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<jats:title>Abstract</jats:title> <jats:p>We studied the weak-lined T Tauri star Hubble 4, a known long-period binary, and its star-spot phenomena. We used optical radial velocity (RV) data taken over a span of 14 yr (2004–2010, 2017–2019) at the McDonald Observatory 2.7 m Harlan J. Smith Telescope and single epoch imaging from the Hubble Space Telescope (HST)/Wide Field Camera 3 instrument. The observed and apparent RV variations show contributions, respectively, from the binary motion as well as from a large spot group on one of the stars, presumed to be the primary. Fitting and removing the orbital signal from the RVs, we found the lower bound on the lifetime of a previously identified large spot group on the surface of the star to be at least 5.1 yr. An ∼5 yr lower limit is a long, but not unprecedented, duration for a single spot group. The later epoch data indicate significant spot evolution has occurred, placing an upper bound on the spot group lifetime at 12 yr. We find that pre-main-sequence evolutionary models for the age of Taurus (∼2 Myr), combined with component mass estimates from the literature, permit us to reproduce the HST relative photometry and the binary-induced contribution to the apparent RV variations. The long-lived star spot we find on Hubble 4 has significant implications for dynamo models in young stars, as it adds evidence for long lifetimes of magnetic field topologies. There are also significant implications for young star exoplanet searches, as long-lived coherent RV signals may be spot induced and not the result of planetary motion.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The search for astronomical pulsed signals within noisy data in the radio band is usually performed through an initial Fourier analysis to find “candidate” frequencies and then refined through the folding of the time series using trial frequencies close to the candidate. In order to establish the significance of the pulsed profiles found at these trial frequencies, pulsed profiles are evaluated with a <jats:italic>χ</jats:italic> <jats:sup>2</jats:sup> test to establish how much they depart from a null hypothesis where the signal is consistent with a flat distribution of noisy measurements. In high-energy astronomy, the <jats:italic>χ</jats:italic> <jats:sup>2</jats:sup> statistic has widely been replaced by the <jats:inline-formula> <jats:tex-math> <?CDATA ${Z}_{n}^{2}$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabda4aieqn1.gif" xlink:type="simple" /> </jats:inline-formula> statistic and <jats:italic>H</jats:italic>-test, as they are more sensitive to extra information, such as the harmonic content of the pulsed profile. The <jats:inline-formula> <jats:tex-math> <?CDATA ${Z}_{n}^{2}$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabda4aieqn2.gif" xlink:type="simple" /> </jats:inline-formula> statistic and <jats:italic>H</jats:italic>-test were originally developed for use with “event data” composed of arrival times of single photons, leaving it unclear how these methods could be used in radio astronomy. In this paper, we present a version of the <jats:inline-formula> <jats:tex-math> <?CDATA ${Z}_{n}^{2}$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabda4aieqn3.gif" xlink:type="simple" /> </jats:inline-formula> statistic and <jats:italic>H</jats:italic>-test for pulse profiles with Gaussian uncertainties appropriate for radio or even optical pulse profiles. We show how these statistical indicators provide better sensitivity to low-significance pulsar candidates with respect to the usual <jats:italic>χ</jats:italic> <jats:sup>2</jats:sup> method and a straightforward way to discriminate between pulse profile shapes. Moreover, they provide an additional tool for radio frequency interference rejection.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Mixing between convective zones is quite uncertain in the envelopes of A-type stars. To study the mixing in A-type envelopes, we use a new convection model, the <jats:italic>k</jats:italic>-<jats:italic>ω</jats:italic> model, in the MESA stellar evolution code. Using the <jats:italic>k</jats:italic>-<jats:italic>ω</jats:italic> model, we find that the overshooting regions of the He <jats:sc>ii</jats:sc> and H/He <jats:sc>i</jats:sc> convection zones are integrated with each other. There is material exchange between the He <jats:sc>ii</jats:sc> and H/He <jats:sc>i</jats:sc> convection zones through overshooting, in agreement with recent numerical simulations.We obtain the overshooting distance of about 3.5<jats:italic>H</jats:italic> <jats:sub>p</jats:sub> below the base of the H/He <jats:sc>i</jats:sc> convection zone in a 2.3 <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub> star. There are two overshooting regions beyond the He <jats:sc>ii</jats:sc> convection zone in the same stellar model. We obtain that the overshooting distance of the upper one is about 3.9<jats:italic>H</jats:italic> <jats:sub>p</jats:sub> and about 2.0<jats:italic>H</jats:italic> <jats:sub>p</jats:sub> for the lower one. We find that the turbulent diffusion effect is particularly strong in the convective envelopes of A-type stars. In addition, we find that the typical size of the convective rolling cells is restricted by the actual thickness of the convective zones, because the thickness of the convection zones are usually smaller than or approximately equal to the local pressure scale height. Through comparisons with the results of the <jats:italic>k</jats:italic>-<jats:italic>ω</jats:italic> model, we find that a suitable value of <jats:italic>f</jats:italic> <jats:sub>ov</jats:sub> is about 0.45 for the H/He <jats:sc>i</jats:sc> convective overshooting region. It is about 0.27 for the upper He <jats:sc>ii</jats:sc> convective overshooting region and about 0.25 for the lower one.