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<jats:title>Abstract</jats:title> <jats:p>The population properties of intermediate-mass black holes remain largely unknown, and understanding their distribution could provide a missing link in the formation of supermassive black holes and galaxies. Gravitational-wave observations can help fill in the gap from stellar mass black holes to supermassive black holes with masses between ∼100–10<jats:sup>4</jats:sup> <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub>. In our work, we propose a new method for examining lens populations through lensing statistics of gravitational waves, here focusing on inferring the number density of intermediate-mass black holes through hierarchical Bayesian inference. Simulating ∼200 lensed gravitational-wave signals, we find that existing gravitational-wave observatories at their design sensitivity could either constrain the number density of 10<jats:sup>6</jats:sup> Mpc<jats:sup>−3</jats:sup> within a factor of 10, or place an upper bound of ≲10<jats:sup>4</jats:sup> Mpc<jats:sup>−3</jats:sup> if the true number density is 10<jats:sup>3</jats:sup> Mpc<jats:sup>−3</jats:sup>. More broadly, our method leaves room for incorporation of additional lens populations, providing a general framework for probing the population properties of lenses in the universe.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Two high-cadence surveys aiming for rotation period measurements of asteroids have been conducted in 2019 January and October using the Zwicky Transient Facility. From the surveys, 25 large superfast rotators (SFRs) were discovered and they are all main-belt asteroids (MBAs), except for one Mars crosser. These large SFRs have a diameter ranging from 0.43 to 7.87 km and a rotation period between 0.48 and 1.95 hr. Considering their diameters and fast rotations, they cannot be explained by rubble-pile structure unless using extraordinary high bulk densities. Cohesion, if available, can conserve these large SFRs. The estimated cohesion for these large SFRs could be up to thousands of pascals, much higher than the currently estimated cohesion for asteroids and that generated by the regolith of Moon and Mars. Such high-level cohesion can be produced from fine-grain regolith, like clay. However, the availability of such fine-grain regolith for asteroids is still unknown. Although the possibility of these large SFRs being large monolithic objects cannot be ruled out, this scenario is very unlikely given that the timescale of disruptive impact for MBAs in a similar diameter range is 10<jats:sup>7</jats:sup>–10<jats:sup>8</jats:sup> yr.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We present a large ∼30″ × 30″ spectroscopic survey of the Galactic Center using the SINFONI IFU at the VLT. Combining observations of the last two decades we compile spectra of over 2800 stars. Using the Bracket-<jats:italic>γ</jats:italic> absorption lines, we identify 195 young stars, extending the list of known young stars by 79. In order to explore the angular momentum distribution of the young stars, we introduce an isotropic cluster prior. This prior reproduces an isotropic cluster in a mathematically exact way, which we test through numerical simulations. We calculate the posterior angular momentum space as a function of projected separation from Sgr A*. We find that the observed young star distribution is substantially different from an isotropic cluster. We identify the previously reported feature of the clockwise disk and find that its angular momentum changes as a function of separation from the black hole and thus confirm a warp of the clockwise disk (<jats:italic>p</jats:italic> ∼ 99.2%). At large separations, we discover three prominent overdensities of the angular momentum. One overdensity has been reported previously, the counterclockwise disk. The other two are new. Determining the likely members of these structures, we find that as many as 75% of stars can be associated with one of these features. Stars belonging to the warped clockwise disk show a top-heavy <jats:italic>K</jats:italic>-band luminosity function, while stars belonging to the larger separation features do not. Our observations are in good agreement with the predictions of simulations of in situ star formation and argue for the common formation of these structures.