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<jats:title>Abstract</jats:title> <jats:p>PolarLight is a gas pixel X-ray polarimeter mounted on a CubeSat, which was launched into a Sun-synchronous orbit in 2018 October. We build a mass model of the whole CubeSat with the Geant4 toolkit to simulate the background induced by the cosmic X-ray background (CXB) and high-energy charged particles in the orbit. The simulated energy spectra and morphologies of event images both suggest that the background measured with PolarLight is dominated by high-energy electrons, with a minor contribution from protons and the CXB. The simulation reveals that, in the energy range 2–8 keV, roughly 28% of background events are caused by energy deposited by a secondary electron with an energy of a few keV, in a physical process identical to the detection of X-rays. Thus, this fraction of the background cannot be discriminated from X-ray events. The background distribution is uneven on the detector plane, with an enhancement near the edges. The edge effect occurs because high-energy electrons tend to produce long tracks, which are discarded by the readout electronics unless energy is partially deposited near the edges. The internal background rate is expected to be around 6 × 10<jats:sup>−3</jats:sup> counts s<jats:sup>−1</jats:sup> cm<jats:sup>−2</jats:sup> at 2–8 keV if an effective particle discrimination algorithm can be applied. This indicates that the internal background should be negligible for future focusing X-ray polarimeters with a focal size of the order of millimeters.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We study the formation of transient loops in the core of the AR 11890. For this purpose, we have used the observations recorded by the Atmospheric Imaging Assembly (AIA) and the Interface Region Imaging Spectrograph (IRIS). For photospheric field configuration, we have used the line-of-sight (LOS) magnetograms obtained from the Helioseismic and Magnetic Imager. The transient is simultaneously observed in all the UV and EUV channels of AIA and the three slit-jaw images from IRIS. The coexistence of the transient in all AIA and IRIS SJI channels suggests the transient’s multithermal nature. The transient consists of short loops located at the base of the transient, as well as long loops. A differential emission measure analysis shows that the transient has a clumpy structure. The highest emission observed at the base is within the temperature bin of <jats:inline-formula> <jats:tex-math> <?CDATA $\mathrm{log}T=6.65\mbox{--}6.95$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabdd2eieqn1.gif" xlink:type="simple" /> </jats:inline-formula>. We observe the longer loops at a similar temperature, albeit very feeble. Using LOS magnetograms, we conclude that the magnetic reconnection may have caused the transient. Our observations further suggest that the physics of the formation of such transients may be similar to those of typical coronal jets, albeit in different topological configurations. Such multiwavelength observations shed light on the formation of hot plasma in the solar corona and provide further essential constraints on modeling the thermodynamics of such transients.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We report the detection and follow-up of a superstellar flare GWAC 181229A with an amplitude of Δ<jats:italic>R</jats:italic> ∼ 9.5 mag on an M9-type star by SVOM/GWAC and the dedicated follow-up telescopes. The estimated bolometric energy <jats:italic>E</jats:italic> <jats:sub>bol</jats:sub> is (5.56–9.25) × 10<jats:sup>34</jats:sup> erg, which makes the event one of the most powerful flares seen on ultracool stars. The magnetic strength is inferred to be 3.6–4.7 kG. Thanks to sampling with a cadence of 15 s, a new component near the peak time with a very steep decay is detected in the <jats:italic>R</jats:italic>-band light curve, followed by the two-component flare template given by Davenport et al. An effective temperature of 5340 ± 40 K is measured by fitting a blackbody shape to the spectrum in the shallower phase during the flare. The filling factors of the flare are estimated to be ∼30% and 19% at the peak time and at 54 minutes after the first detection. The detection of this particular event with large amplitude, huge emitted energy, and a new component demonstrates that high-cadence sky monitoring cooperation with fast follow-up observations is very important for understanding the violent magnetic activity.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The energy spectrum of electrons produced in molecular gas by interstellar cosmic rays (CRs) is rigorously calculated as a function of gas column density <jats:italic>N</jats:italic> traversed by the CRs. This allows us to accurately compute the local value of the secondary ionization rate of molecular hydrogen, <jats:inline-formula> <jats:tex-math> <?CDATA ${\zeta }_{\sec }(N)$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabdc27ieqn1.gif" xlink:type="simple" /> </jats:inline-formula>, as a function of the local primary ionization rate, <jats:italic>ζ</jats:italic> <jats:sub> <jats:italic>p</jats:italic> </jats:sub>(<jats:italic>N</jats:italic>). The ratio <jats:inline-formula> <jats:tex-math> <?CDATA ${\zeta }_{\sec }/{\zeta }_{p}$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabdc27ieqn2.gif" xlink:type="simple" /> </jats:inline-formula> increases monotonically with <jats:italic>N</jats:italic>, and can considerably exceed the value of ≈0.67 commonly adopted in the literature. For sufficiently soft interstellar spectra, the dependence <jats:inline-formula> <jats:tex-math> <?CDATA ${\zeta }_{\sec }/{\zeta }_{p}$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabdc27ieqn3.gif" xlink:type="simple" /> </jats:inline-formula> versus <jats:italic>N</jats:italic> is practically insensitive to their particular shape and thus is a general characteristic of the secondary CR ionization in dense gas.