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<jats:title>Abstract</jats:title> <jats:p>How microflares behave and differ from large flares is an important question in flare studies. Although they have been extensively investigated, microflares are not fully understood in terms of their detailed energy release processes and the role of energetic electrons. A recent study on an A-class microflare suggests the existence of a nonthermal component down to 6.5 keV, indicating that accelerated electrons play an important role in microflares, as in large flares. Here, we revisit this event, and present a comprehensive, quantitative analysis of the energy release and plasma heating processes. Using careful differential emission measure (DEM) analysis, we calculate the thermal X-ray fluxes. By subtracting this multithermal component from the observed data, we confirm the existence of the remaining nonthermal component. In addition, we analyze the clear evaporation process and report the first imaging evidence for a low-energy cutoff of energetic electrons in EM maps of &gt;10 MK plasma, which first appeared as two coronal sources significantly above the chromospheric footpoints. Detailed calculations of electron transport, based on the electron parameters and the evolution of loop dynamics, provide strong evidence of a beam-driven plasma heating process with a low-energy cutoff consistent with that derived independently from DEM analysis. This study reveals the important role of electron thermalization and low-energy cutoffs in the physical processes of microflares.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>HAWC J1826−128 is one of the brightest Galactic TeV <jats:italic>γ</jats:italic>-ray sources detected by the High Altitude Water Cherenkov (HAWC) observatory, with photon energies extending up to nearly ∼100 TeV. This HAWC source spatially coincides with the H.E.S.S. TeV source HESS J1826−130 and the “Eel” pulsar wind nebula (PWN), which is associated with the GeV pulsar PSR J1826−1256. In the X-ray band, Chandra and XMM-Newton revealed that the Eel PWN is composed of both a compact nebula (∼15″) and diffuse X-ray emission (∼6′ × 2′) extending away from the pulsar. Our NuSTAR observation detected hard X-ray emission from the compact PWN up to ∼20 keV and evidence of the synchrotron burn-off effect. In addition to the spatial coincidence between HESS J1826−130 and the diffuse X-ray PWN, our multiwavelength spectral energy distribution (SED) analysis using X-ray and <jats:italic>γ</jats:italic>-ray data establishes a leptonic origin of the TeV emission associated with the Eel PWN. Furthermore, our evolutionary PWN SED model suggests (1) a low PWN <jats:italic>B</jats:italic>-field of ∼1 <jats:italic>μ</jats:italic>G, (2) a significantly younger pulsar age (<jats:italic>t</jats:italic> ∼ 5.7 kyr) than the characteristic age (<jats:italic>τ</jats:italic> = 14.4 kyr), and (3) a maximum electron energy of <jats:inline-formula> <jats:tex-math> <?CDATA ${E}_{\max }=2$?> </jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mrow> <mml:mi>E</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>max</mml:mi> </mml:mrow> </mml:msub> <mml:mo>=</mml:mo> <mml:mn>2</mml:mn> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjac650aieqn1.gif" xlink:type="simple" /> </jats:inline-formula> PeV. The low <jats:italic>B</jats:italic>-field, as well as the putative supersonic motion of the pulsar, may account for the asymmetric morphology of the diffuse X-ray emission. Our results suggest that the Eel PWN may be a leptonic PeVatron particle accelerator powered by the ∼6 kyr old pulsar PSR J1826−1256 with a spin-down power of 3.6 × 10<jats:sup>36</jats:sup> erg s<jats:sup>−1</jats:sup>.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We demonstrate the utility of the bispectrum, the Fourier three-point correlation function, for studying driving scales of magnetohydrodynamic (MHD) turbulence in the interstellar medium. We calculate the bispectrum by implementing a parallelized Monte Carlo direct measurement method, which we have made publicly available. In previous works, the bispectrum has been used to identify nonlinear scaling correlations and break degeneracies in lower-order statistics like the power spectrum. We find that the bicoherence, a related statistic which measures phase coupling of Fourier modes, identifies turbulence-driving scales using density and column density fields. In particular, it shows that the driving scale is phase-coupled to scales present in the turbulent cascade. We also find that the presence of an ordered magnetic field at large scales enhances phase coupling as compared to a pure hydrodynamic case. We therefore suggest the bispectrum and bicoherence as tools for searching for non-locality for wave interactions in MHD turbulence.