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Recent observations revealed a coherence between the spin vector of a galaxy and the orbital motion of its neighbors. We refer to the phenomenon as “the spin–orbit alignment (SOA)” and explore its physical origin via the IllustrisTNG simulation. This is the first study to utilize a cosmological hydrodynamic simulation to investigate the SOA of galaxy pairs. In particular, we identify paired galaxies at <jats:italic>z</jats:italic> = 0 having the nearest neighbor with mass ratios from 1/10 to 10 and calculate the spin–orbit angle for each pair. Our results are as follows. (a) There exists a clear preference for prograde orientations (i.e., SOA) for galaxy pairs, qualitatively consistent with observations. (b) The SOA is significant for both baryonic and dark matter spins, being the strongest for gas and the weakest for dark matter. (c) The SOA is stronger for less massive targets and for targets having closer neighbors. (d) The SOA strengthens for galaxies in low-density regions, and the signal is dominated by central–satellite pairs in low-mass halos. (e) There is an explicit dependence of the SOA on the duration of interaction with its current neighbor. Taken together, we propose that the SOA witnessed at <jats:italic>z</jats:italic> = 0 has been developed mainly by interactions with a neighbor for an extended period of time, rather than tidal torque from the ambient large-scale structure.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>In recent studies, several active galactic nuclei (AGNs) have exhibited gradients of the Faraday rotation measure (RM) transverse to their parsec-scale jet direction. Faraday rotation likely occurs as a result of a magnetized sheath wrapped around the jet. In the case of 3C 273, using Very Long Baseline Array multiepoch observations at 5, 8, and 15 GHz in 2009–2010, we observe that the jet RM has changed significantly toward negative values compared with that previously observed. These changes could be explained by a swing of the parsec-scale jet direction, which causes synchrotron emission to pass through different portions of the Faraday screen. We develop a model for the jet–sheath system in 3C 273 where the sheath is wider than the single-epoch narrow relativistic jet. We present our oversized sheath model together with a derived wide-jet full intrinsic opening angle <jats:italic>α</jats:italic> <jats:sub>int</jats:sub> = 2.°1 and magnetic field strength <jats:italic>B</jats:italic> <jats:sub>∣∣</jats:sub> = 3 <jats:italic>μ</jats:italic>G, and thermal particle density <jats:italic>N</jats:italic> <jats:sub>e</jats:sub> = 125 cm<jats:sup>−3</jats:sup> at the wide jet–sheath boundary 230 pc downstream (deprojected) from its beginning. Most of the Faraday rotation occurs within the innermost layers of the sheath. The model brings together the jet direction swing and long-term RM evolution and may be applicable to other AGN jets that exhibit changes in their apparent jet direction.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We analyze the evolution of shock waves in high-resolution 3D radiative MHD simulations of the quiet Sun and their synthetic emission characteristics. The simulations model the dynamics of a 12.8 × 12.8 × 15.2 Mm quiet-Sun region (including a 5.2 Mm layer of the upper convection zone and a 10 Mm atmosphere from the photosphere to corona) with an initially uniform vertical magnetic field of 10 G, naturally driven by convective flows. We synthesize the Mg <jats:sc>II</jats:sc> and C <jats:sc>II</jats:sc> spectral lines observed by the Interface Region Imaging Spectrograph (IRIS) satellite and extreme ultraviolet emission observed by the Solar Dynamics Observatory (SDO)/AIA telescope. Synthetic observations are obtained using the RH1.5D radiative transfer code and temperature response functions at both the numerical and instrumental resolutions. We found that the Doppler velocity jumps of the C <jats:sc>II</jats:sc> 1334.5 Å IRIS line and a relative enhancement of the emission in the 335 Å SDO/AIA channel are the best proxies for the enthalpy deposited by shock waves into the corona (with Kendall’s <jats:italic>τ</jats:italic> correlation coefficients of 0.59 and 0.38, respectively). The synthetic emission of the lines and the extreme ultraviolet passbands are correlated with each other during the shock-wave propagation. All studied shocks are mostly hydrodynamic (i.e., the magnetic energy carried by horizontal fields is ≤2.6% of the enthalpy for all events) and have Mach numbers &gt;1.0–1.2 in the low corona. The study reveals the possibility of diagnosing energy transport by shock waves into the solar corona, as well as their other properties, by using IRIS and SDO/AIA sensing observations.