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<jats:title>Abstract</jats:title> <jats:p>The complex interplay of magnetohydrodynamics, gravity, and supersonic turbulence in the interstellar medium (ISM) introduces a non-Gaussian structure that can complicate a comparison between theory and observation. In this paper, we show that the wavelet scattering transform (WST), in combination with linear discriminant analysis (LDA), is sensitive to non-Gaussian structure in 2D ISM dust maps. WST-LDA classifies magnetohydrodynamic (MHD) turbulence simulations with up to a 97% true positive rate in our testbed of 8 simulations with varying sonic and Alfvénic Mach numbers. We present a side-by-side comparison with two other methods for non-Gaussian characterization, the reduced wavelet scattering transform (RWST) and the three-point correlation function (3PCF). We also demonstrate the 3D-WST-LDA, and apply it to the classification of density fields in position–position–velocity (PPV) space, where density correlations can be studied using velocity coherence as a proxy. WST-LDA is robust to common observational artifacts, such as striping and missing data, while also being sensitive enough to extract the net magnetic field direction for sub-Alfvénic turbulent density fields. We include a brief analysis of the effect of point-spread functions and image pixelization on 2D-WST-LDA applied to density fields, which informs the future goal of applying WST-LDA to 2D or 3D all-sky dust maps to extract hydrodynamic parameters of interest.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We present the first joint NuSTAR and NICER observations of the ultracompact X-ray binary (UCXB) 4U 1543−624 obtained in 2020 April. The source was at a luminosity of <jats:italic>L</jats:italic> <jats:sub>0.5−50 keV</jats:sub> = 4.9(<jats:italic>D</jats:italic>/7 kpc)<jats:sup>2</jats:sup> × 10<jats:sup>36</jats:sup> erg s<jats:sup>−1</jats:sup> and showed evidence of reflected emission in the form of an O <jats:sc>viii</jats:sc> line, Fe K line, and Compton hump within the spectrum. We used a full reflection model, known as <jats:sc>xillverCO</jats:sc>, that is tailored for the atypical abundances found in UCXBs, to account for the reflected emission. We tested the emission radii of the O and Fe line components and conclude that they originate from a common disk radius in the innermost region of the accretion disk (<jats:italic>R</jats:italic> <jats:sub>in</jats:sub> ≤ 1.07 <jats:italic>R</jats:italic> <jats:sub>ISCO</jats:sub>). Assuming that the compact accretor is a neutron star (NS) and the position of the inner disk is the Alfvén radius, we placed an upper limit on the magnetic field strength to be <jats:italic>B</jats:italic> ≤ 0.7(<jats:italic>D</jats:italic>/7 kpc) × 10<jats:sup>8</jats:sup> G at the poles. Given the lack of pulsations detected and position of <jats:italic>R</jats:italic> <jats:sub>in</jats:sub>, it was likely that a boundary layer region had formed between the NS surface and inner edge of the accretion disk with an extent of 1.2 km. This implies a maximum radius of the neutron star accretor of <jats:italic>R</jats:italic> <jats:sub>NS</jats:sub> ≤ 12.1 km when assuming a canonical NS mass of 1.4 <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub>.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>A common feature of electromagnetic emission from solar flares is the presence of intensity pulsations that vary as a function of time. Known as quasi-periodic pulsations (QPPs), these variations in flux appear to include periodic components and characteristic timescales. Here, we analyze a GOES M3.7 class flare exhibiting pronounced QPPs across a broad band of wavelengths using imaging and time series analysis. We identify QPPs in the time series of X-ray, low-frequency radio, and extreme ultraviolet (EUV) wavelengths using wavelet analysis, and localize the region of the flare site from which the QPPs originate via X-ray and EUV imaging. It was found that the pulsations within the 171 Å, 1600 Å, soft X-ray, and hard X-ray light curves yielded similar periods of <jats:inline-formula> <jats:tex-math> <?CDATA ${122}_{-22}^{+26}$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabe463ieqn1.gif" xlink:type="simple" /> </jats:inline-formula> s, <jats:inline-formula> <jats:tex-math> <?CDATA ${131}_{-27}^{+36}$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabe463ieqn2.gif" xlink:type="simple" /> </jats:inline-formula> s, <jats:inline-formula> <jats:tex-math> <?CDATA ${123}_{-26}^{+11}$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabe463ieqn3.gif" xlink:type="simple" /> </jats:inline-formula> s, and <jats:inline-formula> <jats:tex-math> <?CDATA ${137}_{-56}^{+49}$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabe463ieqn4.gif" xlink:type="simple" /> </jats:inline-formula> s, respectively, indicating a common progenitor. The low-frequency radio emission at 2.5 MHz contained a longer period of ∼231 s. Imaging analysis indicates that the location of the X-ray and EUV pulsations originates from a hard X-ray footpoint linked to a system of nearby open magnetic field lines. Our results suggest that intermittent particle acceleration, likely due to “bursty” magnetic reconnection, is responsible for the QPPs. The precipitating electrons accelerated toward the chromosphere produce the X-ray and EUV pulsations, while the escaping electrons result in low-frequency radio pulses in the form of type III radio bursts. The modulation of the reconnection process, resulting in episodic particle acceleration, explains the presence of these QPPs across the entire spatial range of flaring emission.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We perform a series of high-resolution 2D hydrodynamical simulations of equal-mass binary black holes (BBHs) embedded in active galactic nucleus (AGN) accretion disks to study whether these binaries can be driven to merger by the surrounding gas. We find that the gravitational softening adopted for the BBH has a profound impact on this result. When the softening is less than 10% of the binary separation, we show that, in agreement with recent simulations of isolated equal-mass binaries, prograde BBHs expand in time rather than contract. Eventually, however, the binary separation becomes large enough that the tidal force of the central AGN disrupts them. Only when the softening is relatively large do we find that prograde BBHs harden. We determine through detailed analysis of the binary torque, that this dichotomy is due to a loss of spiral structure in the circum-single disks orbiting each black hole when the softening is a significant fraction of the binary separation. Properly resolving these spirals—both with high resolution and small softening—results in a significant source of binary angular momentum. Only for retrograde BBHs do we find consistent hardening, regardless of softening, as these BBHs lack the important spiral structure in their circum-single disks. This suggests that the gas-driven inspiral of retrograde binaries can produce a population of compact BBHs in the gravitational-wave-emitting regime in AGN disks, which may contribute a large fraction to the observed BBH mergers.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The star formation activity of the host galaxies of active galactic nuclei provides valuable insights into the complex interconnections between black hole growth and galaxy evolution. A major obstacle arises from the difficulty of estimating accurate star formation rates (SFRs) in the presence of a strong active galactic nucleus. Analyzing the 1–500<jats:italic> μ</jats:italic>m spectral energy distributions and high-resolution mid-infrared spectra of low-redshift (<jats:italic>z</jats:italic> &lt; 0.5) Palomar–Green quasars with bolometric luminosity of ∼10<jats:sup>44.5</jats:sup>–10<jats:sup>47.5</jats:sup> erg s<jats:sup>−1</jats:sup>, we find, from comparison with an independent SFR indicator based on [Ne <jats:sc>II</jats:sc>] 12.81 <jats:italic>μ</jats:italic>m and [Ne <jats:sc>III</jats:sc>] 15.56<jats:italic> μ</jats:italic>m, that the torus-subtracted, total infrared (8–1000 <jats:italic>μ</jats:italic>m) emission yields robust SFRs in the range of ∼1–250 <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub> yr<jats:sup>−1</jats:sup>. Combined with available stellar mass estimates, the vast majority (∼75%–90%) of the quasars lie on or above the main sequence of local star-forming galaxies, including a significant fraction (∼50%–70%) that would qualify as starburst systems. This is further supported by the high star formation efficiencies derived from the gas content inferred from the dust masses. Inspection of high-resolution Hubble Space Telescope images reveals a wide diversity of morphological types, including a number of starbursting hosts that have not experienced significant recent dynamical perturbations. The origin of the high star formation efficiency is unknown.