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<jats:title>Abstract</jats:title> <jats:p>Type Ia supernovae (SNe Ia) are more precise standardizable candles when measured in the near-infrared (NIR) than in the optical. With this motivation, from 2012 to 2017 we embarked on the RAISIN program with the Hubble Space Telescope (HST) to obtain rest-frame NIR light curves for a cosmologically distant sample of 37 SNe Ia (0.2 ≲ <jats:italic>z</jats:italic> ≲ 0.6) discovered by Pan-STARRS and the Dark Energy Survey. By comparing higher-<jats:italic>z</jats:italic> HST data with 42 SNe Ia at <jats:italic>z</jats:italic> &lt; 0.1 observed in the NIR by the Carnegie Supernova Project, we construct a Hubble diagram from NIR observations (with only time of maximum light and some selection cuts from optical photometry) to pursue a unique avenue to constrain the dark energy equation-of-state parameter, <jats:italic>w</jats:italic>. We analyze the dependence of the full set of Hubble residuals on the SN Ia host galaxy mass and find Hubble residual steps of size ∼0.06-0.1 mag with 1.5<jats:italic>σ</jats:italic>−2.5<jats:italic>σ</jats:italic> significance depending on the method and step location used. Combining our NIR sample with cosmic microwave background constraints, we find 1 + <jats:italic>w</jats:italic> = −0.17 ± 0.12 (statistical + systematic errors). The largest systematic errors are the redshift-dependent SN selection biases and the properties of the NIR mass step. We also use these data to measure <jats:italic>H</jats:italic> <jats:sub>0</jats:sub> = 75.9 ± 2.2 km s<jats:sup>−1</jats:sup> Mpc<jats:sup>−1</jats:sup> from stars with geometric distance calibration in the hosts of eight SNe Ia observed in the NIR versus <jats:italic>H</jats:italic> <jats:sub>0</jats:sub> = 71.2 ± 3.8 km s<jats:sup>−1</jats:sup> Mpc<jats:sup>−1</jats:sup> using an inverse distance ladder approach tied to Planck. Using optical data, we find 1 + <jats:italic>w</jats:italic> = −0.10 ± 0.09, and with optical and NIR data combined, we find 1 + <jats:italic>w</jats:italic> = −0.06 ± 0.07; these shifts of up to ∼0.11 in <jats:italic>w</jats:italic> could point to inconsistency in the optical versus NIR SN models. There will be many opportunities to improve this NIR measurement and better understand systematic uncertainties through larger low-<jats:italic>z</jats:italic> samples, new light-curve models, calibration improvements, and eventually by building high-<jats:italic>z</jats:italic> samples from the Roman Space Telescope.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We conduct a wide-band X-ray spectral analysis in the energy range of 1.5–100 keV to study the time evolution of the M7.6-class flare of 2016 July 23, with the Miniature X-ray Solar Spectrometer (MinXSS) CubeSat and the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) spacecraft. With the combination of MinXSS for soft X-rays and RHESSI for hard X-rays, a nonthermal component and three-temperature multithermal component—“cool” (<jats:italic>T</jats:italic> ≈ 3 MK), “hot” (<jats:italic>T</jats:italic> ≈ 15 MK), and “superhot” (<jats:italic>T</jats:italic> ≈ 30 MK)—were measured simultaneously. In addition, we successfully obtained the spectral evolution of the multithermal and nonthermal components with a 10 s cadence, which corresponds to the Alfvén timescale in the solar corona. We find that the emission measures of the cool and hot thermal components are drastically increasing more than hundreds of times and the superhot thermal component is gradually appearing after the peak of the nonthermal emission. We also study the microwave spectra obtained by the Nobeyama Radio Polarimeters, and we find that there is continuous gyrosynchrotron emission from mildly relativistic nonthermal electrons. In addition, we conducted a differential emission measure (DEM) analysis by using Atmospheric Imaging Assembly on board the Solar Dynamics Observatory and determined that the DEM of cool plasma increases within the flaring loop. We find that the cool and hot plasma components are associated with chromospheric evaporation. The superhot plasma component could be explained by the thermalization of the nonthermal electrons trapped in the flaring loop.