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Initially classified as a Type Ib supernova (SN), ∼100 days after the explosion SN 2014C made a transition to a Type II SN, presenting a gradual increase in the H<jats:italic>α</jats:italic> emission. This has been interpreted as evidence of interaction between the SN shock wave and a massive shell previously ejected from the progenitor star. In this paper we present numerical simulations of the propagation of the SN shock through the progenitor star and its wind, as well as the interaction of the SN ejecta with the massive shell. To determine with high precision the structure and location of the shell, we couple a genetic algorithm to a hydrodynamic and a bremsstrahlung radiation transfer code. We iteratively modify the density stratification and location of the shell by minimizing the variance between X-ray observations and synthetic predictions computed from the numerical model, allowing the shell structure to be completely arbitrary. By assuming spherical symmetry, we found that our best-fit model has a shell mass of 2.6 <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub>; extends from 1.6 × 10<jats:sup>16</jats:sup> cm to 1.87 × 10<jats:sup>17</jats:sup> cm, implying that it was ejected ∼ 60/(<jats:italic>v</jats:italic> <jats:sub> <jats:italic>w</jats:italic> </jats:sub>/100 km s<jats:sup>−1</jats:sup>) yr before the SN explosion; and has a density stratification with an average behavior ∼<jats:italic>r</jats:italic> <jats:sup>−3</jats:sup> but presenting density fluctuations larger than one order of magnitude. Finally, we predict that if the density stratification follows the same power-law behavior, the SN will break out from the shell by mid-2022, i.e., 8.5 yr after explosion.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We report a possible <jats:italic>γ</jats:italic>-ray enhancement event detected from Tycho’s supernova remnant (SNR), the outcome of a type Ia supernova explosion that occurred in the year 1572. The event lasted for 1.5 yr and showed a factor of 3.6 flux increase mainly in the energy range of 4–100 GeV, while notably accompanied with two 478 GeV photons. Several young SNRs (including Tycho’s SNR) were previously found to show peculiar X-ray structures with flux variations in one- or several-year timescales, such an event at <jats:italic>γ</jats:italic>-ray energies is for the first time seen. The year-long timescale of the event suggests a synchrotron radiation process, but the hard <jats:italic>γ</jats:italic>-ray emission requires extreme conditions of either ultrahigh energies for the electrons up to ∼10 PeV (well above the cosmic-ray <jats:italic>knee</jats:italic> energy) or high inhomogeneity of the magnetic field in the SNR. This event in Tycho’s SNR is likely analogous to the <jats:italic>γ</jats:italic>-ray flares observed in the Crab Nebula, the comparably short timescales of them both requiring a synchrotron process, and similar magnetohydrodynamic processes such as magnetic reconnection would be at work as well in the SNR to accelerate particles to ultrarelativistic energies. The event, if confirmed, helps reveal the more complicated side of the physical processes that can occur in young SNRs.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Using a three-dimensional general circulation model, we show that the atmospheric dynamics on a tidally locked Earth-like exoplanet, simulated with the planetary and orbital parameters of Proxima Centauri b, support a longitudinally asymmetric stratospheric wind oscillation (LASO), analogous to Earth’s quasi-biennial oscillation (QBO). In our simulations, the LASO has a vertical extent of 35–55 km, a period of 5–6.5 months, and a peak-to-peak wind speed amplitude of −70 to +130 m s<jats:sup>−1</jats:sup> with a maximum at an altitude of 41 km. Unlike the QBO, the LASO displays longitudinal asymmetries related to the asymmetric thermal forcing of the planet and to interactions with the resulting stationary Rossby waves. The equatorial gravity wave sources driving the LASO are localized in the deep convection region at the substellar point and in a jet exit