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<jats:title>Abstract</jats:title> <jats:p>The emission mechanism for hard <jats:italic>γ</jats:italic>-ray spectra from supernova remnants (SNRs) is still a matter of debate. Recent multiwavelength observations of the TeV source HESS J1912+101 show that it is associated with an SNR with an age of ∼100 kyr, making it unlikely produce the TeV <jats:italic>γ</jats:italic>-ray emission via leptonic processes. We analyzed Fermi observations of it and found an extended source with a hard spectrum. HESS J1912+101 may represent a peculiar stage of SNR evolution that dominates the acceleration of TeV cosmic rays. By fitting the multiwavelength spectra of 13 SNRs with hard GeV <jats:italic>γ</jats:italic>-ray spectra with simple emission models with a density ratio of GeV electrons to protons of ∼10<jats:sup>−2</jats:sup>, we obtain reasonable mean densities and magnetic fields with a total energy of ∼10<jats:sup>50</jats:sup> erg for relativistic ions in each SNR. Among these sources, only two of them, namely SN 1006 and RCW 86, favor a leptonic origin for the <jats:italic>γ</jats:italic>-ray emission. The magnetic field energy is found to be comparable to that of accelerated relativistic ions and their ratio has a tendency to increase with the age of SNRs. These results suggest that TeV cosmic rays mainly originate from SNRs with hard <jats:italic>γ</jats:italic>-ray spectra.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We investigate what drives the redshift evolution of the typical electron density (<jats:italic>n</jats:italic> <jats:sub> <jats:italic>e</jats:italic> </jats:sub>) in star-forming galaxies, using a sample of 140 galaxies drawn primarily from KMOS<jats:sup>3D</jats:sup> (0.6 &lt; <jats:italic>z</jats:italic> &lt; 2.6) and 471 galaxies from SAMI (<jats:italic>z</jats:italic> &lt; 0.113). We select galaxies that do not show evidence of active galactic nucleus activity or outflows to constrain the average conditions within H <jats:sc>ii</jats:sc> regions. Measurements of the [S <jats:sc>ii</jats:sc>]<jats:italic>λ</jats:italic>6716/[S <jats:sc>ii</jats:sc>]<jats:italic>λ</jats:italic>6731 ratio in four redshift bins indicate that the local <jats:italic>n</jats:italic> <jats:sub> <jats:italic>e</jats:italic> </jats:sub> in the line-emitting material decreases from 187<jats:inline-formula> <jats:tex-math> <?CDATA ${}_{-132}^{+140}$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabd551ieqn1.gif" xlink:type="simple" /> </jats:inline-formula> cm<jats:sup>−3</jats:sup> at <jats:italic>z</jats:italic> ∼ 2.2 to 32<jats:inline-formula> <jats:tex-math> <?CDATA ${}_{-9}^{+4}$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabd551ieqn2.gif" xlink:type="simple" /> </jats:inline-formula> cm<jats:sup>−3</jats:sup> at <jats:italic>z</jats:italic> ∼ 0, consistent with previous results. We use the H<jats:italic>α</jats:italic> luminosity to estimate the rms <jats:italic>n</jats:italic> <jats:sub> <jats:italic>e</jats:italic> </jats:sub> averaged over the volumes of star-forming disks at each redshift. The local and volume-averaged <jats:italic>n</jats:italic> <jats:sub> <jats:italic>e</jats:italic> </jats:sub> evolve at similar rates, hinting that the volume filling factor of the line-emitting gas may be approximately constant across 0 ≲ <jats:italic>z</jats:italic> ≲ 2.6. The KMOS<jats:sup>3D</jats:sup> and SAMI galaxies follow a roughly monotonic trend between <jats:italic>n</jats:italic> <jats:sub> <jats:italic>e</jats:italic> </jats:sub> and star formation rate, but the KMOS<jats:sup>3D</jats:sup> galaxies have systematically higher <jats:italic>n</jats:italic> <jats:sub> <jats:italic>e</jats:italic> </jats:sub> than the SAMI galaxies at a fixed offset from the star-forming main sequence, suggesting a link between the <jats:italic>n</jats:italic> <jats:sub> <jats:italic>e</jats:italic> </jats:sub> evolution and the evolving main sequence normalization. We quantitatively test potential drivers of the density evolution and find that <jats:italic>n</jats:italic> <jats:sub> <jats:italic>e</jats:italic> </jats:sub>(rms) <jats:inline-formula> <jats:tex-math> <?CDATA $\simeq {n}_{{{\rm{H}}}_{2}}$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabd551ieqn3.gif" xlink:type="simple" /> </jats:inline-formula>, suggesting that the elevated <jats:italic>n</jats:italic> <jats:sub> <jats:italic>e</jats:italic> </jats:sub> in high-<jats:italic>z</jats:italic> H <jats:sc>ii</jats:sc> regions could plausibly be the direct result of higher densities in the parent molecular clouds. There is also tentative evidence that <jats:italic>n</jats:italic> <jats:sub> <jats:italic>e</jats:italic> </jats:sub> could be influenced by the balance between stellar feedback, which drives the expansion of H <jats:sc>ii</jats:sc> regions, and the ambient pressure, which resists their expansion.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We find that, under certain conditions, protoplanetary disks may spontaneously generate multiple, concentric gas rings without an embedded planet through an eccentric cooling instability. Using both linear theory and nonlinear hydrodynamics simulations, we show that a variety of background states may trap a slowly precessing, one-armed spiral mode that becomes unstable when a gravitationally stable disk rapidly cools. The angular momentum required to excite this spiral comes at the expense of nonuniform mass transport that generically results in multiple rings. For example, one long-term hydrodynamics simulation exhibits four long-lived, axisymmetric gas rings. We verify the instability evolution and ring-formation mechanism from first principles with our linear theory, which shows remarkable agreement with the simulation results. Dust trapped in these rings may produce observable features consistent with observed disks. Additionally, direct detection of the eccentric gas motions may be possible when the instability saturates, and any residual eccentricity left over in the rings at later times may also provide direct observational evidence of this mechanism.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We conduct an all-sky search for continuous gravitational waves in the LIGO O2 data from the Hanford and Livingston detectors. We search for nearly monochromatic signals with frequency 20.0 Hz ≤ <jats:italic>f</jats:italic> ≤ 585.15 Hz and spin-down <jats:inline-formula> <jats:tex-math> <?CDATA $-2.6\times {10}^{-9}\,\mathrm{Hz}\,{{\rm{s}}}^{-1}\leqslant \dot{f}\leqslant 2.6\times {10}^{-10}$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabc7c9ieqn1.gif" xlink:type="simple" /> </jats:inline-formula> Hz s<jats:sup>−1</jats:sup>. We deploy the search on the Einstein@Home volunteer-computing project and follow-up the waveforms associated with the most significant results with eight further search stages, reaching the best sensitivity ever achieved by an all-sky survey up to 500 Hz. Six of the inspected waveforms pass all the stages but they are all associated with hardware injections, which are fake signals simulated at the LIGO detector for validation purposes. We recover all these fake signals with consistent parameters. No other waveform survives, so we find no evidence of a continuous gravitational wave signal at the detectability level of our search. We constrain the <jats:italic>h</jats:italic> <jats:sub>0</jats:sub> amplitude of continuous gravitational waves at the detector as a function of the signal frequency, in half-Hz bins. The most constraining upper limit at 163.0 Hz is <jats:italic>h</jats:italic> <jats:sub>0</jats:sub> = 1.3 × 10<jats:sup>−25</jats:sup>, at the 90% confidence level. Our results exclude neutron stars rotating faster than 5 ms with equatorial ellipticities larger than 10<jats:sup>−7</jats:sup> closer than 100 pc. These are deformations that neutron star crusts could easily support, according to some models.