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<jats:title>Abstract</jats:title> <jats:p>Recent Parker Solar Probe (PSP) observations of inner heliospheric plasma have shown an abundant presence of Alfvénic polarity reversal of the magnetic field, known as “switchbacks.” While their origin is still debated, their role in driving the solar wind turbulence has been suggested through analysis of the spectral properties of magnetic fluctuations. Here, we provide a complementary assessment of their role in the turbulent cascade. The validation of the third-order linear scaling of velocity and magnetic fluctuations in intervals characterized by a high occurrence of switchbacks suggests that, irrespective of their local or remote origin, these structures are actively embedded in the turbulent cascade, at least at the radial distances sampled by PSP during its first perihelion. The stronger positive energy transfer rate observed in periods with a predominance of switchbacks indicates that they act as a mechanism injecting additional energy in the turbulence cascade.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Solar variability occurs over a broad range of spatial and temporal scales, from the Sun’s brightening over its lifetime to the fluctuations commonly associated with magnetic activity over minutes to years. The latter activity includes most prominently the 11 yr sunspot solar cycle and its modulations. Space weather events, in the form of solar flares, solar energetic particles, coronal mass ejections, and geomagnetic storms, have long been known to approximately follow the solar cycle occurring more frequently at solar maximum than solar minimum. These events can significantly impact our advanced technologies and critical infrastructures, making the prediction for the strength of future solar cycles particularly important. Several methods have been proposed to predict the strength of the next solar cycle, cycle 25, with results that are generally not always consistent. Most of these methods are based on the international sunspot number time series, or other indicators of solar activity. We present here a new approach that uses more than 100 yr of measured fractional areas of the visible solar disk covered by sunspots and plages and an empirical relationship for each of these two indices of solar activity in even–odd cycles. We anticipate that cycle 25 will peak in 2024 and will last for about 12 yr, slightly longer than cycle 24. We also found that, in terms of sunspot and plage areas coverage, the amplitude of cycle 25 will be substantially similar or slightly higher than cycle 24.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We report on an unusually bright observation of PSR J1720−0533 using the Five-hundred-meter Aperture Spherical radio Telescope (FAST). The pulsar is in a black widow system that was discovered by the Commensal Radio Astronomy FAST Survey (CRAFTS). By coincidence, a bright scintillation maximum was simultaneous with the eclipse in our observation, which allowed for precise measurements of flux density variations, as well as dispersion measure (DM) and polarization. We found that there are quasi-periodic pulse emission variations with a modulation period of ∼22 s during the ingress of the eclipse, which could be caused by plasma lensing. No such periodic modulation was found during the egress of the eclipse. The linear polarization of the pulsar disappears before the eclipse, even before there is a visually obvious change in DM. We also found that the pulse scattering may play an important role in the eclipse of PSR J1720−0533.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>GW200115 was the second merger of a black hole and a neutron star confidently detected through gravitational waves. Inference on the signal allows for a large black hole spin misaligned with the orbital angular momentum, but shows little support for aligned spin values. We show that this is a natural consequence of measuring the parameters of a black hole–neutron star binary with nonspinning components while assuming the priors used in the LIGO–Virgo–KAGRA analysis. We suggest that, a priori, a nonspinning binary is more consistent with current astrophysical understanding.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We discuss the prospects for identifying the nearest isolated black holes (IBHs) in our Galaxy. IBHs accreting gas from the interstellar medium likely form magnetically arrested disks (MADs). We show that thermal electrons in the MADs emit optical signals through the thermal synchrotron process while nonthermal electrons accelerated via magnetic reconnections emit a flat-spectrum synchrotron radiation in the X-ray to MeV gamma-ray ranges. The Gaia catalog will include at most a thousand IBHs within ≲1 kpc that are distributed on and around the cooling sequence of white dwarfs (WDs) in the Hertzsprung–Russell diagram. These IBH candidates should also be detected by eROSITA, with which they can be distinguished from isolated WDs and neutron stars. Follow-up observations with hard X-ray and MeV gamma-ray satellites will be useful to unambiguously identify IBHs.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We report millimeter-VLBI results of low-luminosity active galactic nuclei (M84 and M87) up to 88 GHz with source-frequency phase-referencing observations. We detected the weak VLBI core and obtained the first image of M84 at 88 GHz. The derived brightness temperature of the M84 core was about 7.2 × 10<jats:sup>9</jats:sup> K, which could serve as a lower limit because the core down to 30 Schwarzschild radii was still unresolved in our 88 GHz observations. We successfully determined the core shifts of M87 at 22–44 GHz and 44–88 GHz through the source-frequency phase-referencing technique. The jet apex of M87 could be deduced at ∼46 <jats:italic>μ</jats:italic>as upstream of the 43 GHz core from core-shift measurements. The estimated magnetic field strength of the 88 GHz core of M87 is 4.8 ± 2.4 G, which is at the same magnitude of 1–30 G near the event horizon probed by the Event Horizon Telescope.