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<jats:title>Abstract</jats:title> <jats:p>We present optical photometry and spectroscopy of SN 2019stc (=ZTF19acbonaa), an unusual Type Ic supernova (SN Ic) at a redshift of <jats:italic>z</jats:italic> = 0.117. SN 2019stc exhibits a broad double-peaked light curve, with the first peak having an absolute magnitude of <jats:italic>M</jats:italic> <jats:sub> <jats:italic>r</jats:italic> </jats:sub> = −20.0 mag, and the second peak, about 80 rest-frame days later, <jats:italic>M</jats:italic> <jats:sub> <jats:italic>r</jats:italic> </jats:sub> = −19.2 mag. The total radiated energy is large, <jats:italic>E</jats:italic> <jats:sub>rad</jats:sub> ≈ 2.5 × 10<jats:sup>50</jats:sup> erg. Despite its large luminosity, approaching those of Type I superluminous supernovae (SLSNe), SN 2019stc exhibits a typical SN Ic spectrum, bridging the gap between SLSNe and SNe Ic. The spectra indicate the presence of Fe-peak elements, but modeling of the first light-curve peak with radioactive heating alone leads to an unusually high nickel mass fraction of <jats:italic>f</jats:italic> <jats:sub>Ni</jats:sub> ≈ 0.31 (<jats:italic>M</jats:italic> <jats:sub>Ni</jats:sub> ≈ 3.2 <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub>). Instead, if we model the first peak with a combined magnetar spin-down and radioactive heating model we find a better match with <jats:italic>M</jats:italic> <jats:sub>ej</jats:sub> ≈ 4 <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub>, a magnetar spin period of <jats:italic>P</jats:italic> <jats:sub>spin</jats:sub> ≈ 7.2 ms, and magnetic field of <jats:italic>B</jats:italic> ≈ 10<jats:sup>14</jats:sup> G, and <jats:italic>f</jats:italic> <jats:sub>Ni</jats:sub> ≲ 0.2 (consistent with SNe Ic). The prominent second peak cannot be naturally accommodated with radioactive heating or magnetar spin-down, but instead can be explained as circumstellar interaction with ≈0.7 <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub> of hydrogen-free material located ≈400 au from the progenitor. Accounting for the ejecta mass, circumstellar shell mass, and remnant neutron star mass, we infer a CO core mass prior to explosion of ≈6.5 <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub>. The host galaxy has a metallicity of ≈0.26 <jats:italic>Z</jats:italic> <jats:sub>⊙</jats:sub>, low for SNe Ic but consistent with SLSNe. Overall, we find that SN 2019stc is a transition object between normal SNe Ic and SLSNe.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The “Acceleration, Reconnection, Turbulence and Electrodynamics of the Moon's Interaction with the Sun” mission provides a unique opportunity to study the structure of interplanetary shocks and the associated generation of plasma waves with frequencies between ∼50 and 8000 Hz due to its long duration electric and magnetic field burst waveform captures. We compare wave properties and occurrence rates at 11 quasi-perpendicular interplanetary shocks with burst data within 10 minutes (∼3200 proton gyroradii upstream, ∼1900 downstream) of the shock ramp. A perturbed shock is defined as possessing a large amplitude whistler precursor in the quasi-static magnetic field with an amplitude greater than 1/3 the difference between the upstream and downstream average magnetic field magnitudes; laminar shocks lack these large precursors and have a smooth, step function-like transition. In addition to wave modes previously observed, including ion acoustic, whistler, and electrostatic solitary waves, waves in the ion acoustic frequency range that show rapid temporal frequency change are common. Three shocks had burst captures in the ramp; of these, the two laminar shocks contained a wide range of large amplitude wave modes in the ramp whereas the one perturbed shock contained no such waves. Thus, energy dissipation through wave–particle interactions is more prominent in these two laminar shocks than in the perturbed shock. Based on observations from all 11 shocks, the wave occurrence rates for laminar shocks are higher in the transition region, especially the ramp, than downstream. In contrast, perturbed shocks have approximately 2–3 times the wave occurrence rate downstream than laminar shocks.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Using the Monte Carlo code SEDONA, multiband photometry and spectra are calculated for supernovae derived from stripped helium stars with presupernova masses of 2.2 to 10.0 <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub>. The models are representative of evolution in close binaries and have previously been exploded using a parameterized one-dimensional model for neutrino transport. A subset, those with presupernova masses in the range of 2.2–5.6 <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub>, have many properties in common with observed Type Ib and Ic supernovae, including a median ejected mass near 2 <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub>, explosion energies near 1 × 10<jats:sup>51</jats:sup> erg, typical <jats:sup>56</jats:sup>Ni masses of 0.07–0.09 <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub>, peak times of about 20 days, and a narrow range for the <jats:italic>V</jats:italic> − <jats:italic>R</jats:italic> color index 10 days post-<jats:italic>V</jats:italic>-maximum near 0.3 mag. The median peak bolometric luminosity, near 10<jats:sup>42.3</jats:sup> erg s<jats:sup>−1</jats:sup>, is fainter, however, than several observational tabulations, and the brightest explosion has a bolometric luminosity of only 10<jats:sup>42.50</jats:sup> erg s<jats:sup>−1</jats:sup>. The brightest absolute <jats:italic>B</jats:italic>, <jats:italic>V</jats:italic>, and <jats:italic>R</jats:italic> magnitudes at peak are −17.2, −17.8, and −18.0. These limits are fainter than some allegedly typical Type Ib and Ic supernovae and could reflect problems in our models or in the observational analysis. Helium stars with lower and higher masses also produce interesting transients that may have been observed, including fast, faint, blue transients and long, red, faint Type Ic supernovae. New models are specifically presented for SN 2007Y, SN 2007gr, SN 2009jf, LSQ 13abf, SN 2008D, and SN 2010X.