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<jats:title>Abstract</jats:title> <jats:p>Modern observational surveys allow us to probe the distribution function (DF) of the Keplerian orbital elements of wide binaries in the solar neighborhood. This DF exhibits nontrivial features, in particular a superthermal distribution of eccentricities for semimajor axes <jats:italic>a</jats:italic> ≳ 10<jats:sup>3</jats:sup> au. To interpret such features we must first understand how the binary DF is affected by dynamical perturbations, which typically fall into two classes: (i) stochastic kicks from passing stars, molecular clouds, etc. and (ii) secular torques from the Galactic tide. Here we isolate effect (ii) and calculate the time-asymptotic, phase-mixed DF for an ensemble of wide binaries under quadrupole-order tides. For binaries wide enough that the phase-mixing assumption is valid, none of our results depend explicitly on semimajor axes, masses, etc. We show that unless the initial DF is both isotropic in binary orientation <jats:italic>and</jats:italic> thermal in eccentricity, then the final phase-mixed DF is always both anisotropic and nonthermal. However, the only way to produce a superthermal DF under phase mixing is for the initial DF to itself be superthermal.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The hot intracluster medium (ICM) is thought to be quiescent with low observed velocity dispersions. Surface brightness fluctuations of the ICM also suggest that its turbulence is subsonic with a Kolmogorov scaling relation, indicating that the viscosity is suppressed and the kinetic energy cascades to small scales unscathed. However, recent observations of the cold gas filaments in galaxy clusters find that the scaling relations are steeper than that of the hot plasma, signaling kinetic energy losses and the presence of supersonic flows. In this work we use high-resolution simulations to explore the turbulent velocity structure of the cold filaments at the cores of galaxy clusters. Our results indicate that supersonic turbulent structures can be “frozen” in the cold gas that cools and fragments out of a fast, ∼10<jats:sup>7</jats:sup> K outflow driven by the central active galactic nucleus (AGN), when the radiative cooling time is shorter than the dynamical sound-crossing time. After the cold gas formation, however, the slope of the velocity structure function (VSF) flattens significantly over short, ∼10 Myr timescales. The lack of flattened VSF in observations of H<jats:italic>α</jats:italic> filaments indicates that the H<jats:italic>α</jats:italic>-emitting phase is short-lived for the cold gas in galaxy clusters. On the other hand, the ubiquity of supersonic turbulence revealed by observed filaments strongly suggests that supersonic outflows are an integral part of AGN–ICM interaction, and that AGN activity plays a crucial role at driving turbulence in galaxy clusters.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Primordial black holes of planetary masses captured by compact stars are widely studied to constrain their composition fraction of dark matter. Such a capture may lead to an inspiral process and be detected through gravitational-wave signals. In this Letter, we study the postcapture inspiral process by considering two different kinds of compact stars, i.e., strange stars and neutron stars. The dynamical equations are numerically solved, and the gravitational-wave emission is calculated. It is found that the Advanced LIGO can detect the inspiraling of a 10<jats:sup>−5</jats:sup> <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub> primordial black hole at a distance of 10 kpc, while a Jovian-mass case can even be detected at megaparsecs. Promisingly, the next generation of gravitational-wave detectors can detect cases of 10<jats:sup>−5</jats:sup> <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub> primordial black holes up to ∼1Mpc and Jovian-mass cases at several hundred megaparsecs. Moreover, the kilohertz gravitational-wave signal shows significant differences for strange stars and neutron stars, potentially making it a novel probe to the dense matter equation of state.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Tidally tilted pulsators (TTPs) are an intriguing new class of oscillating stars in binary systems; in such stars, the pulsation axis coincides with the line of apsides, or tidal axis, of the binary. All three TTPs discovered so far have been <jats:italic>δ</jats:italic> Scuti stars. In this Letter, we report the first conclusive discovery of tidally tilted pulsations in a subdwarf B (sdB) star. HD 265435 is an sdB–white dwarf binary with a 1.65 hr period that has been identified and characterized as the nearest potential Type Ia supernova progenitor. Using TESS 20 s cadence data from Sectors 44 and 45, we show that the pulsation axis of the sdB star has been tidally tilted into the orbital plane and aligned with the tidal axis of the binary. We identify 31 independent pulsation frequencies, 27 of which have between 1 and 7 sidebands separated by the orbital frequency (<jats:italic>ν</jats:italic> <jats:sub>orb</jats:sub>) or multiples thereof. Using the observed amplitude and phase variability due to tidal tilting, we assign <jats:italic>ℓ</jats:italic> and <jats:italic>m</jats:italic> values to most of the observed oscillation modes and use these mode identifications to generate preliminary asteroseismic constraints. Our work significantly expands our understanding of TTPs, as we now know that (i) they can be found in stars other than <jats:italic>δ</jats:italic> Scuti pulsators, especially highly evolved stars that have lost their H-rich envelopes, and (ii) tidally tilted pulsations can be used to probe the interiors of stars in very tight binaries.