of the magnetospheric size and plasma dynamics. The pulsar light cylinder, the magnetic field intensity nearby it, and the curvature radius all depend on these timing properties, and shape the observed high-energy synchro-curvature emission. Therefore, the radiative properties of pulsars are inextricably linked to them. This fact poses the question of how well does a given pulsar’s spectral energy distribution embed information of the timing parameters, and if so, whether we can deduce them if they have not been measured directly. This is relevant to possibly constrain the timing properties of potential pulsar candidates among unidentified <jats:italic>γ</jats:italic>-ray sources. We consider well-measured pulsar spectra blinding us from the knowledge of their timing properties, and address this question by using our radiative synchro-curvature model that was proven able to fit the observed spectra of the pulsar population. We find that in the majority of the cases studied (8 out of 13), the spin period is constrained within a range of about 1 order of magnitude, within which the real period lies. In the other cases, there is degeneracy and no period range can be constrained. This can be used to facilitate the blind search of pulsed signals.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Accurately measuring and modeling the Ly<jats:italic>α</jats:italic> (Ly<jats:italic>α</jats:italic>; <jats:italic>λ</jats:italic>1215.67 Å) emission line from low-mass stars is vital for our ability to build predictive high energy stellar spectra, yet interstellar medium (ISM) absorption of this line typically prevents model-measurement comparisons. Ly<jats:italic>α</jats:italic> also controls the photodissociation of important molecules, like water and methane, in exoplanet atmospheres such that any photochemical models assessing potential biosignatures or atmospheric abundances require accurate Ly<jats:italic>α</jats:italic> host star flux estimates. Recent observations of three early M and K stars (K3, M0, M1) with exceptionally high radial velocities (&gt;100 km s<jats:sup>−1</jats:sup>) reveal the intrinsic profiles of these types of stars as most of their Ly<jats:italic>α</jats:italic> flux is shifted away from the geocoronal line core and contamination from the ISM. These observations indicate that previous stellar spectra computed with the <jats:monospace>PHOENIX</jats:monospace> atmosphere code have underpredicted the core of Ly<jats:italic>α</jats:italic> in these types of stars. With these observations, we have been able to better understand the microphysics in the upper atmosphere and improve the predictive capabilities of the <jats:monospace>PHOENIX</jats:monospace> atmosphere code. Since these wavelengths drive the photolysis of key molecular species, we also present results analyzing the impact of the resulting changes to the synthetic stellar spectra on observable chemistry in terrestrial planet atmospheres.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The first measurements of the 21 cm brightness temperature power spectrum from the epoch of reionization will very likely be achieved in the near future by radio interferometric array experiments such as the Hydrogen Epoch of Reionization Array (HERA) and the Square Kilometre Array (SKA). Standard MCMC analyses use an explicit likelihood approximation to infer the reionization parameters from the 21 cm power spectrum. In this paper, we present a new Bayesian inference of the reionization parameters where the likelihood is implicitly defined through forward simulations using density estimation likelihood-free inference (DELFI). Realistic effects, including thermal noise and foreground avoidance, are also applied to the mock observations from the HERA and SKA. We demonstrate that this method recovers accurate posterior distributions for the reionization parameters, and it outperforms the standard MCMC analysis in terms of the location and size of credible parameter regions. With the minute-level processing time once the network is trained, this technique is a promising approach for the scientific interpretation of future 21 cm power spectrum observation data. Our code <jats:monospace>21cmDELFI-PS</jats:monospace> is publicly available at this link (<jats:ext-link xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="uri" xlink:href="https://github.com/Xiaosheng-Zhao/21cmDELFI" xlink:type="simple">https://github.com/Xiaosheng-Zhao/21cmDELFI</jats:ext-link>).</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Coronal holes (CHs) are regions with unbalanced magnetic flux and have been associated with open magnetic field (OMF) structures. However, it has been reported that some CHs do not intersect with OMF regions. To investigate the inconsistency, we apply a potential-field (PF) model to construct the magnetic fields of the CHs. As a comparison, we also use a thermodynamic magnetohydrodynamic (MHD) model to synthesize coronal images and identify CHs from the synthetic images. The results from both the potential-field CHs and synthetic MHD CHs reveal that there is a significant percentage of closed field lines extending beyond the CH boundaries and more than 50% (17%) of PF (MHD) CHs do not contain OMF lines. The boundary-crossing field lines are more likely to be found in the lower latitudes during active times. While they tend to be located slightly closer than the non-boundary-crossing ones to the CH boundaries, nearly 40% (20%) of them in PF (MHD) CHs are not located in the boundary regions. The CHs without open field lines are often smaller and less unipolar than those with open field lines. The MHD model indicates higher temperature variations along the boundary-crossing field lines than the non-boundary-crossing ones. The main difference between the results of the two models is that the dominant field lines in the PF and MHD CHs are closed and open field lines, respectively.