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<jats:title>Abstract</jats:title> <jats:p>Keck-telescope spectrophotometry of the companion of PSR J1810+1744 shows a flat, but asymmetric light-curve maximum and a deep, narrow minimum. The maximum indicates strong gravity darkening (GD) near the <jats:italic>L</jats:italic> <jats:sub>1</jats:sub> point, along with a heated pole and surface winds. The minimum indicates a low underlying temperature and substantial limb darkening. The GD is a consequence of extreme pulsar heating and the near-filling of the Roche lobe. Light-curve modeling gives a binary inclination <jats:italic>i</jats:italic> = 65.°7 ± 0.°4. With the Keck-measured radial-velocity amplitude <jats:italic>K</jats:italic> <jats:sub>c</jats:sub> = 462.3 ± 2.2 km s<jats:sup>−1</jats:sup>, this gives an accurate neutron star mass <jats:italic>M</jats:italic> <jats:sub>NS</jats:sub> = 2.13 ± 0.04 <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub>, with important implications for the dense-matter equation of state. A classic direct-heating model, ignoring the <jats:italic>L</jats:italic> <jats:sub>1</jats:sub> gravitational darkening, would predict an unphysical <jats:italic>M</jats:italic> <jats:sub>NS</jats:sub> &gt; 3 <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub>. A few other “spider” pulsar binaries have similar large heating and fill factor; thus, they should be checked for such effects.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The dynamical structure of the Kuiper Belt can be used as a clue to the formation and evolution of the solar system, planetary systems in general, and Neptune’s early orbital history in particular. The problem is best addressed by forward modeling where different initial conditions and Neptune’s orbital evolutions are tested, and the model predictions are compared to orbits of known Kuiper Belt objects (KBOs). It has previously been established that Neptune radially migrated, by gravitationally interacting with an outer disk of planetesimals, from the original radial distance <jats:italic>r</jats:italic> ≲ 25 au to its current orbit at 30 au. Here we show that the migration models with a very low orbital eccentricity of Neptune (<jats:italic>e</jats:italic> <jats:sub>N</jats:sub> ≲ 0.03) do not explain KBOs with semimajor axes 50 &lt; <jats:italic>a</jats:italic> &lt; 60 au, perihelion distances <jats:italic>q</jats:italic> &gt; 35 au, and inclinations <jats:italic>i</jats:italic> &lt; 10°. If <jats:italic>e</jats:italic> <jats:sub>N</jats:sub> ≲ 0.03 at all times, the Kozai cycles control the implantation process and the orbits with <jats:italic>q</jats:italic> &gt; 35 au end up having, due to the angular momentum’s <jats:italic>z</jats:italic>-component conservation, <jats:italic>i</jats:italic> &gt; 10°. Better results are obtained when Neptune’s eccentricity is excited to <jats:italic>e</jats:italic> <jats:sub>N</jats:sub> ≃ 0.1 and subsequently damped by dynamical friction. The low-<jats:italic>e</jats:italic> and low-<jats:italic>i</jats:italic> orbits at 50–60 au are produced in this model when KBOs are lifted from the scattered disk by secular cycles—mainly the apsidal resonance <jats:italic>ν</jats:italic> <jats:sub>8</jats:sub>—near various mean motion resonances. These results give support to a (mild) dynamical instability that presumably excited the orbits of giant planets during Neptune’s early migration.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The tectonic regime of rocky planets fundamentally influences their long-term evolution and cycling of volatiles between interior and atmosphere. Earth is the only known planet with active plate tectonics, but observations of exoplanets may deliver insights into the diversity of tectonic regimes beyond the solar system. Observations of the thermal phase curve of super-Earth LHS 3844b reveal a solid surface and lack of a substantial atmosphere, with a temperature contrast between the substellar and antistellar point of around 1000 K. Here, we use these constraints on the planet’s surface to constrain the interior dynamics and tectonic regimes of LHS 3844b using numerical models of interior flow. We investigate the style of interior convection by assessing how upwellings and downwellings are organized and how tectonic regimes manifest. We discover three viable convective regimes with a mobile surface: (1) spatially uniform distribution of upwellings and downwellings, (2) prominent downwelling on the dayside and upwellings on the nightside, and (3) prominent downwelling on the nightside and upwellings on the dayside. Hemispheric tectonics is observed for regimes (2) and (3) as a direct consequence of the day-to-night temperature contrast. Such a tectonic mode is absent in the present-day solar system and has never been inferred from astrophysical observations of exoplanets. Our models offer distinct predictions for volcanism and outgassing linked to the tectonic regime, which may explain secondary features in phase curves and allow future observations to constrain the diversity of super-Earth interiors.