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<jats:title>Abstract</jats:title> <jats:p>The center of the Milky Way hosts the closest supermassive black hole, Sgr A*. Decades of near-infrared observations of our Galactic Center have shown the presence of a small population of stars (the so-called S-star cluster) orbiting Sgr A*, which were recently reported to be arranged into two orthogonal disks. In this case, the timescale for the Lense–Thirring precession of S stars should be longer than their age, implying a low spin for Sgr A*. In contrast, the recent results by the Event Horizon Telescope favor a highly spinning Sgr A*, which seems to suggest that the S stars could not be arranged in disks. Alternatively, the spin of Sgr A* must be small, suggesting that the models for its observed image are incomplete.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We investigate the eruptive process of two filaments, which is associated with an M-class flare that occurred in 2011 August 4. The filaments are partly overlapped, one in the active region and the other just beside it, and erupt together as a halo coronal mass ejection. For this study, we used the Atmospheric Imaging Assembly and the Heliospheric Magnetic Imager on board the Solar Dynamics Observatory, the Nobeyama Radioheliograph 17 GHz, and the RHESSI Hard X-ray satellite. We found three distinct phases in the microwave flux profile and in the rising pattern of the filaments during the event. In the first phase, there was weak nonthermal emission at 17 GHz and hard X-rays. Those nonthermal sources appeared on one edge of the western filament (F2) in the active region. The F2 began to be bright and rose upward rapidly, while the eastern filament (F1), which was extended to the quiet region, started to brighten from the peak time of the 17 GHz flux. In the second phase, the nonthermal emission weakened and the F2 rose up slowly, while the F1 began to rise up. In the third phase, two filaments erupted together. Since the F1 was stable for a long time in the quiet region, breaking the equilibrium state of the F1 would be decisive for the successful eruption of two filaments and it seems clear that the evolution of the F2 provoked the unstable F1. We suggest that tether-cutting reconnection between two overlapped filaments triggers the eruption of the two filaments as a tangled identity.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The population-level distributions of the masses, spins, and redshifts of binary black holes (BBHs) observed using gravitational waves can shed light on how these systems form and evolve. Because of the complex astrophysical processes shaping the inferred BBH population, models allowing for correlations among these parameters will be necessary to fully characterize these sources. We hierarchically analyze the BBH population detected by LIGO and Virgo with a model allowing for correlations between the effective aligned spin and the primary mass and redshift. We find that the width of the effective spin distribution grows with redshift at 98.6% credibility. We determine this trend to be robust under the application of several alternative models and additionally verify that such a correlation is unlikely to be spuriously introduced using a simulated population. We discuss the possibility that this correlation could be due to a change in the natal black hole spin distribution with redshift.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>One scenario for the generation of fast radio bursts (FRBs) is magnetic reconnection in a current sheet of the magnetar wind. Compressed by a strong magnetic pulse induced by a magnetar flare, the current sheet fragments into a self-similar chain of magnetic islands. Time-dependent plasma currents at their interfaces produce coherent radiation during their hierarchical coalescence. We investigate this scenario using 2D radiative relativistic particle-in-cell simulations to compute the efficiency of the coherent emission and to obtain frequency scalings. Consistent with expectations, a fraction of the reconnected magnetic field energy, <jats:italic>f</jats:italic> ∼ 0.002, is converted to packets of high-frequency fast magnetosonic waves, which can escape from the magnetar wind as radio emission. In agreement with analytical estimates, we find that magnetic pulses of 10<jats:sup>47</jats:sup> erg s<jats:sup>−1</jats:sup> can trigger relatively narrowband GHz emission with luminosities of approximately 10<jats:sup>42</jats:sup> erg s<jats:sup>−1</jats:sup>, sufficient to explain bright extragalactic FRBs. The mechanism provides a natural explanation for a downward frequency drift of burst signals, as well as the ∼100 ns substructure recently detected in <jats:named-content xmlns:xlink="http://www.w3.org/1999/xlink" content-type="object" xlink:href="FRB 20200120E" xlink:type="simple">FRB 20200120E</jats:named-content> .