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<jats:title>Abstract</jats:title> <jats:p>We present new measurements of the UV spectral slope <jats:italic>β</jats:italic> for galaxies at <jats:italic>z</jats:italic> = 6−9 in the Frontier Field cluster MACS J0416.1–2403 and its parallel field to an unprecedented level of low stellar mass. We fit synthetic stellar population models to the observed spectral energy distribution and calculate <jats:italic>β</jats:italic> by fitting a power law to the best-fit spectrum. With this method, we report the derivation of rest-frame UV colors of galaxies for the Frontier Fields program extending out to <jats:italic>z</jats:italic> = 9, probing magnitudes as faint as <jats:italic>M</jats:italic> <jats:sub>UV</jats:sub> = −13.5 at <jats:italic>z</jats:italic> = 6. We find no significant correlation between <jats:italic>β</jats:italic> and rest-frame UV magnitude <jats:italic>M</jats:italic> <jats:sub>1500</jats:sub> all redshifts, but we do find a strong correlation between <jats:italic>β</jats:italic> and stellar mass, with lower-mass galaxies exhibiting bluer UV slopes. At <jats:italic>z</jats:italic> = 7, the bluest median value of our sample is redder than the previously reported values in the literature, whereas at <jats:italic>z</jats:italic> = 9, our bluest data point has a median value of <jats:inline-formula> <jats:tex-math> <?CDATA $\beta =-{2.63}_{-0.43}^{+0.52}$?> </jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>β</mml:mi> <mml:mo>=</mml:mo> <mml:mo>−</mml:mo> <mml:msubsup> <mml:mrow> <mml:mn>2.63</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>0.43</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>0.52</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabdd3fieqn1.gif" xlink:type="simple" /> </jats:inline-formula>. Thus, we find no evidence for extreme stellar populations at <jats:italic>z</jats:italic> &gt; 6. We also observe a strong correlation between <jats:italic>β</jats:italic> and star formation rate (SFR), such that galaxies with low SFRs exhibit bluer slopes. Additionally, there exists a star formation main sequence up to <jats:italic>z</jats:italic> = 9 with SFRs correlating with stellar mass. All of these relations show that <jats:italic>β</jats:italic> values correlate with a process that drives both the overall SFR and stellar mass assembly. Furthermore, we observe no trend between <jats:italic>β</jats:italic> and specific SFR, suggesting that <jats:italic>β</jats:italic> is getting set by a global process driven by the scale of the galaxy.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We present a study of the optical and near-infrared (NIR) spectra of SN 2013ai along with its light curves. These data range from discovery until 380 days after explosion. SN 2013ai is a fast declining Type II supernova (SN II) with an unusually long rise time, 18.9 ± 2.7 days in the <jats:italic>V</jats:italic>-band, and a bright <jats:italic>V</jats:italic>-band peak absolute magnitude of −18.7 ± 0.06 mag. The spectra are dominated by hydrogen features in the optical and NIR. The spectral features of SN 2013ai are unique in their expansion velocities, which, when compared to large samples of SNe II, are more than 1,000 km s<jats:sup>−1</jats:sup> faster at 50 days past explosion. In addition, the long rise time of the light curve more closely resembles SNe IIb rather than SNe II. If SN 2013ai is coeval with a nearby compact cluster, we infer a progenitor zero-age main-sequence mass of ∼17 <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub>. After performing light-curve modeling, we find that SN 2013ai could be the result of the explosion of a star with little hydrogen mass, a large amount of synthesized <jats:sup>56</jats:sup>Ni, 0.3–0.4 <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub>, and an explosion energy of 2.5–3.0 × 10<jats:sup>51</jats:sup> erg. The density structure and expansion velocities of SN 2013ai are similar to those of the prototypical SN IIb, SN 1993J. However, SN 2013ai shows no strong helium features in the optical, likely due to the presence of a dense core that prevents the majority of <jats:italic>γ</jats:italic>-rays from escaping to excite helium. Our analysis suggests that SN 2013ai could be a link between SNe II and stripped-envelope SNe.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We present the analysis of X-ray observations of the black hole binary 4U 1630−47 using relativistic reflection spectroscopy. We use archival data from the Rossi X-ray Timing Explorer, Neil Gehrels Swift Observatory, and Nuclear Spectroscopic Telescope Array observatories, taken during different outbursts of the source between 1998 and 2015. Our modeling includes two relatively new advances in modern reflection codes: high-density disks, and returning thermal disk radiation. Accretion disks around stellar-mass black holes are expected to have densities well above the standard value assumed in traditional reflection models (i.e., <jats:italic>n</jats:italic> <jats:sub>e</jats:sub> ∼ 10<jats:sup>15</jats:sup> cm<jats:sup>−3</jats:sup>). New high-density reflection models have important implications in the determination of disk truncation (i.e., the disk inner radius). This is because one must retain self-consistency in the irradiating flux and corresponding disk ionization state, which is a function of disk density and system geometry. We find that the disk density is <jats:italic>n</jats:italic> <jats:sub>e</jats:sub> ≥ 10<jats:sup>20</jats:sup> cm<jats:sup>−3</jats:sup> across all spectral states. This density, combined with our constraints on the ionization state of the material, implies an irradiating flux impinging on the disk that is consistent with the expected theoretical estimates. Returning thermal disk radiation—the fraction of disk photons that bend back to the disk producing additional reflection components—is expected predominantly in the soft state. We show that returning radiation models indeed provide a better fit to the soft-state data, reinforcing previous results that show that in the soft state, the irradiating continuum may be blackbody emission from the disk itself.