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Dense circumstellar material (CSM) is thought to play an important role in observed luminous optical transients: if such CSM is shocked, e.g., by ejecta expelled from the progenitor during core-collapse, then radiation produced by the shock-heated CSM can power bright UV/optical emission. If the initial CSM has an “outer edge” where most of the mass is contained and at which the optical depth is large, then shock breakout—when photons are first able to escape the shocked CSM—occurs near it. The rather thin shell of shocked CSM subsequently expands, and in the ensuing cooling-envelope phase, radiative and adiabatic losses compete to expend the CSM thermal energy. Here we derive an analytic solution to the bolometric light curve produced by such shocked CSM. For the first time, we provide an analytic solution to the cooling-envelope phase that is applicable starting from shock breakout and until the expanding CSM becomes optically thin. In particular, we account for the planar CSM geometry that is relevant at early times and properly treat radiative losses within this planar phase. We show that these effects can dramatically impact the resulting light curves, particularly if the CSM optical depth is only marginally larger than <jats:italic>c</jats:italic>/<jats:italic>v</jats:italic> <jats:sub>sh</jats:sub> (where <jats:italic>v</jats:italic> <jats:sub>sh</jats:sub> is the shock velocity). This has important implications for interpreting observed fast optical transients, which have previously been modeled using either computationally expensive numerical simulations or more simplified models that do not properly capture the early light-curve evolution.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We aim to determine why there exists anisotropic H <jats:sc>i</jats:sc> absorption around quasars; i.e., the environments around quasars are highly biased toward producing strong H <jats:sc>i</jats:sc> absorption in the transverse direction while there exists a significant deficit of H <jats:sc>i</jats:sc> absorption within a few megaparsecs of quasars along the line of sight. The most plausible explanation for this opposite trend is that the transverse direction is shielded from quasar UV radiation by dust torus. However, a critical weakness of this explanation is that we do not have any information on the inclination angle of our sightline relative to the torus. In this study, we examine environments of quasars with broad-absorption troughs in their spectra (i.e., BAL quasars) because it is widely believed that BAL troughs are observed if the central continuum is viewed from the side through their powerful outflows near the dust torus. With closely separated 12 projected quasar pairs at different redshifts with a separation angle of <jats:italic>θ</jats:italic> &lt; 120″, we examine H <jats:sc>i</jats:sc> absorption at foreground BAL quasars in the spectra of background quasars. We confirm that there exists optically thick gas around two of 12 BAL quasars, and that the mean H <jats:sc>i</jats:sc> absorption strength is EW<jats:sub>rest</jats:sub> ∼ 1 Å. This is consistent with past results of studies of non-BAL quasars, although not statistically significant. The origins of optically thick H <jats:sc>i</jats:sc> absorbers around BAL and non-BAL quasars could be different since their column densities are different by ∼3 orders of magnitude. A larger sample is required to narrow down possible scenarios explaining the anisotropic H <jats:sc>i</jats:sc> absorption around quasars.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We report on analysis of X-ray, optical, and radio observations of the previously overlooked X-ray source 2CXO J174517.0–321356 located just 3.°2 away from the Galactic center. Timing analysis of X-ray observations of the source with XMM-Newton reveals periodic pulsations with periods of 1228 and 614 s, with the latter being tentatively considered fundamental. On the other hand, an observation of the object with NuSTAR reveals a hard thermal-bremsstrahlung spectrum. Inspection of the archival Very Large Telescope image reveals, however, no obvious optical counterpart down to <jats:italic>R</jats:italic> &gt; 25 mag. Observations made with ATCA showed a possible faint radio counterpart with a positive spectral index (<jats:italic>α</jats:italic> &gt; 0.51) between 1 and 3 GHz, but follow-up ATCA and Very Large Array observations at frequencies between 4.5–10 GHz and 3–22 GHz, respectively, could not detect it. Given the properties in these three bands, we argue that the most likely origin of the X-ray source is emission from a new intermediate polar close to the Galactic center. Alternatively, and less likely, it is an ultracompact X-ray binary, which is one of the most compact X-ray binaries.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Stellar variability is driven by a multitude of internal physical processes that depend on fundamental stellar properties. These properties are our bridge to reconciling stellar observations with stellar physics and to understand the distribution of stellar populations within the context of galaxy formation. Numerous ongoing and upcoming missions are charting brightness fluctuations of stars over time, which encode information about physical processes such as the rotation period, evolutionary state (such as effective temperature and surface gravity), and mass (via asteroseismic parameters). Here, we explore how well we can predict these stellar properties, across different evolutionary states, using only photometric time-series data. To do this, we implement a convolutional neural network, and with data-driven modeling we predict stellar properties from light curves of various baselines and cadences. Based on a single quarter of Kepler data, we recover the stellar properties, including the surface gravity for red giant stars (with an uncertainty of ≲0.06 dex) and rotation period for main-sequence stars (with an uncertainty of ≲5.2 days, and unbiased from ≈5 to 40 days). Shortening the Kepler data to a 27 days Transiting Exoplanet Survey Satellite–like baseline, we recover the stellar properties with a small decrease in precision, ∼0.07 for log <jats:italic>g</jats:italic> and ∼5.5 days for <jats:italic>P</jats:italic> <jats:sub>rot</jats:sub>, unbiased from ≈5 to 35 days. Our flexible data-driven approach leverages the full information content of the data, requires minimal or no feature engineering, and can be generalized to other surveys and data sets. This has the potential to provide stellar property estimates for many millions of stars in current and future surveys.</jats:p>