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<jats:title>Abstract</jats:title> <jats:p>We report on the hard X-ray burst and the first ∼100 days of NICER monitoring of the soft X-ray temporal and spectral evolution of the newly discovered magnetar Swift J1818.0−1607. The burst properties are typical of magnetars with a duration of <jats:italic>T</jats:italic> <jats:sub>90</jats:sub> = 10 ± 4 ms and a temperature of <jats:italic>kT</jats:italic> = 8.4 ± 0.7 keV. The 2–8 keV pulse shows a broad, single-peak profile with a pulse fraction increasing with time from 30% to 43%. The NICER observations reveal strong timing noise with <jats:inline-formula> <jats:tex-math> <?CDATA $\dot{\nu }$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabb3c9ieqn1.gif" xlink:type="simple" /> </jats:inline-formula> varying erratically by a factor of 10, with an average long-term spin-down rate of <jats:inline-formula> <jats:tex-math> <?CDATA $\dot{\nu }=(-2.48\pm 0.03)\times {10}^{-11}$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabb3c9ieqn2.gif" xlink:type="simple" /> </jats:inline-formula> s<jats:sup>−2</jats:sup>, implying an equatorial surface magnetic field of 2.5 × 10<jats:sup>14</jats:sup> G and a young characteristic age of ∼470 yr. We detect a large spin-up glitch at MJD 58928.56 followed by a candidate spin-down glitch at MJD 58934.81, with no accompanying flux enhancements. The persistent soft X-ray spectrum of Swift J1818.0−1607 can be modeled as an absorbed blackbody with a temperature of ∼1 keV. Its flux decayed by ∼60% while the modeled emitting area decreased by ∼30% over the NICER observing campaign. This decrease, coupled with the increase in the pulse fraction, points to a shrinking hot spot on the neutron star surface. Assuming a distance of 6.5 kpc, we measure a peak X-ray luminosity of 1.9 × 10<jats:sup>35</jats:sup> erg s<jats:sup>−1</jats:sup>, lower than its spin-down luminosity of 7.2 × 10<jats:sup>35</jats:sup> erg s<jats:sup>−1</jats:sup>. Its quiescent thermal luminosity is ≲1.7 × 10<jats:sup>34</jats:sup> erg s<jats:sup>−1</jats:sup>, lower than those of canonical young magnetars. We conclude that Swift J1818.0−1607 is an important link between regular magnetars and high-magnetic-field, rotation-powered pulsars.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We report on the spectral variability of the blazar 3C 279 in the optical to X-ray band between MJD 55100 and 58400 during which long-term radio variability was observed. We construct light curves and band spectra in each of the optical (2 × 10<jats:sup>14</jats:sup>–1.5 × 10<jats:sup>15</jats:sup> Hz) and X-ray (0.3–10 keV) bands, measure the spectral parameters (flux <jats:italic>F</jats:italic> and spectral index <jats:italic>α</jats:italic>), and investigate the correlation between <jats:italic>F</jats:italic> and <jats:italic>α</jats:italic> within and across the bands. We find that the correlation of the optical properties dramatically change after ∼MJD 55500 and the light curves show more frequent activity after ∼MJD 57700. We therefore divide the time interval into three “states” based on the correlation properties and source activity in the light curves, and analyze each of the three states separately. We find various correlations between the spectral parameters in the states and an intriguing 65 day delay of the optical emission with respect to the X-ray one in state 2 (MJD 55500–57700). We attempt to explain these findings using a one-zone synchro-Compton emission scenario.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Extreme-ultraviolet and X-ray emission from stellar coronae drives mass loss from exoplanet atmospheres, and ultraviolet emission from stellar chromospheres drives photochemistry in exoplanet atmospheres. Comparisons of the spectral energy distributions of host stars are, therefore, essential for understanding the evolution and habitability of exoplanets. The large number of stars observed with the MUSCLES, Mega-MUSCLES, and other recent Hubble Space Telescope observing programs has provided for the first time a large sample (79 stars) of reconstructed Ly<jats:italic>α</jats:italic> fluxes that we compare with X-ray fluxes to identify significant patterns in the relative emission from these two atmospheric regions as a function of stellar age and effective temperature. We find that as stars age on the main sequence, the emissions from their chromospheres and coronae follow a pattern in response to the amount of magnetic heating in these atmospheric layers. A single trend-line slope describes the pattern of X-ray versus Ly<jats:italic>α</jats:italic> emission for G and K dwarfs, but the different trend lines for M dwarf stars show that the Ly<jats:italic>α</jats:italic> fluxes of M stars are significantly smaller than those of