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>In the absence of solar radiation, precipitating electrons from the solar wind (SW) are generally thought to be the dominant source of energy deposition in the nightside Martian upper atmosphere, creating a patchy ionosphere and possibly also affecting the nightside thermal budget of various neutral and ionized species. Previous model calculations have not taken into account in situ heating via SW electron impact. In the present study, we utilize extensive measurements made by several instruments on board the Mars Atmosphere and Volatile Evolution spacecraft, in order to perform data-driven computations of the nightside neutral, ion, and electron heating rates. Considering the large range of energetic electron intensity observed on the nightside of Mars, we divide the entire data set into two subsamples, either with or without energetic electron depletion, a notable feature of the nightside Martian ionosphere. Our calculations indicate that in situ nightside neutral heating is dominated by exothermic chemistry and Maxwell interaction with thermal ions for regions with depletion, and by direct SW impact for regions without. Collisional quenching of excited state species produced from a variety of channels, such as electron impact excitation, dissociation, and ionization, as well as <jats:inline-formula> <jats:tex-math> <?CDATA ${{\rm{O}}}_{2}^{+}$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabdbb0ieqn1.gif" xlink:type="simple" /> </jats:inline-formula> dissociative recombination, makes a substantial contribution to neutral heating, except during depletion. For comparison, nightside ion heating is mainly driven by energetic ion production under all circumstances, which occurs mainly via ion-neutral reaction O<jats:sup>+</jats:sup> + CO<jats:sub>2</jats:sub> and CO<jats:sub>2</jats:sub> <jats:sup>+</jats:sup> predissociation.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We aim to understand the effect of stellar evolution on the evolution of protoplanetary disks. We focus in particular on the disk evolution around intermediate-mass (IM) stars, which evolve more rapidly than low-mass ones. We numerically solve the long-term evolution of disks around 0.5–5 <jats:italic>M</jats:italic> <jats:sub>☉</jats:sub> stars considering viscous accretion and photoevaporation (PE) driven by stellar far-ultraviolet (FUV), extreme-ultraviolet (EUV), and X-ray emission. We also take stellar evolution into account and consider the time evolution of the PE rate. We find that the FUV, EUV, and X-ray luminosities of IM stars evolve by orders of magnitude within a few million years, along with the time evolution of stellar structure, stellar effective temperature, or accretion rate. Therefore, the PE rate also evolves with time by orders of magnitude, and we conclude that stellar evolution is crucial for the disk evolution around IM stars.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>In this work, reservoir phenomenon in the decay phase of gradual solar energetic particle (SEP) events are investigated with two Helios and the IMP 8 spacecraft from 1976 January to 1980 March and two STEREO and the SOHO spacecraft from 2010 January to 2014 September. Using these data, 62 reservoir events of solar energetic protons were identified, and the effects of perpendicular diffusion and magnetic mirroring on the formation of the reservoir phenomenon have been studied. We find that the reservoir events could be observed in almost all longitudes in the ecliptic at 1 au; thus, the perpendicular diffusion in interplanetary space is an important mechanism to explain the uniform distribution of SEPs. Furthermore, in the 1976 April 30 event, the effects of magnetic mirroring associated with an interplanetary coronal mass ejection were observed during the reservoir phenomenon. Therefore, the effects of magnetic mirroring could also help to form the reservoir phenomenon. This study could improve the understanding of the propagation of SEPs in interplanetary space.