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>For the first time, Parker Solar Probe (PSP) observed the sub-Alfvénic solar wind where the solar wind bulk speed drops below the local Alfvén speed for an extended period of time. Here, we report on the turbulent properties of the sub-Alfvénic region. We analyze the turbulence correlation length and the energy transfer and compare the results with nearby super-Alfvénic regions. As the Alfvén speed is larger or comparable to the solar wind speed in the intervals studied, we use a modified Taylor’s hypothesis to account for wave propagation. We find that the wave propagation speed affects the analysis of the correlation lengths of the forward and backward propagating <jats:bold> <jats:italic>z</jats:italic> </jats:bold> <jats:sup>±</jats:sup> modes. In the sub-Alfvénic region, the correlation length of the <jats:bold> <jats:italic>z</jats:italic> </jats:bold> <jats:sup>−</jats:sup> mode is shorter than that of the outward propagating <jats:bold> <jats:italic>z</jats:italic> </jats:bold> <jats:sup>+</jats:sup> mode, although the correlation time of the <jats:bold> <jats:italic>z</jats:italic> </jats:bold> <jats:sup>−</jats:sup> mode is about 10 times larger than that of the <jats:bold> <jats:italic>z</jats:italic> </jats:bold> <jats:sup>+</jats:sup> mode. For the energy transfer, we use both incompressible and compressible formulations to calculate the energy flux based on third-order structure functions. The incompressible energy cascade rates for the forward and backward propagating modes are computed separately using the modified Taylor’s hypothesis. The averaged compressible cascade rate is higher in the sub-Alfvénic interval than the nearby downstream super-Alfvénic region, which may be due to the lower fluctuation amplitude in the latter super-Alfvénic interval. Longer incursions of the PSP in the sub-Alfvénic winds in the future will give us better statistics.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Some repeating fast radio burst (FRB) sources exhibit complex polarization behaviors, including frequency-dependent depolarization, variation of rotation measure (RM), and oscillating spectral structures of polarized components. Very recently, Feng et al. reported that active repeaters exhibit conspicuous frequency-dependent depolarization and a strong correlation between RM scatter (<jats:italic>σ</jats:italic> <jats:sub>RM</jats:sub>) and the temporal scattering time (<jats:italic>τ</jats:italic> <jats:sub>s</jats:sub>), <jats:inline-formula> <jats:tex-math> <?CDATA ${\sigma }_{\mathrm{RM}}\propto {\tau }_{{\rm{s}}}^{1.0\pm 0.2}$?> </jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mrow> <mml:mi>σ</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>RM</mml:mi> </mml:mrow> </mml:msub> <mml:mo>∝</mml:mo> <mml:msubsup> <mml:mrow> <mml:mi>τ</mml:mi> </mml:mrow> <mml:mrow> <mml:mi mathvariant="normal">s</mml:mi> </mml:mrow> <mml:mrow> <mml:mn>1.0</mml:mn> <mml:mo>±</mml:mo> <mml:mn>0.2</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjlac5f46ieqn1.gif" xlink:type="simple" /> </jats:inline-formula>, both of which can be well described by multipath propagation through a magnetized inhomogeneous plasma screen. This observation strongly suggests that the temporal scattering and RM scatter originate from the same region. Besides, a particular finding of note in Feng et al. is that the FRBs with compact persistent radio sources (PRSs) tend to have extreme <jats:italic>σ</jats:italic> <jats:sub>RM</jats:sub>. In this work, we focus on some theoretical predictions of the relations among temporal scattering, depolarization by RM scatter, and PRSs contributed by the magnetized plasma environment close to a repeating FRB source. The behaviors of the RM scatter imply that the magnetized plasma environment is consistent with a supernova remnant or pulsar wind nebula, and the predicted <jats:italic>σ</jats:italic> <jats:sub>RM</jats:sub>–<jats:italic>τ</jats:italic> <jats:sub>s</jats:sub> relation is <jats:inline-formula> <jats:tex-math> <?CDATA ${\sigma }_{\mathrm{RM}}\propto {\tau }_{{\rm{s}}}^{(0.54-0.83)}$?> </jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mrow> <mml:mi>σ</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>RM</mml:mi> </mml:mrow> </mml:msub> <mml:mo>∝</mml:mo> <mml:msubsup> <mml:mrow> <mml:mi>τ</mml:mi> </mml:mrow> <mml:mrow> <mml:mi mathvariant="normal">s</mml:mi> </mml:mrow> <mml:mrow> <mml:mo stretchy="false">(</mml:mo> <mml:mn>0.54</mml:mn> <mml:mo>−</mml:mo> <mml:mn>0.83</mml:mn> <mml:mo stretchy="false">)</mml:mo> </mml:mrow> </mml:msubsup> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjlac5f46ieqn2.gif" xlink:type="simple" /> </jats:inline-formula> for different astrophysical scenarios. We further make a general discussion of PRSs that does not depend on specific astrophysical scenarios. We show that the specific luminosity of a PRS should have a positive correlation with the RM contributed by the plasma screen. This is consistent with the observations of FRB 121102 and FRB 190520B.