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We present comprehensive characterization of the Galactic open cluster M 36. Some 200 member candidates, with an estimated contamination rate of ∼8%, have been identified on the basis of proper motion and parallax measured by the Gaia DR2. The cluster has a proper motion grouping around (<jats:inline-formula> <jats:tex-math> <?CDATA ${\mu }_{\alpha }\cos \delta =-0.15\pm 0.01$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabded4ieqn1.gif" xlink:type="simple" /> </jats:inline-formula> mas yr<jats:sup>−1</jats:sup>, and <jats:italic>μ</jats:italic> <jats:sub> <jats:italic>δ</jats:italic> </jats:sub> = − 3.35 ± 0.02 mas yr<jats:sup>−1</jats:sup>), distinctly separated from the field population. Most member candidates have parallax values 0.7–0.9 mas, with a median value of 0.82 ± 0.07 mas (distance ∼1.20 ± 0.13 kpc). The angular diameter of <jats:inline-formula> <jats:tex-math> <?CDATA $27^{\prime} \pm 0\buildrel{\,\prime}\over{.} 4$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabded4ieqn2.gif" xlink:type="simple" /> </jats:inline-formula> determined from the radial density profile then corresponds to a linear extent of 9.42 ± 0.14 pc. With an estimated age of ∼15 Myr, M 36 is free of nebulosity. To the southwest of the cluster, we discover a highly obscured (<jats:italic>A</jats:italic> <jats:sub> <jats:italic>V</jats:italic> </jats:sub> up to ∼23 mag), compact (∼<jats:inline-formula> <jats:tex-math> <?CDATA $1\buildrel{\,\prime}\over{.} 9\times 1\buildrel{\,\prime}\over{.} 2$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabded4ieqn3.gif" xlink:type="simple" /> </jats:inline-formula>) dense cloud, within which three young stellar objects in their infancy (ages ≲0.2 Myr) are identified. The molecular gas, 3.6 pc in extent, contains a total mass of (2–3) × 10<jats:sup>2</jats:sup> <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub>, and has a uniform velocity continuity across the cloud, with a velocity range of −20 to −22 km s<jats:sup>−1</jats:sup>, consistent with the radial velocities of known star members. In addition, the cloud has a derived kinematic distance marginally in agreement with that of the star cluster. If physical association between M 36 and the young stellar population can be unambiguously established, this manifests a convincing example of prolonged star formation activity spanning up to tens of Myr in molecular clouds.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Radio sources at the highest redshifts can provide unique information on the first massive galaxies and black holes, the densest primordial environments, and the epoch of reionization. The number of astronomical objects identified at <jats:italic>z</jats:italic> &gt; 6 has increased dramatically over the last few years, but previously only three radio-loud (<jats:italic>R</jats:italic> <jats:sub>2500</jats:sub> = <jats:italic>f</jats:italic> <jats:sub> <jats:italic>ν</jats:italic>,5 GHz</jats:sub>/<jats:italic>f</jats:italic> <jats:sub> <jats:italic>ν</jats:italic>,2500 Å</jats:sub> &gt; 10) sources had been reported at <jats:italic>z</jats:italic> &gt; 6, with the most distant being a quasar at <jats:italic>z</jats:italic> = 6.18. Here we present the discovery and characterization of PSO J172.3556+18.7734, a radio-loud quasar at <jats:italic>z</jats:italic> = 6.823. This source has an Mg <jats:sc>ii</jats:sc>-based black hole mass of ∼3 × 10<jats:sup>8</jats:sup> <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub> and is one of the fastest accreting quasars, consistent with super-Eddington accretion. The ionized region around the quasar is among the largest measured at these redshifts, implying an active phase longer than the average lifetime of the <jats:italic>z</jats:italic> ≳ 6 quasar population. From archival data, there is evidence that its 1.4 GHz emission has decreased by a factor of two over the last two decades. The quasar’s radio spectrum between 1.4 and 3.0 GHz is steep (<jats:italic>α</jats:italic> = −1.31). Assuming the measured radio slope and extrapolating to rest-frame 5 GHz, the quasar has a radio-loudness parameter <jats:italic>R</jats:italic> <jats:sub>2500</jats:sub> ∼ 90. A second steep radio source (<jats:italic>α</jats:italic> = −0.83) of comparable brightness to the quasar is only 23.″1 away (∼120 kpc at <jats:italic>z</jats:italic> = 6.82; projection probability &lt;2%), but shows no optical or near-infrared counterpart. Further follow-up is required to establish whether these two sources are physically associated.