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The association of GW170817/GRB 170817A/AT2017gfo provides the first direct evidence for neutron star mergers as significant sources of <jats:italic>r</jats:italic>-process nucleosynthesis. A gamma-ray transient (GRT) would be powered by the radioactive decay of the freshly synthesized <jats:italic>r</jats:italic>-process elements. By analyzing the composition and gamma-ray opacity of the kilonova ejecta in detail, we calculate the lightcurve and spectrum of the GRT for a range of spherically symmetric merger ejecta models with mass <jats:italic>M</jats:italic> <jats:sub>ej</jats:sub> = 0.001 to ∼0.05<jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub> and expansion velocity <jats:italic>v</jats:italic> <jats:sub>ej</jats:sub> = 0.1<jats:italic>c</jats:italic> to ∼0.4<jats:italic>c</jats:italic>. It is found that the peak of the GRT lightcurve depends on <jats:italic>M</jats:italic> <jats:sub>ej</jats:sub> and <jats:italic>v</jats:italic> <jats:sub>ej</jats:sub> as <jats:inline-formula> <jats:tex-math> <?CDATA ${t}_{\mathrm{pk}}\approx 0.5\,\mathrm{days}\,{({M}_{\mathrm{ej}}/0.01{M}_{\odot })}^{1/2}{({v}_{\mathrm{ej}}/0.1c)}^{-1}$?> </jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mrow> <mml:mi>t</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>pk</mml:mi> </mml:mrow> </mml:msub> <mml:mo>≈</mml:mo> <mml:mn>0.5</mml:mn> <mml:mspace width="0.25em" /> <mml:mi>days</mml:mi> <mml:mspace width="0.25em" /> <mml:msup> <mml:mrow> <mml:mo stretchy="false">(</mml:mo> <mml:msub> <mml:mrow> <mml:mi>M</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>ej</mml:mi> </mml:mrow> </mml:msub> <mml:mrow> <mml:mo stretchy="true">/</mml:mo> </mml:mrow> <mml:mn>0.01</mml:mn> <mml:msub> <mml:mrow> <mml:mi>M</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>⊙</mml:mo> </mml:mrow> </mml:msub> <mml:mo stretchy="false">)</mml:mo> </mml:mrow> <mml:mrow> <mml:mn>1</mml:mn> <mml:mrow> <mml:mo stretchy="true">/</mml:mo> </mml:mrow> <mml:mn>2</mml:mn> </mml:mrow> </mml:msup> <mml:msup> <mml:mrow> <mml:mo stretchy="false">(</mml:mo> <mml:msub> <mml:mrow> <mml:mi>v</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>ej</mml:mi> </mml:mrow> </mml:msub> <mml:mrow> <mml:mo stretchy="true">/</mml:mo> </mml:mrow> <mml:mn>0.1</mml:mn> <mml:mi>c</mml:mi> <mml:mo stretchy="false">)</mml:mo> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>1</mml:mn> </mml:mrow> </mml:msup> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjlac7470ieqn1.gif" xlink:type="simple" /> </jats:inline-formula> and <jats:inline-formula> <jats:tex-math> <?CDATA ${L}_{\mathrm{pk}}\approx 2.0\times {10}^{41}\,\mathrm{erg}\,{{\rm{s}}}^{-1}{({M}_{\mathrm{ej}}/0.01{M}_{\odot })}^{1/2}({v}_{\mathrm{ej}}/0.1c)$?> </jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mrow> <mml:mi>L</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>pk</mml:mi> </mml:mrow> </mml:msub> <mml:mo>≈</mml:mo> <mml:mn>2.0</mml:mn> <mml:mo>×</mml:mo> <mml:msup> <mml:mrow> <mml:mn>10</mml:mn> </mml:mrow> <mml:mrow> <mml:mn>41</mml:mn> </mml:mrow> </mml:msup> <mml:mspace width="0.25em" /> <mml:mi>erg</mml:mi> <mml:mspace width="0.25em" /> <mml:msup> <mml:mrow> <mml:mi mathvariant="normal">s</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>1</mml:mn> </mml:mrow> </mml:msup> <mml:msup> <mml:mrow> <mml:mo stretchy="false">(</mml:mo> <mml:msub> <mml:mrow> <mml:mi>M</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>ej</mml:mi> </mml:mrow> </mml:msub> <mml:mrow> <mml:mo stretchy="true">/</mml:mo> </mml:mrow> <mml:mn>0.01</mml:mn> <mml:msub> <mml:mrow> <mml:mi>M</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>⊙</mml:mo> </mml:mrow> </mml:msub> <mml:mo stretchy="false">)</mml:mo> </mml:mrow> <mml:mrow> <mml:mn>1</mml:mn> <mml:mrow> <mml:mo stretchy="true">/</mml:mo> </mml:mrow> <mml:mn>2</mml:mn> </mml:mrow> </mml:msup> <mml:mo stretchy="false">(</mml:mo> <mml:msub> <mml:mrow> <mml:mi>v</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>ej</mml:mi> </mml:mrow> </mml:msub> <mml:mrow> <mml:mo stretchy="true">/</mml:mo> </mml:mrow> <mml:mn>0.1</mml:mn> <mml:mi>c</mml:mi> <mml:mo stretchy="false">)</mml:mo> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjlac7470ieqn2.gif" xlink:type="simple" /> </jats:inline-formula>. Most radiating photons are in the 100–3000 keV band and the spectrum peaks at about 800 keV for different nuclear physics inputs. The line features are blurred out by the Doppler broadening effect. Adopting the ejecta parameters reported in the literature, we examine the detection probability of the possible GRT associated with AT2017gfo. We show that the GRT cannot be convincingly detected with either current or proposed missions in the MeV band, such as ETCC and AMEGO. The low gamma-ray flux, together with the extremely low event rate at local universe, makes a discovery of GRTs a great challenge.