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>It is generally held that &gt;100 TeV emission from astrophysical objects unambiguously demonstrates the presence of PeV protons or nuclei, due to the unavoidable Klein–Nishina suppression of inverse Compton emission from electrons. However, in the presence of inverse Compton dominated cooling, hard high-energy electron spectra are possible. We show that the environmental requirements for such spectra can naturally be met in spiral arms, and in particular in regions of enhanced star formation activity, the natural locations for the most promising electron accelerators: powerful young pulsars. Our scenario suggests a population of hard ultra-high energy sources is likely to be revealed in future searches, and may also provide a natural explanation for the 100 TeV sources recently reported by the High-Altitude Water Cherenkov Observatory.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The results of determinations of the azimuthal and meridional velocities by time–distance helioseismology from Helioseismic and Magnetic Imager on board Solar Dynamics Observatory from 2010 May to 2020 September at latitudes and Stonyhurst longitudes from − 60° to + 60° and depths to about 19 Mm below the photosphere are used to analyze spatiotemporal variations of the solar differential rotation and meridional circulation. The pattern of torsional oscillations, or latitudinal belts of alternating “fast” and “slow” zonal flows migrating from high latitudes toward the equator, is found to extend in the time–latitude diagrams over the whole time interval. The oscillation period is comparable with a doubled solar-activity-cycle and can be described as an extended solar cycle. The zonal-velocity variations are related to the solar-activity level, the local-velocity increases corresponding to the sunspot-number increases and being localized at latitudes where the strongest magnetic fields are recorded. The dramatic growth of the zonal velocities in 2018 appears to be a precursor of the beginning of Solar Cycle 25. The strong symmetrization of the zonal-velocity field by 2020 can be considered another precursor. The general pattern of poleward meridional flows is modulated by latitudinal variations similar to the extended-solar-cycle behavior of the zonal flows. During the activity maximum, these variations are superposed with a higher harmonic corresponding to meridional flows converging to the spot-formation latitudes. Our results indicate that variations of both the zonal and meridional flows exhibit the extended-solar-cycle behavior, which is an intrinsic feature of the solar dynamo.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We study the large-scale anisotropy of the universe by measuring the dipole in the angular distribution of a flux-limited, all-sky sample of 1.36 million quasars observed by the Wide-field Infrared Survey Explorer (WISE). This sample is derived from the new CatWISE2020 catalog, which contains deep photometric measurements at 3.4 and 4.6 <jats:italic>μ</jats:italic>m from the cryogenic, post-cryogenic, and reactivation phases of the WISE mission. While the direction of the dipole in the quasar sky is similar to that of the cosmic microwave background (CMB), its amplitude is over twice as large as expected, rejecting the canonical, exclusively kinematic interpretation of the CMB dipole with a <jats:italic>p</jats:italic>-value of 5 × 10<jats:sup>−7</jats:sup> (4.9<jats:italic>σ</jats:italic> for a normal distribution, one-sided), the highest significance achieved to date in such studies. Our results are in conflict with the cosmological principle, a foundational assumption of the concordance ΛCDM model.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Collisionless shocks are common features in space and astrophysical systems where supersonic plasma flows interact, such as in the solar wind, the heliopause, and supernova remnants. Recent experimental capabilities and diagnostics allow detailed laboratory investigations of high-Mach-number shocks, which therefore can become a valuable way to understand shock dynamics in various astrophysical environments. Using 2D particle-in-cell simulations with a Coulomb binary collision operator, we demonstrate the mechanism for generation of energetic electrons and experimental requirements for detecting this process in the laboratory high-Mach-number collisionless shocks. We show through a parameter study that electron acceleration by magnetized collisionless shocks is feasible in laboratory experiments with laser-driven expanding plasmas.