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The most common form of magnetar activity is short X-ray bursts, with durations from milliseconds to seconds, and luminosities ranging from 10<jats:sup>36</jats:sup>–10<jats:sup>43</jats:sup> erg s<jats:sup>−1</jats:sup>. Recently, an X-ray burst from the galactic magnetar SGR 1935+2154 was detected to be coincident with two fast radio burst (FRB) like events from the same source, providing evidence that FRBs may be linked to magnetar bursts. Using fully 3D force-free electrodynamics simulations, we show that such magnetar bursts may be produced by Alfvén waves launched from localized magnetar quakes: a wave packet propagates to the outer magnetosphere, becomes nonlinear, and escapes the magnetosphere, forming an ultra-relativistic ejecta. The ejecta pushes open the magnetospheric field lines, creating current sheets behind it. Magnetic reconnection can happen at these current sheets, leading to plasma energization and X-ray emission. The angular size of the ejecta can be compact, ≲1 sr if the quake launching region is small, ≲0.01 sr at the stellar surface. We discuss implications for the FRBs and the coincident X-ray burst from SGR 1935+2154.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>FUV spectra of <jats:italic>η</jats:italic> Car, recorded across two decades with HST/STIS, document multiple changes in resonant lines caused by dissipating extinction in our line of sight. The FUV flux has increased nearly tenfold, which has led to increased ionization of the multiple shells within the Homunculus and photodestruction of H<jats:sub>2</jats:sub>. Comparison of observed resonant line profiles with CMFGEN model profiles allows separation of wind–wind collision and shell absorptions from the primary wind P Cygni profiles. The dissipating occulter preferentially obscured the central binary and interacting winds relative to the very extended primary wind. We are now able to monitor changes in the colliding winds with orbital phase. High-velocity transient absorptions occurred across the most recent periastron passage, indicating acceleration of the primary wind by the secondary wind, which leads to a downstream, high-velocity bow shock that is newly generated every orbital period. There is no evidence of changes in the properties of the binary winds.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We report here radio follow-up observations of the optical tidal disruption event (TDE) AT 2019azh. Previously reported X-ray observations of this TDE showed variability at early times and a dramatic increase in luminosity, by a factor of ∼10, about 8 months after optical discovery. The X-ray emission is mainly dominated by intermediate hard-soft X-rays and is exceptionally soft around the X-ray peak, which is <jats:italic>L</jats:italic> <jats:sub> <jats:italic>X</jats:italic> </jats:sub> ∼ 10<jats:sup>43</jats:sup> erg s<jats:sup>−1</jats:sup>. The high cadence 15.5 GHz observations reported here show an early rise in radio emission followed by an approximately constant light curve, and a late-time flare. This flare starts roughly at the time of the observed X-ray peak luminosity and reaches its peak about 110 days after the peak in the X-ray, and a year after optical discovery. The radio flare peaks at <jats:italic>ν</jats:italic> <jats:italic>L</jats:italic> <jats:sub> <jats:italic>ν</jats:italic> </jats:sub> ∼ 10<jats:sup>38</jats:sup> erg s<jats:sup>−1</jats:sup>, a factor of two higher than the emission preceding the flare. In light of the late-time radio and X-ray flares, and the X-ray spectral evolution, we speculate a possible transition in the accretion state of this TDE, similar to the observed behavior in black hole X-ray binaries. We compare the radio properties of AT 2019azh to other known TDEs, and focus on the similarities to the late-time radio flare of the TDE ASASSN-15oi.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The Legacy Survey of Space and Time (LSST) with the Vera Rubin Observatory will provide strong microlensing constraints on dark compact objects (DCOs) in our Galaxy. However, most current forecasts limit their analysis to Primordial Black Holes (PBH). It is unclear how well LSST microlensing will be able to constrain alternative models of DCOs with different Galactic spatial profile distributions at a subdominant DM fraction. In this work, we investigate how well LSST microlensing will constrain spherical or disk-like Galactic spatial distributions of DCOs, taking into account extended observing times, baryonic microlensing background, and sky distribution of LSST sources. These extensions represent significant improvements over existing microlensing forecasts in terms of both accuracy and versatility. We demonstrate this power by deriving new LSST sensitivity projections for DCOs in spherical and disk-like