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We present numerical simulations of misaligned disks around a spinning black hole covering a range of parameters. Previous simulations have shown that disks that are strongly warped by a forced precession—in this case, the Lense–Thirring effect from the spinning black hole—can break apart into discrete disks or rings that can behave quasi-independently for short timescales. With the simulations we present here, we confirm that thin and highly inclined disks are more susceptible to disk tearing than thicker disks or those with lower inclination, and we show that lower values of the disk viscosity parameter lead to instability at lower warp amplitudes. This is consistent with detailed stability analysis of the warped disk equations. We find that the growth rates of the instability seen in the numerical simulations are similar across a broad range of parameters, and are of the same order as the predicted growth rates. However, we did not find the expected trend of growth rates with viscosity parameter. This may indicate that the growth rates are affected by numerical resolution, or that the wavelength of the fastest-growing mode is a function of local disk parameters. Finally, we also find that disk tearing can occur for disks with a viscosity parameter that is higher than predicted by a local stability analysis of the warped disk equations. In this case, the instability manifests differently, producing large changes in the disk tilt locally in the disk, rather than the large changes in disk twist that typically occur in lower-viscosity disks.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Accretion disks around black holes power some of the most luminous objects in the universe. Disks that are misaligned to the black hole spin can become warped over time by Lense–Thirring precession. Recent work has shown that strongly warped disks can become unstable, causing the disk to break into discrete rings producing a more dynamic and variable accretion flow. In a companion paper, we present numerical simulations of this instability and the resulting dynamics. In this paper, we discuss the implications of this dynamics for accreting black hole systems, with particular focus on the variability of active galactic nuclei (AGN). We discuss the timescales on which variability might manifest, as well as the impact of the observer orientation with respect to the black hole spin axis. When the disk warp is unstable near the inner edge of the disk, we find quasi-periodic behavior of the inner disk, which may explain the recent quasi-periodic eruptions observed in, for example, the Seyfert 2 galaxy GSN 069 and in the galactic nucleus of RX J1301.9+2747. These eruptions are thought to be similar to the “heartbeat” modes observed in some X-ray binaries (e.g., GRS 1915+105 and IGR J17091-3624). When the instability manifests at larger radii in the disk, we find that the central accretion rate can vary on timescales that may be commensurate with, e.g., changing-look AGN. We therefore suggest that some of the variability properties of accreting black hole systems may be explained by the disk being significantly warped, leading to disk tearing.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Magnetars are highly magnetized neutron stars that are characterized by recurrent emission of short-duration bursts in soft gamma-rays/hard X-rays. Recently, FRB 200428 was found to be associated with an X-ray burst from a Galactic magnetar. Two fast radio bursts show mysterious periodic activity. However, it is unclear whether magnetar X-ray bursts are periodic phenomena. In this paper, we investigate the activity period of SGR 1806-20. More than 3000 short bursts observed by different telescopes are collected, including the observations of RXTE, HETE-2, ICE, and Konus. We consider the observation windows and divide the data into two subsamples to alleviate the effect of uneven sampling. The epoch-folding and Lomb–Scargle methods are used to derive the period of the short bursts. We find a possible period of about 398.20 ± 25.45 days, but other peaks exist in the periodograms. If the period is real, the connection between short bursts of magnetars and fast radio bursts should be investigated extensively.</jats:p>