warmer stars with the same X-ray flux. The X-ray and Ly<jats:italic>α</jats:italic> luminosities divided by the stellar bolometric luminosities show different patterns depending on stellar age. The <jats:italic>L</jats:italic>(Ly<jats:italic>α</jats:italic>)/<jats:italic>L</jats:italic>(bol) ratios increase smoothly to cooler stars of all ages, but the <jats:italic>L</jats:italic>(<jats:italic>X</jats:italic>)/<jats:italic>L</jats:italic>(bol) ratios show different trends. For older stars, the increase in coronal emission with decreasing <jats:inline-formula> <jats:tex-math> <?CDATA ${T}_{\mathrm{eff}}$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabb36fieqn1.gif" xlink:type="simple" /> </jats:inline-formula> is much steeper than that of chromospheric emission. We suggest a fundamental link between atmospheric properties and trend lines relating coronal and chromospheric heating,</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Foreshock transients are ion kinetic structures in the ion foreshock. Due to their dynamic pressure perturbations, they can disturb the bow shock and magnetosphere–ionosphere system. They can also accelerate particles contributing to shock acceleration. However, it is still unclear how exactly they form. Recent particle-in-cell simulations point out the important role of the electric field and Hall current in the formation process. To further examine this, we use data from the Magnetospheric Multiscale (MMS) mission to apply case studies on two small (1000–2000 km) foreshock transient events that just started to form. In event 1 where MMS was in a tetrahedral formation, we show that the current density configuration, which determined the magnetic field profile, was mainly driven by Hall currents generated by demagnetized foreshock ions. The resulting time variation of the magnetic field induced an electric field that drove cold plasma moving outward with magnetic field lines. In event 2 where MMS was in a string-of-pearls formation, we analyze the evolution of field and plasma parameters. We show that the magnetic flux and mass flux were transported outward from the core, resulting in the steepening of the boundary. The steepened boundary, which trapped more foreshock ions and caused stronger demagnetization of foreshock ions, nonlinearly further enhanced the Hall current. Based on our observations, we propose a physical formation process wherein the positive feedback of foreshock ions on the varying magnetic field caused by the foreshock ion Hall current enables an “instability” and the growth of the structure.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Velocity profiles of a line of <jats:inline-formula> <jats:tex-math> <?CDATA ${{\rm{H}}}_{3}^{+}$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabb1b5ieqn3.gif" xlink:type="simple" /> </jats:inline-formula> at 3.7 <jats:italic>μ</jats:italic>m produced in warm diffuse gas have been observed toward 18 stars in the Central Molecular Zone (CMZ) of the Galaxy. Their longitude–velocity diagram indicates that the gas is radially expanding within the CMZ at speeds up to a maximum of ∼150 km s<jats:sup>−1</jats:sup>. The current momentum and energy in the gas are ∼5 × 10<jats:sup>8</jats:sup> <jats:italic> M</jats:italic> <jats:sub>⊙</jats:sub> km s<jats:sup>−1</jats:sup> and ∼5 × 10<jats:sup>53</jats:sup> erg. The motion is similar to that of the Expanding Molecular Ring discovered in 1972 by Kaifu et al. and by Scoville. We propose that the expanding gas seen in <jats:inline-formula> <jats:tex-math> <?CDATA ${{\rm{H}}}_{3}^{+}$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabb1b5ieqn4.gif" xlink:type="simple" /> </jats:inline-formula> is part of the same phenomenon, despite differences in estimates of density, morphology, and degree of rotation. The outward motion suggests that one or more ejection events occurred near the center of the CMZ (0.5–1) × 10<jats:sup>6</jats:sup> yr ago, which may be related to creation of the recently observed microwave bubble. These observations revive the circular face-on view of the CMZ proposed in 1972, which fell out of favor after 1991 when Binney et al. proposed that a face-on view of the CMZ would show it to have an elliptical shape, with high eccentricity. While that model may apply on kiloparsec scales, we argue that it is incorrect to apply it to the much smaller CMZ. We discuss the fate of the expanding gas, which appears to be eventual infall into the center, leading to episodes of star formation and violent events associated with accretion onto Sgr A*.