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>I obtained time-series photometry of the compact binary candidate for the Fermi source 4FGL J0935.3+0901. Superposed on the 2.44 hr orbital modulation are day-to-day variations and frequent flaring as seen in several redback and black widow millisecond pulsars (MSPs). The short orbital period favors a black widow. While the modulation of ≤1 mag is smaller than that of most black widows, it could indicate a low orbital inclination. Although a published optical spectrum shows strong emission lines, the light curve evinces pulsar heating of the companion star rather than accretion-disk emission of a transitional MSP. Emission lines and flaring occur in the same objects, probably powered by shocks between the relativistic pulsar wind and a wind driven off the companion star. I also recovered the period in photometry from the Zwicky Transient Facility (ZTF). A phase-connected ephemeris derived from MDM Observatory and ZTF data spanning 4 yr yields a period of 0.10153276(36) days and an epoch for the ascending node of the putative pulsar.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We report a decaying kink oscillation of a flux rope during a confined eruptive flare, observed off the solar limb by the Solar Dynamics Observatory’s Atmospheric Imaging Assembly (AIA), which lacked a detectable white-light coronal mass ejection. The erupting flux rope underwent kinking, rotation, and apparent leg–leg interaction during the event. The oscillations were observed simultaneously in multiple AIA channels at 304, 171, and 193 Å, indicating that multithermal plasma was entrained in the rope. After reaching the overlying loops in the active region, the flux rope exhibited large-amplitude, decaying kink oscillations with an apparent initial amplitude of 30 Mm, a period of about 16 minutes, and a decay time of about 17 minutes. We interpret these oscillations as a fundamental standing kink mode of the flux rope. The oscillation polarization has a clear vertical component, while the departure of the detected waveform from a sinusoidal signal suggests that the oscillation could be circularly or elliptically polarized. The estimated kink speed is 1080 km s<jats:sup>−1</jats:sup>, corresponding to an Alfvén speed of about 760 km s<jats:sup>−1</jats:sup>. This speed, together with the estimated electron density in the rope from our differential emission measure analysis, <jats:italic>n</jats:italic> <jats:sub> <jats:italic>e</jats:italic> </jats:sub> ≈ (1.5–2.0) × 10<jats:sup>9</jats:sup> cm<jats:sup>−3</jats:sup>, yields a magnetic-field strength of about 15 G. To the best of our knowledge, decaying kink oscillations of a flux rope with nonhorizontal polarization during a confined eruptive flare have not been reported before. These oscillations provide unique opportunities for indirect measurements of the magnetic-field strength in low-coronal flux ropes during failed eruptions.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>In this Letter, we report observations of magnetic switchback (SB) features near 1 au using data from the Wind spacecraft. These features appear to be strikingly similar to the ones observed by the Parker Solar Probe mission closer to the Sun: namely, one-sided spikes (or enhancements) in the solar-wind bulk speed <jats:italic>V</jats:italic> that correlate/anticorrelate with the spikes seen in the radial-field component <jats:italic>B</jats:italic> <jats:sub> <jats:italic>R</jats:italic> </jats:sub>. In the solar-wind streams that we analyzed, these specific SB features near 1 au are associated with large-amplitude Alfvénic oscillations that propagate outward from the Sun along a local background (prevalent) magnetic field <jats:bold> <jats:italic>B</jats:italic> </jats:bold> <jats:sub>0</jats:sub> that is nearly radial. We also show that, when <jats:bold> <jats:italic>B</jats:italic> </jats:bold> <jats:sub>0</jats:sub> is nearly perpendicular to the radial direction, the large-amplitude Alfvénic oscillations display variations in <jats:italic>V</jats:italic> that are two sided (i.e., <jats:italic>V</jats:italic> alternately increases and decreases depending on the vector Δ<jats:bold> <jats:italic>B</jats:italic> </jats:bold> = <jats:bold> <jats:italic>B</jats:italic> </jats:bold> − <jats:bold> <jats:italic>B</jats:italic> </jats:bold> <jats:sub>0</jats:sub>). As a consequence, SBs may not always appear as one-sided spikes in <jats:italic>V</jats:italic>, especially at larger heliocentric distances where the local background field statistically departs from the radial direction. We suggest that SBs can be well described by large-amplitude Alfvénic fluctuations if the field rotation is computed with respect to a well-determined local background field that, in some cases, may deviate from the large-scale Parker field.