</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We compare the 230 GHz near-horizon emission from Sagittarius A* to simulations representing three classes of accretion flows. Using the structure function to capture the variability statistics of the light curve, we find a noticeable discrepancy between the observations and models based on torus-fed accretion disks, whether those disks bring in a small or large amount of net magnetic flux. On the other hand, the simulations that are fed more realistically by stellar winds match the observed structure function very well. We describe the differences between models, arguing that feeding by stellar winds may be a critical component in constructing theoretical models for accretion in the Galactic Center.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Pulsar timing array experiments have recently reported strong evidence for a common-spectrum stochastic process with a strain spectral index consistent with that expected of a nanohertz-frequency gravitational-wave background, but with negligible yet non-zero evidence for spatial correlations required for a definitive detection. However, it was pointed out by the Parkes Pulsar Timing Array (PPTA) collaboration that the same models used in recent analyses resulted in strong evidence for a common-spectrum process in simulations where none is present. In this work, we introduce a methodology to distinguish pulsar power spectra with the same amplitude from noise power spectra of similar but distinct amplitudes. The former is the signature of a spatially uncorrelated pulsar term of a nanohertz gravitational-wave background, whereas the latter could represent ensemble pulsar noise properties. We test the methodology on simulated data sets. We find that the reported common process in PPTA pulsars is indeed consistent with the spectral feature of a pulsar term. We recommend this methodology as one of the validity tests that the real astrophysical and cosmological backgrounds should pass, as well as for inferences about the spatially uncorrelated component of the background.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The Milky Way (MW) galaxy is in focus, thanks to new observational data. Here we shed new light on the MW’s past by studying the structural evolution of MW progenitors, which we identify from extragalactic surveys. Specifically, we constrain the stellar-mass growth history (SMGH) of the MW with two methods: (i) direct measurement of the MW’s star formation history, and (ii) assuming the MW is a typical star-forming galaxy that remains on the star-forming main sequence. We select MW progenitors based on these two SMGHs at <jats:italic>z</jats:italic> = 0.2–2.0 from the CANDELS/3D-HST data. We estimate the structural parameters (including half-mass radius <jats:italic>r</jats:italic> <jats:sub>50</jats:sub> and Sérsic index) from the stellar-mass profiles. Our key finding is that the progenitors of the MW galaxy grow self-similarly on spatially resolved scales with roughly a constant half-mass radius (∼2–3 kpc) over the past 10 Gyr, while their stellar masses increase by about 1 dex, implying little-to-no inside-out growth. We discover that the radius containing 20% of the stellar mass (<jats:italic>r</jats:italic> <jats:sub>20</jats:sub>) decreases by 60% between redshifts of <jats:italic>z</jats:italic> = 2.0 and <jats:italic>z</jats:italic> = 0.7, while the central stellar-mass density (Σ<jats:sub>1</jats:sub>) increases by a factor of 1.3 dex over the same time and the Sérsic index changes as <jats:inline-formula> <jats:tex-math> <?CDATA $n\propto {\left(1+z\right)}^{-1.41\pm 0.19}$?> </jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>n</mml:mi> <mml:mo>∝</mml:mo> <mml:msup> <mml:mrow> <mml:mfenced close=")" open="("> <mml:mrow> <mml:mn>1</mml:mn> <mml:mo>+</mml:mo> <mml:mi>z</mml:mi> </mml:mrow> </mml:mfenced> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>1.41</mml:mn> <mml:mo>±</mml:mo> <mml:mn>0.19</mml:mn> </mml:mrow> </mml:msup> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjlac76c8ieqn1.gif" xlink:type="simple" /> </jats:inline-formula>. This is consistent with an early (<jats:italic>z</jats:italic> &gt; 1) formation of a thick disk, followed by the formation of a bar that led to an increase in the mass in the core. The formation and evolution of the thin disk had only little impact on the overall half-mass size. We also show that the constant-size evolution of the MW progenitors challenges semiempirical approaches and numerical simulations.