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We present nearly self-consistent stellar-halo models of the Fornax dwarf spheroidal galaxy associated with the Milky Way galaxy. Such galaxies are dominated by dark matter and have almost no gas in the system. Therefore, they are excellent objects for <jats:italic>N</jats:italic>-body modeling that takes into account visible and dark matter halo components. In order to model the dark matter halo inferred from the analysis of the measured velocities of Fornax's stars, we constructed several self-consistent quasi-equilibrium models based on two source code sets. One of them (GalactICS Software, NEMO) deals with the self-consistent distribution function modeling that depends on the energy <jats:italic>E</jats:italic> and vertical component of the angular momentum <jats:italic>L</jats:italic> <jats:sub> <jats:italic>z</jats:italic> </jats:sub>. The other is included in the AGAMA framework and is based on Schwarzschild's calculation of orbits. It can reproduce the nonspherical self-consistent structure of Fornax as the weighted sum of orbit contributions to the galactic density even though the inferred dark halo parameters come from Jeans analysis, which does not require that any distribution functions be positive. To guess the parameters that make the <jats:italic>N</jats:italic>-body models close to the visible object, we use the stellar dark matter model of the Fornax galaxy based on hydrodynamic axisymmetric Jeans equations taking into account the velocity anisotropy parameter. Then, we studied the evolution of the models by performing <jats:italic>N</jats:italic>-body simulations with the falcON code in order to test their stability. The variability of the model parameters over time was obtained during simulations. The AGAMA models show the best agreement of the resulting velocity dispersion profiles with the observed data.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>While magnetized turbulence is ubiquitous in many astrophysical and terrestrial systems, our understanding of even the simplest physical description of this phenomena, ideal magnetohydrodynamic (MHD) turbulence, remains substantially incomplete. In this work, we highlight the shortcomings of existing theoretical and phenomenological descriptions of MHD turbulence that focus on the joint (kinetic and magnetic) energy fluxes and spectra by demonstrating that treating these quantities separately enables fundamental insights into the dynamics of MHD turbulence. This is accomplished through the analysis of the scale-wise energy transfer over time within an implicit large eddy simulation of subsonic, super-Alfvénic MHD turbulence. Our key finding is that the kinetic energy spectrum develops a scaling of approximately <jats:italic>k</jats:italic> <jats:sup>−4/3</jats:sup> in the stationary regime as magnetic tension mediates large-scale kinetic to magnetic energy conversion and significantly suppresses the kinetic energy cascade. This motivates a reevaluation of existing MHD turbulence theories with respect to a more differentiated modeling of the energy fluxes.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The observed emission lines of Be stars originate from a circumstellar Keplerian disk, which is generally well explained by the viscous decretion disk model. In an earlier work we performed modeling of the full light curve of the bright Be star <jats:italic>ω</jats:italic> CMa with the 1D time-dependent hydrodynamic code <jats:monospace>SINGLEBE</jats:monospace> and the Monte Carlo radiative transfer code <jats:monospace>HDUST</jats:monospace>. We used a <jats:italic>V</jats:italic>-band light curve that probes the inner disk through four disk formation and dissipation cycles. This new study compares predictions of the same set of model parameters with time-resolved photometry from the near-UV through the mid-IR, comprehensive series of optical spectra, and optical broadband polarimetry, which overall represent a larger volume of the disk. Qualitatively, the models reproduce the trends in the observed data due to the growth and decay of the disk. However, quantitative differences exist, e.g., an overprediction of flux increasing with wavelength, too slow decreases in Balmer emission line strength during disk dissipation, and a discrepancy between the range of polarimetric data and the model. We find that a larger value of the viscosity parameter alone or a disk truncated by a companion star reduces these discrepancies by increasing the dissipation rate in the outer regions of the disk.