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We investigate ways to produce the bifurcation observed in the stellar stream of the Sagittarius dwarf galaxy (Sgr). Our method consists of running <jats:italic>N</jats:italic>-body simulations of Sgr falling into the Milky Way for the last 3 Gyr, with added test particles on disk orbits that span a wide range of initial positions, energies, and angular momenta. We find that particles that end up in the faint branch are predominantly high-angular-momentum particles that can all originate from a single plane within the progenitor, nearly perpendicular both to the orbital plane of the progenitor and to the Milky Way stellar disk. Their original configuration at the start of the simulation corresponds to spiral features already present 3 Gyr ago, which could be, e.g., the result of a disk-like component being tidally perturbed, or the tidal tails of a satellite being disrupted within Sgr. We then run a simulation including the self-gravity of this disky component. Despite the remaining ambiguity of its origin, this disk component of the Sgr dwarf with spiral overdensities provides a first step toward a working model to reproduce the observed faint branch of the bifurcated Sgr stream.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The physical processes of gamma-ray emission and particle acceleration during the prompt phase in gamma-ray bursts (GRBs) are still unsettled. In order to perform unambiguous physical modeling of observations, a clear identification of the emission mechanism is needed. An instance of a clear identification is the synchrotron emission during the very strong flare in GRB 160821A, which occurred during the prompt phase at 135 s. Here we show that the distribution of the radiating electrons in this flare is initially very narrow but later develops a power-law tail of accelerated electrons. We thus identify for the first time the onset of particle acceleration in a GRB jet. The flare is consistent with a late energy release from the central engine causing an external shock as it encounters a preexisting ring nebula of a progenitor Wolf–Rayet star. Relativistic forward and reverse shocks develop, leading to two distinct emission zones with similar properties. The particle acceleration only occurs in the forward shock, moving into the dense nebula matter. Here, the magnetization also decreases below the critical value, which allows for Fermi acceleration to operate. Using this fact, we find a bulk Lorentz factor of 420 ≲ Γ ≲ 770 and an emission radius of <jats:italic>R</jats:italic> ∼ 10<jats:sup>18</jats:sup> cm, indicating a tenuous gas of the immediate circumburst surroundings. The observation of the onset of particle acceleration thus gives new and independent constraints on the properties of the flow as well as on theories of particle acceleration in collisionless astrophysical shocks.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We use halo dwarf stars with photometrically determined metallicities that are located within 2 kpc of the Sun to identify local halo substructure. The kinematic properties of these stars do not indicate a single, dominant radial merger event (RME). The retrograde Virgo Radial Merger (VRM) component has [Fe/H] = −1.7. A second, nonrotating RME component we name Nereus is identified with [Fe/H] = −2.1 and has similar energy to the VRM. We identify a possible third RME, which we name Cronus, that is corotating with the disk, has lower energy than the VRM, and has [Fe/H] = −1.2. We identify the Nyx Stream in the data. In addition to these substructures, we observe metal-poor halo stars ([Fe/H] ∼ −2.0 and <jats:italic>σ</jats:italic> <jats:sub> <jats:italic>v</jats:italic> </jats:sub> ∼ 180 km s<jats:sup>−1</jats:sup>) and a disk/Splash component with lower rotational velocity than the disk and lower metallicity than typically associated with the Splash. An additional excess of halo stars with low velocity and metallicity of [Fe/H] = −1.5 could be associated with the shell of a lower-energy RME or indicate that lower-energy halo stars have higher metallicity. Stars that comprise the “Gaia Sausage” velocity structure are a combination of the components identified in this work.</jats:p>