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We present a new kinematic model for the Small Magellanic Cloud (SMC), using data from the Gaia Data Release 2 catalog. We identify a sample of astrometrically well-behaved red giant (RG) stars belonging to the SMC and cross-match with publicly available radial velocity (RV) catalogs. We create a 3D spatial model for the RGs, using RR Lyrae for distance distributions, and apply kinematic models with varying rotation properties and a novel tidal expansion prescription to generate mock proper motion (PM) catalogs. When we compare this series of mock catalogs to the observed RG data, we find that a combination of moderate rotation (with a magnitude of ∼10–20 km s<jats:sup>−1</jats:sup> at 1 kpc from the SMC center, inclination between ∼50 and 80°, and a predominantly north-to-south line-of-nodes position angle of ∼180°) and tidal expansion (with a scaling of ∼10 km s<jats:sup>−1</jats:sup> kpc<jats:sup>−1</jats:sup>) is required to explain the PM signatures. The exact best-fit parameters depend somewhat on whether we assess only the PMs or include the RVs as a qualitative check, leaving some tension remaining between the PM and RV conclusions. In either case, the parameter space preferred by our model is different from previously inferred rotational geometries, including from the SMC H <jats:sc>i</jats:sc> gas, and from the RG RV-only analyses and new SMC PM analyses, which conclude that a rotation signature is not detectable. Taken together this underscores the need to treat the SMC as a series of different populations with distinct kinematics.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The fast multipole method (FMM) obeys periodic boundary conditions “natively” if it uses a periodic Green’s function for computing the multipole expansion in the interaction zone of each FMM oct-tree node. One can define the “optimal” Green’s function for such a method that results in the numerical solution that converges to the equivalent particle-mesh (PM) solution in the limit of sufficiently high order of multipoles. A discrete functional equation for the optimal Green’s function can be derived, but is not practically useful as methods for its solution are not known. Instead, this paper presents an approximation for the optimal Green’s function that is accurate to better than 10<jats:sup>−3</jats:sup> in <jats:inline-formula> <jats:tex-math> <?CDATA ${L}_{\mathrm{MAX}}$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabd9c2ieqn1.gif" xlink:type="simple" /> </jats:inline-formula> norm and 10<jats:sup>−4</jats:sup> in <jats:italic>L</jats:italic> <jats:sub>2</jats:sub> norm for practically useful multipole counts. Such an approximately optimal Green’s function offers a practical way for implementing the FMM with periodic boundary conditions natively, without the need to compute lattice sums or to rely on hybrid FMM-PM approaches.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We report the temporal and spectral analysis of three thermonuclear X-ray bursts from 4U 1608−52, observed by the Neutron Star Interior Composition Explorer (NICER) during and just after the outburst observed from the source in 2020. In two of the X-ray bursts, we detect secondary peaks 30 and 18 s after the initial peaks. The secondary peaks show a fast rise exponential decay-like shape resembling a thermonuclear X-ray burst. Time-resolved X-ray spectral analysis reveals that the peak flux, blackbody temperature, and apparent emitting radius values of the initial peaks are in agreement with X-ray bursts previously observed from 4U 1608−52, while the same values for the secondary peaks tend toward the lower end of the distribution of bursts observed from this source. The third X-ray burst, which happened during much lower accretion rates, did not show any evidence for a deviation from an exponential decay and was significantly brighter than the previous bursts. We present the properties of the secondary peaks and discuss the events within the framework of short recurrence time bursts or bursts with secondary peaks. We find that the current observations do not fit in standard scenarios and challenge our understanding of flame spreading.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Without an existing large-scale coherent magnetic field in the early universe, Population III stars would likely rotate at or near breakup speed. In this work, focusing on the accretion phase of Population III stars, we investigate the possibility of generating a coherent magnetic field through large-scale dynamo processes, as well as the corresponding field saturation level. Using results from hydrodynamic simulations performed with a cylindrical grid, we demonstrate that primordial accretion disks are turbulent with a Shakura–Sunyaev disk parameter <jats:italic>α</jats:italic> <jats:sub>ss</jats:sub> ≳ 10<jats:sup>−3</jats:sup> and evidence for helical turbulence with a dynamo number ∣<jats:italic>D</jats:italic> <jats:sub> <jats:italic>α</jats:italic>Ω</jats:sub>∣ ≫ 10. The presence of helical turbulence at these levels allows large-scale dynamo modes to grow, and the saturation level is determined by the amount of net helicity remaining in the dynamo active regions (i.e., the quenching problem). We demonstrate that if the accretion could successfully alleviate the quenching problem, the magnetic field can reach approximate equipartition with <jats:italic>B</jats:italic>/<jats:italic>B</jats:italic> <jats:sub>eq</jats:sub> ∼ 3.</jats:p>