distributions. We forecast that LSST will be able to constrain one-solar-mass PBHs to have a DM fraction under 4.1 × 10<jats:sup>−4</jats:sup>. One-solar-mass objects in a dark disk distribution with the same dimensions as the Galactic disk will be constrained below 3.1 × 10<jats:sup>−4</jats:sup>, while those with <jats:italic>m</jats:italic> = 10<jats:sup>5</jats:sup> <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub> will be constrained to below 3.4 × 10<jats:sup>−5</jats:sup>. We find that compressed dark disks can be constrained up to a factor of ∼10 better than ones with identical dimensions to the baryonic disk. We also find that dark disks become less tightly constrained when they are tilted with respect to our own disk. This forecasting software is a versatile tool, capable of constraining any model of DCOs in the Milky Way with microlensing, <jats:ext-link xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="uri" xlink:href="https://github.com/HarrisonWinch96/DarkDisk_Microlensing" xlink:type="simple">and is made publicly available</jats:ext-link>.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We present Atacama Large Millimeter Array band 6/7 (1.3 mm/0.87 mm) and Very Large Array Ka-band (9 mm) observations toward NGC 2071 IR, an intermediate-mass star-forming region. We characterize the continuum and associated molecular line emission toward the most luminous protostars, i.e., IRS1 and IRS3, on ∼100 au (0.″2) scales. IRS1 is partly resolved in the millimeter and centimeter continuum, which shows a potential disk. IRS3 has a well-resolved disk appearance in the millimeter continuum and is further resolved into a close binary system separated by ∼40 au at 9 mm. Both sources exhibit clear velocity gradients across their disk major axes in multiple spectral lines including C<jats:sup>18</jats:sup>O, H<jats:sub>2</jats:sub>CO, SO, SO<jats:sub>2</jats:sub>, and complex organic molecules like CH<jats:sub>3</jats:sub>OH, <jats:sup>13</jats:sup>CH<jats:sub>3</jats:sub>OH, and CH<jats:sub>3</jats:sub>OCHO. We use an analytic method to fit the Keplerian rotation of the disks and give constraints on physical parameters with a Markov Chain Monte Carlo routine. The IRS3 binary system is estimated to have a total mass of 1.4–1.5 <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub>. IRS1 has a central mass of 3–5 <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub> based on both kinematic modeling and its spectral energy distribution, assuming that it is dominated by a single protostar. For both IRS1 and IRS3, the inferred ejection directions from different tracers, including radio jet, water maser, molecular outflow, and H<jats:sub>2</jats:sub> emission, are not always consistent, and for IRS1 these can be misaligned by ∼50°. IRS3 is better explained by a single precessing jet. A similar mechanism may be present in IRS1 as well but an unresolved multiple system in IRS1 is also possible.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Stars form within molecular clouds, so characterizing the physical states of molecular clouds is key to understanding the process of star formation. Cloud structure and stability are frequently assessed using metrics including the virial parameter and Larson scaling relationships between cloud radius, velocity dispersion, and surface density. Departures from the typical Galactic relationships between these quantities have been observed in low-metallicity environments. The amount of H<jats:sub>2</jats:sub> gas in cloud envelopes without corresponding CO emission is expected to be high under these conditions; therefore, this CO-dark gas could plausibly be responsible for the observed variations in cloud properties. We derive simple corrections that can be applied to empirical clump properties (mass, radius, velocity dispersion, surface density, and virial parameter) to account for CO-dark gas in clumps following power-law and Plummer mass density profiles. We find that CO-dark gas is not likely to be the cause of departures from Larson’s relationships in low-metallicity regions, but that virial parameters may be systematically overestimated. We demonstrate that correcting for CO-dark gas is critical for accurately comparing the dynamical state and evolution of molecular clouds across diverse environments.