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Alignment of nonspherical grains with magnetic fields is an important problem, as it lays the foundation of probing magnetic fields with polarized dust thermal emissions. In this paper, we investigate the feasibility of magnetic alignment in protoplanetary disks (PPDs). We use an alignment condition that Larmor precession should be fast compared with the damping timescale. We first show that the Larmor precession timescale is some 3 orders of magnitude longer than the damping time for millimeter-sized grains under conditions typical of PPDs, making the magnetic alignment unlikely. The precession time can be shortened by superparamagnetic inclusions (SPIs), but the reduction factor strongly depends on the size of the SPI clusters, which we find is limited by the so-called “Néel’s relaxation process.” In particular, the size limit of SPIs is set by the so-called “anisotropic energy constant” of the SPI material, which describes the energy barrier needed to change the direction of the magnetic moment of an SPI. For the most common iron-bearing materials, we find maximum SPI sizes corresponding to a reduction factor of the Larmor precession timescale of order 10<jats:sup>3</jats:sup>. We also find that reaching this maximum reduction factor requires fine-tuning on the SPI sizes. Lastly, we illustrate the effects of the SPI size limits on magnetic alignment of dust grains with a simple disk model, and we conclude that it is unlikely for relatively large grains of order 100 <jats:italic>μ</jats:italic>m or more to be aligned with magnetic fields, even with SPIs.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We present the discovery that ASASSN-14ko is a periodically flaring active galactic nucleus at the center of the galaxy ESO 253-G003. At the time of its discovery by the All-Sky Automated Survey for Supernovae (ASAS-SN), it was classified as a supernova close to the nucleus. The subsequent 6 yr of <jats:italic>V</jats:italic>- and <jats:italic>g</jats:italic>-band ASAS-SN observations revealed that ASASSN-14ko has nuclear flares occurring at regular intervals. The 17 observed outbursts show evidence of a decreasing period over time, with a mean period of <jats:italic>P</jats:italic> <jats:sub>0</jats:sub> = 114.2 ± 0.4 days and a period derivative of <jats:inline-formula> <jats:tex-math> <?CDATA $\dot{P}=-0.0017\pm 0.0003$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabe38dieqn1.gif" xlink:type="simple" /> </jats:inline-formula>. The most recent outburst in 2020 May, which took place as predicted, exhibited spectroscopic changes during the rise and had a UV bright, blackbody spectral energy distribution similar to tidal disruption events (TDEs). The X-ray flux decreased by a factor of 4 at the beginning of the outburst and then returned to its quiescent flux after ∼8 days. The Transiting Exoplanet Survey Satellite observed an outburst during Sectors 4–6, revealing a rise time of 5.60 ± 0.05 days in the optical and a decline that is best fit with an exponential model. We discuss several possible scenarios to explain ASASSN-14ko’s periodic outbursts, but currently favor a repeated partial TDE. The next outbursts should peak in the optical on UT 2020 September 7.4±1.1 and UT 2020 December 26.5±1.4.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The analytical results of Chandrasekhar's semi-infinite diffuse reflection problem is crucial in the context of the stellar or planetary atmosphere. However, the atmospheric emission effect was not taken into account in this model, and the solutions are applicable only for a diffusely scattering atmosphere in the absence of emission. We extend the model of the semi-infinite diffuse reflection problem by including the effects of thermal emission <jats:italic>B</jats:italic>(<jats:italic>T</jats:italic>), and present how this affects Chandrasekhar's analytical end results. Hence, we aim to generalize Chandrasekhar’s model to provide a complete picture of this problem. We use Invariance Principle Method to find the radiative transfer equation accurate for diffuse reflection in the presence of <jats:italic>B</jats:italic>(<jats:italic>T</jats:italic>). Then we derive the modified scattering function <jats:italic>S</jats:italic>(<jats:italic>μ</jats:italic>, <jats:italic>ϕ</jats:italic>; <jats:italic>μ</jats:italic> <jats:sub>0</jats:sub>, <jats:italic>ϕ</jats:italic> <jats:sub>0</jats:sub>) for