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Blazars are active galactic nuclei with their relativistic jets pointing toward the observer, comprising two major subclasses, flat-spectrum radio quasars (FSRQs) and BL Lac objects. We present multiwavelength photometric and spectroscopic monitoring observations of the blazar B2 1420+32, focusing on its outbursts in 2018–2020. Multiepoch spectra show that the blazar exhibited large-scale spectral variability in both its continuum and line emission, accompanied by dramatic gamma-ray and optical variability by factors of up to 40 and 15, respectively, on week to month timescales. Over the last decade, the gamma-ray and optical fluxes increased by factors of 1500 and 100, respectively. B2 1420+32 was an FSRQ with broad emission lines in 1995. Following a series of flares starting in 2018, it transitioned between BL Lac and FSRQ states multiple times, with the emergence of a strong Fe pseudocontinuum. Two spectra also contain components that can be modeled as single-temperature blackbodies of 12,000 and 5200 K. Such a collection of “changing-look” features has never been observed previously in a blazar. We measure gamma-ray–optical and interband optical lags implying emission-region separations of less than 800 and 130 gravitational radii, respectively. Since most emission-line flux variations, except the Fe continuum, are within a factor of 2–3, the transitions between FSRQ and BL Lac classifications are mainly caused by the continuum variability. The large Fe continuum flux increase suggests the occurrence of dust sublimation releasing more Fe ions in the central engine and an energy transfer from the relativistic jet to subrelativistic emission components.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>To better understand the decay of different types of sunspots, we studied the decay of eight <jats:italic>α</jats:italic>-configuration sunspots by using the data that were acquired by the Helioseismic and Magnetic Imager on board the Solar Dynamic Observatory. We followed their decay for about four days and analyzed the evolution of their photospheric area and magnetic field parameters. We found that the area and total magnetic flux of <jats:italic>α</jats:italic> sunspots show a near-linear decrease during their decay. Meanwhile, the area decay rate of an individual sunspot is not constant. The area decay of a sunspot can be divided into two stages, a slow and a rapid decay process. Moreover, according to the difference of the area decay of the penumbra and umbra, the α sunspots decay can be classified in three ways: the penumbra and umbra decay synchronously, the penumbra decays first, and the umbra decays first. In addition, the flux decay of the penumbra is lagging behind the decay of the penumbral area. This finding suggests that the vertical magnetic field of the sunspot penumbra increases significantly in the early stage of sunspot decay.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We describe a numerical scheme for magnetohydrodynamics simulations of dust–gas mixture by extending smoothed particle magnetohydrodynamics. We employ the single-species particle approach to describe dust–gas mixture with several modifications from the previous studies. We assume that the charged and neutral dust can be treated as single-fluid, that the electromagnetic force acts on the gas, and that that acting on the charged dust is negligible. The validity of these assumptions in the context of protostar formation is not obvious and is extensively evaluated. By investigating the electromagnetic force and electric current with terminal velocity approximation, it is found that as the dust size increases, the contribution of dust to them becomes smaller and negligible. We conclude that our assumption that the electromagnetic force on the dusts is negligible is valid for the dust size with <jats:italic>a</jats:italic> <jats:sub> <jats:italic>d</jats:italic> </jats:sub> ≳ 10 <jats:italic>μ</jats:italic>m. On the other hand, they do not produce the numerical artifact for the dust <jats:italic>a</jats:italic> <jats:sub> <jats:italic>d</jats:italic> </jats:sub> ≲ 10 <jats:italic>μ</jats:italic>m in the envelope and disk, where the perfect coupling between gas and dust is realized. However, we also found that our assumptions may break down in outflow (or under an environment with very strong magnetic field and low density) for the dust <jats:italic>a</jats:italic> <jats:sub> <jats:italic>d</jats:italic> </jats:sub> ≲ 10 <jats:italic>μ</jats:italic>m. We conclude that our assumptions are valid in almost all cases where macroscopic dust dynamics is important in the context of protostar formation. We conduct numerical tests of dusty waves, dusty magnetohydrodynamics shocks, and gravitational collapse of magnetized cloud cores with our simulation code. The results show that our numerical scheme well reproduces the dust dynamics in the magnetized medium.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We report the timing and spectral analyses of the type-II X-ray bursts from the rapid burster (MXB 1730–335) observed by the Hard X-ray Modulation Telescope (Insight-HXMT) and Swift/X-Ray Telescope (XRT). By stacking the long-duration bursts, we find for the first time that the hard X-rays are lagging behind the soft X-rays by 3 s. However, such a lag is not visible for the short-duration bursts, probably because of the poor statistics. For all bursts the energy spectrum is found to be nonthermal, thanks to the broadband coverage of Insight-HXMT. These findings provide new insights into the type-II bursts and require a temporally visible corona for possible interpretation.</jats:p>