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Active galactic nuclei (AGN) can vary significantly in their rest-frame optical/UV continuum emission, and with strong associated changes in broad line emission, on much shorter timescales than predicted by standard models of accretion disks around supermassive black holes. Most such <jats:italic>changing-look</jats:italic> or <jats:italic>changing-state</jats:italic> AGN—and at higher luminosities, changing-look quasars (CLQs)—have been found via spectroscopic follow-up of known quasars showing strong photometric variability. The Time Domain Spectroscopic Survey of the Sloan Digital Sky Survey IV (SDSS-IV) includes repeat spectroscopy of large numbers of previously known quasars, many selected irrespective of photometric variability, and with spectral epochs separated by months to decades. Our visual examination of these repeat spectra for strong broad line variability yielded 61 newly discovered CLQ candidates. We quantitatively compare spectral epochs to measure changes in continuum and H<jats:italic>β</jats:italic> broad line emission, finding 19 CLQs, of which 15 are newly recognized. The parent sample includes only broad line quasars, so our study tends to find objects that have dimmed, i.e., turn-off CLQs. However, we nevertheless find four turn-on CLQs that meet our criteria, albeit with broad lines in both dim and bright states. We study the response of H<jats:italic>β</jats:italic> and Mg <jats:sc>ii</jats:sc> emission lines to continuum changes. The Eddington ratios of CLQs are low, and/or their H<jats:italic>β</jats:italic> broad line width is large relative to the overall quasar population. Repeat quasar spectroscopy in the upcoming SDSS-V black hole Mapper program will reveal significant numbers of CLQs, enhancing our understanding of the frequency and duty cycle of such strong variability, and the physics and dynamics of the phenomenon.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Wind spacecraft measurements are analyzed to obtain a current sheet (CS) normal width <jats:italic>d</jats:italic> <jats:sub>cs</jats:sub> distribution of 3374 confirmed magnetic reconnection exhausts in the ecliptic plane of the solar wind at 1 au. The <jats:italic>d</jats:italic> <jats:sub>cs</jats:sub> distribution displays a nearly exponential decay from a peak at <jats:italic>d</jats:italic> <jats:sub>cs</jats:sub> = 25 <jats:italic>d</jats:italic> <jats:sub> <jats:italic>i</jats:italic> </jats:sub> to a median at <jats:italic>d</jats:italic> <jats:sub>cs</jats:sub> = 85 <jats:italic>d</jats:italic> <jats:sub> <jats:italic>i</jats:italic> </jats:sub> and a 95th percentile at <jats:italic>d</jats:italic> <jats:sub>cs</jats:sub> = 905 <jats:italic>d</jats:italic> <jats:sub> <jats:italic>i</jats:italic> </jats:sub> with a maximum exhaust width at <jats:italic>d</jats:italic> <jats:sub>cs</jats:sub> = 8077 <jats:italic>d</jats:italic> <jats:sub> <jats:italic>i</jats:italic> </jats:sub>. A magnetic field <jats:italic>θ</jats:italic>-rotation angle distribution increases linearly from a relatively few high-shear events toward a broad peak at 35° &lt; <jats:italic>θ</jats:italic> &lt; 65°. The azimuthal <jats:italic>ϕ</jats:italic> angles of the CS normal directions of 430 thick <jats:italic>d</jats:italic> <jats:sub>cs</jats:sub> ≥ 500 <jats:italic>d</jats:italic> <jats:sub> <jats:italic>i</jats:italic> </jats:sub> exhausts are consistent with a dominant Parker-spiral magnetic field and a CS normal along the ortho-Parker direction. The CS normal orientations of 370 kinetic-scale <jats:italic>d</jats:italic> <jats:sub>cs</jats:sub> &lt; 25 <jats:italic>d</jats:italic> <jats:sub> <jats:italic>i</jats:italic> </jats:sub> exhausts are isotropic in contrast, and likely associated with Alfvénic solar wind turbulence. We propose that the alignment of exhaust normal directions from narrow <jats:italic>d</jats:italic> <jats:sub>cs</jats:sub> ∼ 15–25 <jats:italic>d</jats:italic> <jats:sub> <jats:italic>i</jats:italic> </jats:sub> widths to well beyond <jats:italic>d</jats:italic> <jats:sub>cs</jats:sub> ∼ 500 <jats:italic>d</jats:italic> <jats:sub> <jats:italic>i</jats:italic> </jats:sub> with an ortho-Parker azimuthal direction of a large-scale heliospheric current sheet (HCS) is a consequence of CS bifurcation and turbulence within the HCS exhaust that may trigger reconnection of the adjacent pair of bifurcated CSs. The proposed HCS-avalanche scenario suggests that the underlying large-scale parent HCS closer to the Sun evolves with heliocentric distance to fracture into many, more or less aligned, secondary CSs due to reconnection. A few wide exhaust-associated HCS-like CSs could represent a population of HCSs that failed to reconnect as frequently between the Sun and 1 au as other HCSs.</jats:p>