different kinds of phase functions. We find that the scattering function <jats:italic>S</jats:italic>(<jats:italic>μ</jats:italic>, <jats:italic>ϕ</jats:italic>; <jats:italic>μ</jats:italic> <jats:sub>0</jats:sub>, <jats:italic>ϕ</jats:italic> <jats:sub>0</jats:sub>) as well as the diffusely reflected specific intensity <jats:italic>I</jats:italic>(0, <jats:italic>μ</jats:italic>; <jats:italic>μ</jats:italic> <jats:sub>0</jats:sub>) for different phase functions are modified due to the emission <jats:bold> <jats:italic>B</jats:italic> </jats:bold>(<jats:bold> <jats:italic>T</jats:italic> </jats:bold>) from layer <jats:italic>τ</jats:italic> = 0. In both cases, <jats:bold> <jats:italic>B</jats:italic> </jats:bold>(<jats:bold> <jats:italic>T</jats:italic> </jats:bold>) is added to the results of the only scattering case derived by Chandrasekhar, with some multiplicative factors. Thus the diffusely reflected spectra will be enriched and carry the temperature information of the <jats:italic>τ</jats:italic> = 0 layer. As the effects are additive in nature, hence our model reduces to the sub-case of Chandrasekhar's scattering model in the case of <jats:bold> <jats:italic>B</jats:italic> </jats:bold>(<jats:bold> <jats:italic>T</jats:italic> </jats:bold>) = 0. We conclude that our generalized model provides more accurate results due to the inclusion of the thermal emission effect in Chandrasekhar's semi-infinite atmosphere problem.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Observations of the <jats:italic>γ</jats:italic>-ray emission around star clusters, isolated supernova remnants, and pulsar wind nebulae indicate that the cosmic-ray (CR) diffusion coefficient near acceleration sites can be suppressed by a large factor compared to the Galaxy average. We explore the effects of such local suppression of CR diffusion on galaxy evolution using simulations of isolated disk galaxies with regular and high gas fractions. Our results show that while CR propagation with constant diffusivity can make gaseous disks more stable by increasing the midplane pressure, large-scale CR pressure gradients cannot prevent local fragmentation when the disk is unstable. In contrast, when CR diffusivity is suppressed in star-forming regions, the accumulation of CRs in these regions results in strong local pressure gradients that prevent the formation of massive gaseous clumps. As a result, the distribution of dense gas and star formation changes qualitatively: a globally unstable gaseous disk does not violently fragment into massive star-forming clumps but maintains a regular grand-design spiral structure. This effect regulates star formation and disk structure and is qualitatively different from and complementary to the global role of CRs in vertical hydrostatic support of the gaseous disk and in driving galactic winds.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>An appearance or disappearance of quasi-periodic oscillations (QPOs) associated with the variation of X-ray flux can be used to decipher the accretion–ejection mechanism of black hole X-ray sources. We searched for and studied such rapid transitions in H1743-322 using archival data from the Rossi X-ray Timing Explorer and found eight such events, where a QPO vanishes suddenly along with the variation of X-ray flux. The appearance/disappearance of QPOs was associated with four events exhibiting type-B QPOs at ∼4.5 Hz: one was a type-A QPO at <jats:italic>ν</jats:italic> ∼ 3.5 Hz, and the other three were connected to type-C QPOs at ∼9.5 Hz. Spectral studies of the data revealed that the inner disk radius remained at the same location around 2–9 <jats:italic>r</jats:italic> <jats:sub> <jats:italic>g</jats:italic> </jats:sub>, depending on the model used, but power-law indices were varying, indicating that either a corona or a jet is responsible for the events. The probable ejection radii of coronas were estimated to be around 4.2–15.4 <jats:italic>r</jats:italic> <jats:sub> <jats:italic>g</jats:italic> </jats:sub> based on the plasma ejection model. Our X-ray and quasi-simultaneous radio correlation studies suggest that the type-B QPOs are probably related to the precession of a weak jet, though a small and weak corona is present at its base, and the type-C QPOs are associated with the base of a relatively strong jet, which is acting like a corona.</jats:p>