region near the western terminator, unlike the QBO, for which these sources are distributed uniformly around the planet. Longitudinally, the western terminator experiences the highest wind speeds and undergoes reversals earlier than other longitudes. The antistellar point only experiences a weak oscillation with a very brief, low-speed westward phase. The QBO on Earth is associated with fluctuations in the abundances of water vapor and trace gases such as ozone, which are also likely to occur on exoplanets if these gases are present. Strong fluctuations in temperature and the abundances of atmospheric species at the terminators will need to be considered when interpreting atmospheric observations of tidally locked exoplanets.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Dynamical models are crucial for uncovering the internal dynamics of galaxies; however, most of the results to date assume axisymmetry, which is not representative of a significant fraction of massive galaxies. Here, we build triaxial Schwarzschild orbit-superposition models of galaxies taken from the SAMI Galaxy Survey, in order to reconstruct their inner orbital structure and mass distribution. The sample consists of 161 passive galaxies with total stellar masses in the range 10<jats:sup>9.5</jats:sup>–10<jats:sup>12</jats:sup> <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub>. We find that the changes in internal structures within 1<jats:italic>R</jats:italic> <jats:sub>e</jats:sub> are correlated with the total stellar mass of the individual galaxies. The majority of the galaxies in the sample (73% ± 3%) are oblate, while 19% ± 3% are mildly triaxial and 8% ± 2% have triaxial/prolate shape. Galaxies with <jats:inline-formula> <jats:tex-math> <?CDATA $\mathrm{log}{M}_{\star }/{M}_{\odot }\gt 10.50$?> </jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>log</mml:mi> <mml:msub> <mml:mrow> <mml:mi>M</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>⋆</mml:mo> </mml:mrow> </mml:msub> <mml:mrow> <mml:mo stretchy="true">/</mml:mo> </mml:mrow> <mml:msub> <mml:mrow> <mml:mi>M</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>⊙</mml:mo> </mml:mrow> </mml:msub> <mml:mo>&gt;</mml:mo> <mml:mn>10.50</mml:mn> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjac5bd5ieqn1.gif" xlink:type="simple" /> </jats:inline-formula> are more likely to be non-oblate. We find a mean dark matter fraction of <jats:italic>f</jats:italic> <jats:sub>DM</jats:sub> = 0.28 ± 0.20, within 1<jats:italic>R</jats:italic> <jats:sub>e</jats:sub>. Galaxies with higher intrinsic ellipticity (flatter) are found to have more negative velocity anisotropy <jats:italic>β</jats:italic> <jats:sub> <jats:italic>r</jats:italic> </jats:sub> (tangential anisotropy). <jats:italic>β</jats:italic> <jats:sub> <jats:italic>r</jats:italic> </jats:sub> also shows an anticorrelation with the edge-on spin parameter <jats:inline-formula> <jats:tex-math> <?CDATA ${\lambda }_{\mathrm{Re},\mathrm{EO}}$?> </jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mrow> <mml:mi>λ</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>Re</mml:mi> <mml:mo>,</mml:mo> <mml:mi>EO</mml:mi> </mml:mrow> </mml:msub> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjac5bd5ieqn2.gif" xlink:type="simple" /> </jats:inline-formula>, so that <jats:italic>β</jats:italic> <jats:sub> <jats:italic>r</jats:italic> </jats:sub> decreases with increasing <jats:inline-formula> <jats:tex-math> <?CDATA ${\lambda }_{\mathrm{Re},\mathrm{EO}}$?> </jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mrow> <mml:mi>λ</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>Re</mml:mi> <mml:mo>,</mml:mo> <mml:mi>EO</mml:mi> </mml:mrow> </mml:msub> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjac5bd5ieqn3.gif" xlink:type="simple" /> </jats:inline-formula>, reflecting the contribution from disk-like orbits in flat, fast-rotating galaxies. We see evidence of an increasing fraction of hot orbits with increasing stellar mass, while warm and cold orbits show a decreasing trend. We also find that galaxies with different (<jats:italic>V</jats:italic>/<jats:italic>σ</jats:italic> – <jats:italic>h</jats:italic> <jats:sub>3</jats:sub>) kinematic signatures have distinct combinations of orbits. These results are in agreement with a formation scenario in which slow- and fast-rotating galaxies form through two main channels.