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The origins of Type Ia supernovae (SNe Ia) are still debated. Some of the leading scenarios involve a double detonation in double white dwarf (WD) systems. In these scenarios, helium shell detonation occurs on top of a carbon-oxygen (CO) WD, which then drives the detonation of the CO core, producing an SN Ia. Extensive studies have been done on the possibility of a double helium detonation, following a dynamical helium mass-transfer phase onto a CO-WD. However, 3D self-consistent modeling of the double-WD system, the mass transfer, and the helium shell detonation have been little studied. Here we use 3D hydrodynamical simulations to explore this case in which a helium detonation occurs near the point of Roche lobe overflow of the donor WD and may lead to an SN Ia through the dynamically driven double-degenerate double-detonation (D6) mechanism. We find that the helium layer of the accreting primary WD does undergo a detonation, while the underlying CO core does not, leading to an extremely rapid and faint nova-like transient instead of a luminous SN Ia event. This failed core detonation suggests that D6 SNe Ia may be restricted to the most massive CO primary WDs. We highlight the nucleosynthesis of the long-lived radioisotope <jats:sup>44</jats:sup>Ti during explosive helium burning, which may serve as a hallmark both of successful as well as failed D6 events, which subsequently detonate as classical double-degenerate mergers.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>A radio-emitting tidal disruption event (AT2019dsg) is proposed as a likely counterpart to the IceCube neutrino event IC 191001A. In this work, we have revisited the Fermi-LAT data in the direction of the neutrino and confirmed no signal at the site of AT2019dsg. Instead, at the edge of the 90% confidence level error region of this neutrino, there is a <jats:italic>γ</jats:italic>-ray transient source associated with the blazar GB6 J2113+1121. In 2019 May, GB6 J2113+1121 was undergoing a <jats:italic>γ</jats:italic>-ray flare that is unprecedented since the start of the Fermi-LAT operation, with a variability amplitude of about 20 fold. Similar violent flares of GB6 J2113+1121, unobserved before, have also been detected in the optical bands. Moreover, the blazar remained in a high-flux state in the infrared bands when IC 191001A arrived, though the blazar ‘s <jats:italic>γ</jats:italic>-ray and optical activities have temporarily ceased. Motivated by this spatial and temporal coincidence, we suggest that GB6 J2113+1121 is a candidate to be the counterpart to IC 191001A. The jet properties of GB6 J2113+1121 are investigated, which are found to be comparable with that of neutrino-emitting blazars (candidates). A specific analysis of archival IceCube data in this direction and future observations would put further constraints on the origin of the neutrino.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The cutoff frequency is an important characteristic parameter of type III radio bursts. Employing the radio data of the Parker Solar Probe (PSP) in the encounter phases of its first five orbits, our previous work revealed that the maximum probability distribution of the cutoff frequency <jats:italic>f</jats:italic> <jats:sub> <jats:italic>lo</jats:italic> </jats:sub> (∼680 kHz) is remarkably higher than that based on Ulysses and WIND (∼100 kHz) investigated by Leblanc et al. and Dulk et al. However, the main influencing factor of the discrepancy is still unknown though the possible reasons are discussed. In this study, we utilize the simultaneous observation by WIND and PSP to analyze statistically the distribution of the cutoff frequency of type III radio bursts, which had not been done before. Based on the automatic Canny edge detection and manual selection, we obtain the <jats:italic>f</jats:italic> <jats:sub> <jats:italic>lo</jats:italic> </jats:sub> of 491 (WIND) and 1194 (PSP) type III bursts from their simultaneous observations in the same solar activity period (from 2019 January 1 to 2020 July 31). The statistical results show that the dominant cutoff frequency measured by PSP (i.e., ∼700 kHz) is still significantly higher than that by WIND (i.e., ∼100 kHz). This implies that radiation attenuation is the main influencing factor for the difference in the statistical results of the cutoff frequency.</jats:p>