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We show that a small but measurable shift in the eclipse midpoint time of eclipsing binary (EBs) stars of ∼0.1 s over a decade baseline can be used to directly measure the Galactic acceleration of stars in the Milky Way at ∼kiloparsec distances from the Sun. We consider contributions to the period drift rate from dynamical mechanisms other than the Galaxy’s gravitational field and show that the Galactic acceleration can be reliably measured using a sample of Kepler EBs with orbital and stellar parameters from the literature. The contribution from tidal decay we estimate here is an upper limit assuming the stars are not tidally synchronized. We find there are about 200 detached EBs that have estimated timing precision better than 0.5 s, and for which other dynamical effects are subdominant to the Galactic signal. We illustrate the method with a prototypical, precisely timed EB using an archival Kepler light curve and a modern synthetic HST light curve (which provides a decade baseline). This novel method establishes a realistic possibility to constrain dark matter substructure and the Galactic potential using eclipse timing to measure Galactic accelerations, along with other emerging new methods, including pulsar timing and extreme-precision radial velocity observations. This acceleration signal grows quadratically with time. Therefore, given baselines established in the near future for distant EBs, we can expect to measure the period drift in the future with space missions like JWST and the Roman Space Telescope.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>GSN 069 is a recently discovered quasi-periodic eruption (QPE) source recurring about every 9 hr. The mechanism for the QPEs of GSN 069 is still unclear. In this work, a disk instability model is constructed to explain GSN 069 based on Pan et al. (PLC21), where the authors proposed a toy model for the repeating changing-look active galactic nuclei. We improve the work of PLC21 by including a nonzero viscous torque condition on the inner boundary of the disk and adopting a general form for the viscous stress torque in the Kerr metric. It is found that the 0.4–2 keV light curves, the light curves at different energy bands, and the phase-resolved X-ray spectrum of GSN 069 can all be qualitatively reproduced by our model. Furthermore, the profiles of light curves in QPEs can be significantly changed by the parameter <jats:italic>μ</jats:italic> in the viscous torque equation, which implies that our model may also be applied to other QPEs.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Binary black hole (BBH) mergers, particularly those with component masses in the pair-instability gap, may be produced by hierarchical mergers in the disks surrounding Active Galactic Nuclei (AGNs). While the interaction of an embedded BBH with an AGN disk is typically assumed to facilitate a merger, recent high-resolution hydrodynamical simulations challenge this assumption. However, these simulations often have simplified treatments for gas thermodynamics. In this work, we model the possible consequence of various feedback from an embedded BBH with a simple model that maintains an enhanced temperature profile around each binary component. We show that when the minidisks around each BH become hotter than the background by a factor of three, the BBH orbital evolution switches from expansion to contraction. By analyzing the gravitational torque profile, we find that this change in direction is driven by a weakening of the minidisk spirals and their positive torque on the binary. Our results highlight the important role of thermodynamics around BBHs and its effect on their orbital evolution, suggesting that AGN disks could be efficient factories for BBH mergers.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The epochs of cosmic dawn and reionization present promising avenues for understanding the role of dark matter (DM) in our cosmos. The first galaxies that populated the universe during these eras resided in DM halos that were much less massive than their counterparts today. Consequently, observations of such galaxies can provide us with a handle on the clustering of DM in an otherwise currently inaccessible regime. In this work, we use high-redshift UV galaxy luminosity function (UV LF) data from the Hubble Space Telescope to study the clustering properties of DM at small scales. In particular, we present new measurements of the matter power spectrum at wavenumbers 0.5 Mpc<jats:sup>−1</jats:sup> &lt; <jats:italic>k</jats:italic> &lt; 10 Mpc<jats:sup>−1</jats:sup> to roughly 30% precision, obtained after marginalizing over the unknown astrophysics. These new data points cover the uncharted redshift range 4 ≤ <jats:italic>z</jats:italic> ≤ 10 and encompass scales beyond those probed by cosmic microwave background and large-scale structure observations. This work establishes the UV LF as a powerful tool to probe the nature of DM in a different regime than other cosmological and astrophysical data sets.</jats:p>