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We present a multi-instrument study of the two precursor brightenings prior to the M6.5 flare (SOL2015-06-22T18:23) in the NOAA Active Region 12371, with a focus on the temperature (<jats:italic>T</jats:italic>), electron number density (<jats:italic>n</jats:italic>), and emission measure (EM). The data used in this study were obtained from four instruments with a variety of wavelengths, i.e., the Solar Dynamics Observatory’s Atmospheric Imaging Assembly (AIA), in six extreme ultraviolet (EUV) passbands; the Expanded Owens Valley Solar Array (EOVSA) in microwave (MW); the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) in hard X-rays (HXR); and the Geostationary Operational Environmental Satellite (GOES) in soft X-rays (SXR). We compare the temporal variations of <jats:italic>T</jats:italic>, <jats:italic>n</jats:italic>, and EM derived from the different data sets. Here are the key results. (1) GOES SXR and AIA EUV have almost identical EM variations (1.5–3 × 10<jats:sup>48</jats:sup> cm<jats:sup>−3</jats:sup>) and very similar <jats:italic>T</jats:italic> variations, from 8 to 15 million Kelvin (MK). (2) Listed from highest to lowest, EOVSA MW provides the highest temperature variations (15–60 MK), followed by RHESSI HXR (10–24 MK), then GOES SXR and AIA EUV (8–15 MK). (3) The EM variation from the RHESSI HXR measurements is always less than the values from AIA EUV and GOES SXR by at most 20 times. The number density variation from EOVSA MW is greater than the value from AIA EUV by at most 100 times. The results quantitatively describe the differences in the thermal parameters at the precursor phase, as measured by different instruments operating at different wavelength regimes and for different emission mechanisms.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Particle acceleration is ubiquitous at shock waves, occurring on scales ranging from supernova remnants in the universe to coronal-mass-ejection-driven shocks and planetary bow shocks in the heliosphere. The most promising mechanism responsible for the almost universally observed power-law spectra is diffusive shock acceleration (DSA). However, how electrons are preaccelerated by different shocks to the energy required by the DSA theory is still unclear. In this paper, we perform two-dimensional particle-in-cell plasma simulations to investigate how the magnetic field orientations, with respect to simulation planes, affect electron preacceleration in moderate-Mach-number low-<jats:bold> <jats:italic>β</jats:italic> </jats:bold> shocks. Simulation results show that instabilities can be different as the simulation planes capture different trajectories of particles. For magnetic fields perpendicular to the simulation plane, electron cyclotron drift instability dominates in the foot. Electrons can be trapped by the electrostatic wave and undergo shock-surfing acceleration. For magnetic fields lying in the simulation plane, whistler waves produced by modified two-stream instability dominate in the foot and scatter the electrons. In both cases, electrons undergo multistage acceleration in the foot, shock surface, and immediate downstream, during which process shock-surfing acceleration takes place as part of the preacceleration mechanism in moderate-Mach-number quasi-perpendicular shocks.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We discuss a spectroscopic survey of the strong lensing cluster A1489 that includes redshifts for 195 cluster members along with central velocity dispersions for 188 cluster members. The caustic technique applied to the redshift survey gives the dynamical parameters <jats:italic>M</jats:italic> <jats:sub>200</jats:sub> = (1.25 ± 0.09) × 10<jats:sup>15</jats:sup> <jats:italic> M</jats:italic> <jats:sub>⊙</jats:sub>, <jats:italic>R</jats:italic> <jats:sub>200</jats:sub> = 1.97 ± 0.05 Mpc, and a cluster line-of-sight velocity dispersion of 1150 ± 72 km s<jats:sup>−1</jats:sup> within <jats:italic>R</jats:italic> <jats:sub>200</jats:sub>. These parameters are very similar to those of other strong lensing systems with comparably large Einstein radii. We use the spectroscopy and deep photometry to demonstrate that A1489 is probably dynamically active; its four brightest cluster galaxies have remarkably different rest-frame radial velocities. Like other massive strong lensing clusters, the velocity dispersion function for members of A1489 shows an excess for dispersions ≳250 km s<jats:sup>−1</jats:sup>. The central dispersions also provide enhanced constraints on future lensing models.</jats:p>