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>X-ray observations are limited by the background, due to the intrinsic faintness or diffuse nature of the sources. The future Athena X-ray observatory has among its goals the characterization of these sources. We aim at characterizing the particle-induced background of the Athena microcalorimeter, in both its low- (soft protons) and high-energy (galactic cosmic rays—GCR) induced components, to assess the instrument capability to characterize background-dominated sources such as the outskirts of clusters of galaxies. We compare two radiation environments, namely the L1 and L2 Lagrangian points, and derive indications against the latter. We estimate the particle-induced background level on the X-IFU microcalorimeter with Monte Carlo simulations, before and after all of the solutions adopted to reduce its level. Concerning the GCR-induced component, the background level is compliant with the mission requirement. Regarding the soft-proton component, the analysis does not predict dramatically different backgrounds in the L1 and L2 orbits. However, the lack of data concerning the L2 environment labels it as very weakly characterizable, and thus we advise against choosing it as the orbit for X-ray missions. We then use these background levels to simulate the observation of a typical galaxy cluster from its center out to 1.2 <jats:italic>R</jats:italic> <jats:sub>200</jats:sub> to probe the characterization capabilities of the instrument out to the outskirts. We find that without any background reduction, it is not possible to characterize the properties of the cluster in the outer regions. We also find no improvement in the observations when carried out during the solar maximum with respect to solar minimum conditions.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The classification of dark matter halos as isolated hosts or subhalos is critical for our understanding of structure formation and the galaxy–halo connection. Most commonly, subhalos are defined to reside inside a spherical overdensity boundary such as the virial radius. The resulting host–subhalo relations depend sensitively on the somewhat arbitrary overdensity threshold, but the impact of this dependence is rarely quantified. The recently proposed splashback radius tends to be larger and to include more subhalos than even the largest spherical overdensity boundaries. We systematically investigate the dependence of the subhalo fraction on the radius definition and show that it can vary by factors of unity between different spherical overdensity definitions. Using splashback radii can yet double the abundance of subhalos compared to the virial definition. We also quantify the abundance of flyby (or backsplash) halos, hosts that used to be subhalos in the past. We show that the majority of these objects are mislabeled satellites that are naturally classified as subhalos when we use the splashback radius. We show that the subhalo fraction can be understood as a universal function of only peak height and the slope of the linear power spectrum. We provide a simple fitting function that captures our simulation results to 20% accuracy across a wide range of halo masses, redshifts, and cosmologies. Finally, we demonstrate that splashback radii significantly change our understanding of satellite and flyby galaxies in the Local Group.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We present Spitzer/InfraRed Array Camera observations of dust formation from six extragalactic carbon-rich Wolf-Rayet (WC) binary candidates in low-metallicity (<jats:italic>Z</jats:italic> ≲ 0.65 <jats:italic>Z</jats:italic> <jats:sub>⊙</jats:sub>) environments using multiepoch mid-infrared (IR) imaging data from the SPitzer InfraRed Intensive Transients Survey (SPIRITS). Optical follow-up spectroscopy of SPIRITS 16ln, 19q, 16df, 18hb, and 14apu reveals emission features from C <jats:sc>iv</jats:sc> <jats:italic>λ</jats:italic>5801−12 and/or the C <jats:sc>iii–iv</jats:sc> <jats:italic>λ</jats:italic>4650/He <jats:sc>ii</jats:sc> <jats:italic>λ</jats:italic>4686 blend that are consistent with early-type WC stars. We identify SPIRITS 16ln as the variable mid-IR counterpart of the recently discovered colliding-wind WC4 + O binary candidate, N604-WRXc, located in the subsolar metallicity NGC 604 H <jats:sc>ii</jats:sc> region in M33. We interpret the mid-IR variability from SPIRITS 16ln as a dust-formation episode in an eccentric colliding-wind WC binary. SPIRITS 19q, 16df, 14apu, and 18hb exhibit absolute [3.6] magnitudes exceeding that of one of the most IR-luminous dust-forming WC systems known, WR 104 (<jats:italic>M</jats:italic> <jats:sub>[3.6]</jats:sub> ≲ −12.3). An analysis of dust formation in the mid-IR outburst from SPIRITS 19q reveals a high dust production rate of <jats:inline-formula> <jats:tex-math> <?CDATA ${\dot{M}}_{d}\gtrsim 2\times {10}^{-6}$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabd8cdieqn1.gif" xlink:type="simple" /> </jats:inline-formula> <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub> yr<jats:sup>−1</jats:sup>, which may therefore exceed that of the most efficient dust-forming WC systems known. We demonstrate that efficient dust formation is feasible from early-type WC binaries in the theoretical framework of colliding-wind binary dust formation if the systems host an O-type companion with high mass-loss rates (<jats:inline-formula> <jats:tex-math> <?CDATA $\dot{M}\gtrsim 1.6\times {10}^{-6}$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabd8cdieqn2.gif" xlink:type="simple" /> </jats:inline-formula> <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub> yr<jats:sup>−1</jats:sup>). This efficient dust formation from early-type WC binaries highlights their potential role as significant sources of dust in low-metallicity environments.</jats:p>