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Electrons contribute a strong radiation component to the surface bombardment of Europa. However, they are not typically considered to contribute to erosion of the surface H<jats:sub>2</jats:sub>O-ice or produce exospheres of radiolytic O<jats:sub>2</jats:sub>, as laboratory studies on sputtering induced by electrons are sparse. Here we have measured the sputtering yield of H<jats:sub>2</jats:sub>O-ice induced by 0.5 keV electrons between 14 and 125 K, estimating the composition of the stable products ejected during irradiation. Combining these measurements with updated electron flux measurements, we estimate for the first time that the global surface production rate of O<jats:sub>2</jats:sub> from electron-induced sputtering is larger than the production rate previously estimated for all of the ionic components combined. Our results emphasize the importance of electrons in producing exospheres on icy satellites and suggest that these exospheres need not be tied to environments where the surface is being bombarded with heavy ions.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Nebular He <jats:sc>ii</jats:sc> emission implies the presence of energetic photons (<jats:italic>E</jats:italic> ≥ 54 eV). Despite the great deal of effort dedicated to understanding He <jats:sc>ii</jats:sc> ionization, its origin has remained mysterious, particularly in metal-deficient star-forming (SF) galaxies. Unfolding He <jats:sc>ii</jats:sc>-emitting, metal-poor starbursts at <jats:italic>z</jats:italic> ∼ 0 can yield insight into the powerful ionization processes occurring in the primordial universe. Here we present a new study on the effects that X-ray sources have on the He <jats:sc>ii</jats:sc> ionization in the extremely metal-poor galaxy IZw18 (<jats:italic>Z</jats:italic> ∼ 3% <jats:italic>Z</jats:italic> <jats:sub>⊙</jats:sub>), whose X-ray emission is dominated by a single high-mass X-ray binary (HMXB). This study uses optical integral field spectroscopy, archival Hubble Space Telescope observations, and all of the X-ray data sets publicly available for IZw18. We investigate the time-variability of the IZw18 HMXB for the first time; its emission shows small variations on timescales from days to decades. The best-fit models for the HMXB X-ray spectra cannot reproduce the observed He <jats:sc>ii</jats:sc> ionization budget of IZw18, nor can recent photoionization models that combine the spectra of both very low metallicity massive stars and the emission from HMXB. We also find that the IZw18 HMXB and the He <jats:sc>ii</jats:sc>-emission peak are spatially displaced at a projected distance of ≃200 pc. These results reduce the relevance of X-ray photons as the dominant He <jats:sc>ii</jats:sc> ionizing mode in IZw18, which leaves uncertain what process is responsible for the bulk of its He <jats:sc>ii</jats:sc> ionization. This is in line with recent work discarding X-ray binaries as the main source responsible for He <jats:sc>ii</jats:sc> ionization in SF galaxies.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Nonthermal sources located above bright flare arcades, referred to as the “above-the-loop-top” sources, have been often suggested as the primary electron acceleration site in major solar flares. The X8.2 limb flare on 2017 September 10 features such an above-the-loop-top source, which was observed in both microwaves and hard X-rays (HXRs) by the Expanded Owens Valley Solar Array and the Reuven Ramaty High Energy Solar Spectroscopic Imager, respectively. By combining the microwave and HXR imaging spectroscopy observations with multifilter extreme ultraviolet and soft X-ray imaging data, we derive the coronal magnetic field and energetic electron distribution of the source over a broad energy range from &lt;10 keV up to ∼MeV during the early impulsive phase of the flare. The source has a strong magnetic field of over 800 G. The best-fit electron distribution consists of a thermal “core” from ∼25 MK plasma. A nonthermal power-law “tail” joins the thermal core at ∼16 keV with a spectral index of ∼3.6, which breaks down at above ∼160 keV to &gt;6.0. Temporally resolved analysis suggests that the electron distribution above the break energy rapidly hardens with the spectral index decreasing from &gt;20 to ∼6.0 within 20 s, or less than ∼10 Alfvén crossing times in the source. These results provide strong support for the above-the-loop-top source as the primary site where an ongoing bulk acceleration of energetic electrons is taking place very early in the flare energy release.</jats:p>