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The abundance of active galactic nuclei (AGN) in cosmic voids is relatively unexplored in the literature, but can potentially provide new constraints on the environmental dependence of AGN activity and the AGN-host coevolution. We investigated AGN fractions in one of the largest samples of optically selected cosmic voids from Sloan Digital Sky Survey Data Release 12 for redshift range 0.2–0.7 for moderately bright and bright AGN. We separated inner and outer void regions based on the void size, given by its effective void radius. We classified galaxies at a distance &lt;0.6 <jats:italic>R</jats:italic> <jats:sub>eff</jats:sub> as inner void members and galaxies in the interval 0.6 &lt; <jats:italic>R</jats:italic>/<jats:italic>R</jats:italic> <jats:sub>eff</jats:sub> &lt; 1.3 as outer void galaxies. We found higher average fractions in the inner voids (4.9 ± 0.7)% than for their outer counterparts (3.1 ± 0.1)% at <jats:italic>z</jats:italic> &gt; 0.42, which clearly indicates an environmental dependence. This conclusion was confirmed upon further separating the data in narrower void-centric distance bins and measured a significant decrease in AGN activity from inner to outer voids for <jats:italic>z</jats:italic> &gt; 0.42. At low redshifts (<jats:italic>z</jats:italic> &lt; 0.42), we find very weak dependence on the environment for the inner and outer regions for two out of three bins. We argue that the higher fraction in low-density regions close to void centers relative to their outer counterparts observed in the two higher-redshift bins suggests that more efficient galaxy interactions may occur at a one-to-one level in voids that may be suppressed in denser environments due to higher velocity dispersions. It could also indicate less prominent ram pressure stripping in voids or some intrinsic host or void environment properties.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We demonstrate an efficient mechanism for generating magnetic fields in turbulent, collisionless plasmas. By using fully kinetic, particle-in-cell simulations of an initially nonmagnetized plasma, we inspect the genesis of magnetization, in a nonlinear regime. The complex motion is initiated via a Taylor–Green vortex, and the plasma locally develops strong electron temperature anisotropy, due to the strain tensor of the turbulent flow. Subsequently, in a domino effect, the anisotropy triggers a Weibel instability, localized in space. In such active wave–particle interaction regions, the seed magnetic field grows exponentially and spreads to larger scales due to the interaction with the underlying stirring motion. Such a self-feeding process might explain magnetogenesis in a variety of astrophysical plasmas, wherever turbulence is present.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The lifetime of the remnant produced by the merger of two neutron stars can provide a wealth of information on the equation of state of nuclear matter and on the processes leading to the electromagnetic counterpart. Hence, it is essential to determine when this lifetime is the shortest, corresponding to when the remnant has a mass equal to the threshold mass, <jats:italic>M</jats:italic> <jats:sub>th</jats:sub>, to prompt collapse to a black hole. We report on the results of more than 360 simulations of merging neutron-star binaries covering 40 different configurations differing in mass ratio and spin of the primary. Using this data, we have derived a quasi-universal relation for <jats:italic>M</jats:italic> <jats:sub>th</jats:sub> and expressed its dependence on the mass ratio and spin of the binary. The new expression recovers the results of Koeppel et al. for equal-mass, irrotational binaries and reveals that <jats:italic>M</jats:italic> <jats:sub>th</jats:sub> can increase (decrease) by 5% (10%) for binaries that have spins aligned (antialigned) with the orbital angular momentum and provides evidence for a nonmonotonic dependence of <jats:italic>M</jats:italic> <jats:sub>th</jats:sub> on the mass asymmetry in the system. Finally, we extend to unequal masses and spinning binaries the lower limits that can be set on the stellar radii once a neutron star binary is detected, illustrating how the merger of an unequal-mass, rapidly spinning binary can significantly constrain the allowed values of the stellar radii.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We place the first constraints on binary planets and exomoons from Doppler monitoring of directly imaged exoplanets. We model radial velocity observations of HR 8799 b, c, and d from Ruffio et al. and determine upper limits on the <jats:inline-formula> <jats:tex-math> <?CDATA $m\sin i$?> </jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>m</mml:mi> <mml:mi>sin</mml:mi> <mml:mi>i</mml:mi> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjlac33b4ieqn1.gif" xlink:type="simple" /> </jats:inline-formula> of short-period binary planets and satellites. At 95% confidence, we rule out companions orbiting the three planets more massive than <jats:inline-formula> <jats:tex-math> <?CDATA $m\sin i=2$?> </jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>m</mml:mi> <mml:mi>sin</mml:mi> <mml:mi>i</mml:mi> <mml:mo>=</mml:mo> <mml:mn>2</mml:mn> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjlac33b4ieqn2.gif" xlink:type="simple" /> </jats:inline-formula> <jats:italic>M</jats:italic> <jats:sub>J</jats:sub> with orbital periods shorter than 5 days. We achieve our tightest constraints on moons orbiting HR 8799c, where with 95% confidence we rule out out edge-on Jupiter-mass companions in periods shorter than 5 days and edge-on half-Jupiter-mass moons in periods shorter than 1 day. These radial velocity observations come from spectra with resolution 20 times lower than typical radial velocity instruments and were taken using a spectrograph that was designed before the first directly imaged exoplanet was discovered. Similar data sets from new and upcoming instruments will probe significantly lower exomoon masses.</jats:p>