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We performed a numerical study for interpreting observations of plasma blobs occurring in the solar corona. Considering all of the previous studies and the presence of magnetic null points together with propagating magnetohydrodynamic waves in the solar corona, we guessed that the interaction of fast magnetoacoustic waves with null points could give rise to blobs under coronal conditions. The outcome of these interactions contributes to coronal jets and flares that directly affects us on Earth. The propagation of magnetoacoustic waves in the vicinity of a magnetic null point contributes to the high current density accumulation at the small scale around the magnetic null point, which has significant magnetic gradients. When nonlinearity becomes dominant, the variation of current density could result in instabilities and thus anomalous resistivity. Moreover, it is demonstrated that plasmoids with eruption events take place in the solar corona without considering the transition region. In our numerical simulation results, it is interesting that plasma blobs manifest themselves in many parameters, including current density, temperature, plasma density, flows, and magnetic fields, simultaneously and consistent with the generation of plasmoids. In this work, it is found that plasmoid instability is the reason for the plasma blobs and tiny blobs are produced by the tearing instability occurring in thin current sheets.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Recent studies of early-type galaxies have suggested that the initial mass function (IMF) slope is bottom-heavy; that is, they contain a larger fraction of low-mass stars than the Milky Way. However, measurements of the IMF remain challenging in unresolved galaxies because features in their observed spectra are sensitive to a number of factors, including the stellar age, metallicity, and elemental abundances, in addition to the IMF. In this paper, we use new high signal-to-noise IMACS (Magellan) spectra to study the elliptical shell galaxy NGC 3923 at optical (3700–6600 Å) and near-infrared (7900–8500 Å) wavelengths, as a function of radius. We have undertaken a number of independent approaches to better understand the uncertainties in our results. (1) We compare two different stellar population model libraries; (2) we undertake spectral index fitting as well as full spectral fitting; (3) we have performed simulations for which we a priori know the input IMF and that closely match our data; (4) we also investigate the effects of including a two-component rather than a single stellar population. We show that our results are sensitive to the assumptions we make and to the methods we use. In addition, we evaluate the accuracy and precision of our results based on simulated mock data. We find some indication (although assumption dependent) for a bottom-heavy IMF in the mass range 0.5–1.0 <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub>, while the IMF in the mass range 0.08–0.5 <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub> appears to be Milky Way–like and constant. Including near-infrared data in our analysis gives consistent results and improves the precision.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Redshifts used in current cosmological supernova samples are measured using two primary techniques, one based on well-measured host galaxy spectral lines and the other based on supernova-dominated spectra. Here, we construct an updated Pantheon catalog with revised redshifts, redshift sources, and estimated uncertainties for the entire sample to investigate whether these two techniques yield consistent results. The best-fit cosmological parameters using these two measurement techniques disagree, with a supernova-only sample producing Ω<jats:sub> <jats:italic>m</jats:italic> </jats:sub> 3.2<jats:italic>σ</jats:italic> higher and <jats:italic>H</jats:italic> <jats:sub>0</jats:sub> 2.5<jats:italic>σ</jats:italic> lower than a hostz-only sample, and we explore several possible sources of bias that could result from using the lower-precision supernova-dominated redshifts. In a pilot study, we show that using a host redshift-only subsample will generically produce lower Ω<jats:sub> <jats:italic>m</jats:italic> </jats:sub> and matter density Ω<jats:sub> <jats:italic>m</jats:italic> </jats:sub> <jats:italic>h</jats:italic> <jats:sup>2</jats:sup> and slightly higher <jats:italic>H</jats:italic> <jats:sub>0</jats:sub> than previous analysis which, for the Pantheon data set, could result in supernova and cosmic microwave background measurements agreeing on Ω<jats:sub> <jats:italic>m</jats:italic> </jats:sub> <jats:italic>h</jats:italic> <jats:sup>2</jats:sup> despite tension in <jats:italic>H</jats:italic> <jats:sub>0</jats:sub>. To obtain rigorous results, though, the Pantheon catalog should be improved by obtaining host spectra for supernovae that have faded, and future surveys should be designed to use host galaxy redshifts rather than lower-precision methods.</jats:p>