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We assemble a catalog of Magellanic Cloud red giants from Data Release 2 of the Gaia mission and, utilizing machine-learning methods, obtain photometric metallicity estimates for them. In doing so, we are able to chemically map the entirety of the Magellanic System at once. Our maps reveal a plethora of substructure within our red giant sample, with the Large Magellanic Cloud (LMC) bar and spiral arm being readily apparent. We uncover a curious spiral-like feature in the southern portion of the LMC disk, hosting relatively metal-rich giants and likely a by-product of historic encounters with the Small Magellanic Cloud (SMC). Modeling the LMC as an inclined thin disk, we find a shallow metallicity gradient of −0.048 ± 0.001 dex kpc<jats:sup>−1</jats:sup> out to ∼12° from the center of the dwarf. We see evidence that the SMC is disrupting, with its outer isodensity contours displaying the S-shape symptomatic of tidal stripping. On studying the proper motions of the SMC giants, we observe a population of them being violently dragged toward the larger Cloud. The perturbed stars predominantly lie in front of the SMC, and we interpret that they exist as a tidal tail of the dwarf, trailing in its motion and undergoing severe disruption from the LMC. We find the metallicity structure in the Magellanic Bridge region to be complex, with evidence for a composite nature in this stellar population, consisting of both LMC and SMC debris.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We present spatially resolved spectroscopy from the Keck Cosmic Web Imager (KCWI) of a star-forming galaxy at <jats:italic>z</jats:italic> = 0.6942, which shows emission from the Mg <jats:sc>ii</jats:sc> <jats:italic>λ</jats:italic> <jats:italic>λ</jats:italic>2796, 2803 doublet in the circumgalactic medium (CGM) extending ∼37 kpc at 3<jats:italic>σ</jats:italic> significance in individual spaxels (1<jats:italic>σ</jats:italic> detection limit 4.8 × 10<jats:sup>−19</jats:sup> erg s<jats:sup>−1</jats:sup> cm<jats:sup>−2</jats:sup> arcsec<jats:sup>−2</jats:sup>). The target galaxy, selected from a near-UV spectroscopic survey of Mg <jats:sc>ii</jats:sc> line profiles at 0.3 &lt; <jats:italic>z</jats:italic> &lt; 1.4, has a stellar mass log (<jats:italic>M</jats:italic> <jats:sub>*</jats:sub>/<jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub>) = 9.9, a star formation rate of 50 <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub> yr<jats:sup>−1</jats:sup>, and a morphology indicative of a merger. After deconvolution with the seeing, we obtain 5<jats:italic>σ</jats:italic> detections of Mg <jats:sc>ii</jats:sc> line emission extending for ∼31 kpc measured in 7-spaxel (1.1 arcsec<jats:sup>2</jats:sup>) apertures. Spaxels covering the galaxy stellar regions show clear P Cygni−like emission/absorption profiles, with the blueshifted absorption extending to relative velocities of <jats:italic>v</jats:italic> = −800 km s<jats:sup>−1</jats:sup>; however, the P Cygni profiles give way to pure emission at large radii from the central galaxy. We have performed 3D radiative transfer modeling to infer the geometry and velocity and density profiles of the outflowing gas. Our observations are most consistent with an isotropic outflow rather than biconical wind models with half-opening angles <jats:italic>ϕ</jats:italic> ≤ 80°. Furthermore, our modeling suggests that a wind velocity profile that decreases with radius is necessary to reproduce the velocity widths and strengths of Mg <jats:sc>ii</jats:sc> line emission profiles at large circumgalactic radii. The extent of the Mg <jats:sc>ii</jats:sc> emission we measure directly is further corroborated by our modeling, where we rule out outflow models with extent &lt;30 kpc.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Several explosions of type Ia supernovae (SNe Ia) have been found to exhibit deviations from spherical symmetry upon closer inspection. Examples are the gamma-ray lines from SN 2014J as measured by INTEGRAL/SPI and morphology information from radioactive isotopes in older remnants such as Tycho. A systematic study of the effects of parameters such as ignition geometry and burning morphology in SNe Ia is still missing. We use a two-dimensional hydrodynamics code with postprocessing nucleosynthesis and simulate the double detonations in a sub-Chandrasekhar-mass carbon–oxygen white dwarf starting from the nuclear runaway in the accumulated He envelope toward disruption of the white dwarf. We explore potential variety through four triggering scenarios that sample main asymmetry drivers. We further investigate their global effects on the aspherical structure of the ejecta based on individual elements. We apply the results to the well-observed SN 2014J and other recently observed SN remnants in order to illustrate how these new observational data, together with other observed quantities, help to constrain the explosion and progenitors of SNe Ia.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Accreting X-ray pulsars (XRPs) undergo luminous X-ray outbursts during which the spectral and timing behavior of the neutron star can be studied in detail. We analyze a NuSTAR observation of the XRP XTE J1858+034 during its outburst in 2019. The spectrum is fit with a phenomenological, a semiempirical, and a physical spectral model. A candidate cyclotron line is found at 48 keV, implying a magnetic field of 5.4 × 10<jats:sup>12</jats:sup> G at the site of emission. This is also supported by the physical best-fit model. We propose an orbital period of about 81 days based on the visual inspection of the X-ray outburst recurrence time. Based on Fermi/GBM data, the standard disk accretion-torque theory allowed us to infer a distance of 10.9 ± 1.0 kpc. Pulse profiles are single-peaked and show a pulsed fraction that is strongly energy-dependent at least up to 40 keV.</jats:p>