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<jats:title>Abstract</jats:title> <jats:p>The scaling of the turbulent spectra provides a key measurement that allows us to discriminate between different theoretical predictions of turbulence. In the solar wind, this has driven a large number of studies dedicated to this issue using in situ data from various orbiting spacecraft. While a semblance of consensus exists regarding the scaling in the magnetohydrodynamic (MHD) and dispersive ranges, the precise scaling in the transition range and the actual physical mechanisms that control it remain open questions. Using the high-resolution data in the inner heliosphere from the Parker Solar Probe mission, we find that the sub-ion scales (i.e., at the frequency <jats:italic>f</jats:italic> ∼ [2, 9] Hz) follow a power-law spectrum <jats:italic>f</jats:italic> <jats:sup> <jats:italic>α</jats:italic> </jats:sup> with a spectral index <jats:italic>α</jats:italic> varying between −3 and −5.7. Our results also show that there is a trend toward an anticorrelation between the spectral slopes and the power amplitudes at the MHD scales, in agreement with previous studies: the higher the power amplitude the steeper the spectrum at sub-ion scales. A similar trend toward an anticorrelation between steep spectra and increasing normalized cross helicity is found, in agreement with previous theoretical predictions about the imbalanced solar wind. We discuss the ubiquitous nature of the ion transition range in solar wind turbulence in the inner heliosphere.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We report three new FRBs discovered by the Five-hundred-meter Aperture Spherical radio Telescope (FAST), namely FRB 181017.J0036+11, FRB 181118, and FRB 181130, through the Commensal Radio Astronomy FAST Survey (CRAFTS). Together with FRB 181123, which was reported earlier, all four FAST-discovered FRBs share the same characteristics of low fluence (≤0.2 Jy ms) and high dispersion measure (DM, &gt;1000 pc cm<jats:sup>−3</jats:sup>), consistent with the anticorrelation between DM and fluence of the entire FRB population. FRB 181118 and FRB 181130 exhibit band-limited features. FRB 181130 is prominently scattered (<jats:italic>τ</jats:italic> <jats:sub> <jats:italic>s</jats:italic> </jats:sub> ≃ 8 ms) at 1.25 GHz. FRB 181017.J0036+11 has full-bandwidth emission with a fluence of 0.042 Jy ms, which is one of the faintest FRB sources detected so far. CRAFTS has started to build a new sample of FRBs that fills the region for more distant and fainter FRBs in the fluence–DM<jats:sub>E</jats:sub> diagram, previously out of reach of other surveys. The implied all-sky event rate of FRBs is <jats:inline-formula> <jats:tex-math> <?CDATA ${1.24}_{-0.90}^{+1.94}\times {10}^{5}$?> </jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msubsup> <mml:mrow> <mml:mn>1.24</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>0.90</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>1.94</mml:mn> </mml:mrow> </mml:msubsup> <mml:mo>×</mml:mo> <mml:msup> <mml:mrow> <mml:mn>10</mml:mn> </mml:mrow> <mml:mrow> <mml:mn>5</mml:mn> </mml:mrow> </mml:msup> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjlabe7f0ieqn1.gif" xlink:type="simple" /> </jats:inline-formula> sky<jats:sup>−1</jats:sup> day<jats:sup>−1</jats:sup> at the 95% confidence interval above 0.0146 Jy ms. We also demonstrate here that the probability density function of CRAFTS FRB detections is sensitive to the assumed intrinsic FRB luminosity function and cosmological evolution, which may be further constrained with more discoveries.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We study X-ray spectra from the outburst rise of the accreting black hole binary MAXI J1820+070. We find that models having the disk inclinations within those of either the binary or the jet imply significant changes of the accretion disk inner radius during the luminous part of the hard spectral state, with that radius changing from &gt;100 to ∼10 gravitational radii. The main trend is a decrease with the decreasing spectral hardness. Our analysis requires the accretion flow to be structured, with at least two components with different spectral slopes. The harder component dominates the bolometric luminosity and produces strong, narrow, X-ray reflection features. The softer component is responsible for the underlying broader reflection features. The data are compatible with the harder component having a large scale height, located downstream the disk truncation radius, and reflecting mostly from remote parts of the disk. The softer component forms a corona above the disk up to some transition radius. Our findings can explain the changes of the characteristic variability timescales, found in other works, as being driven by the changes of the disk characteristic radii.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We report the time variability of the late-time radio emission in a Type I superluminous supernova (SLSN), PTF10hgi, at <jats:italic>z</jats:italic> = 0.0987. The Karl G. Jansky Very Large Array 3 GHz observations at 8.6 and 10 yr after the explosion both detected radio emission with a ∼40% decrease in flux density in the second epoch. This is the first report of a significant variability of the late-time radio light curve in an SLSN. Through combination with previous measurements in two other epochs, we constrained both the rise and decay phases of the radio light curve over three years, peaking at approximately 8–9 yr after the explosion with a peak luminosity of <jats:italic>L</jats:italic> <jats:sub>3 GHz</jats:sub> = 2 × 10<jats:sup>21</jats:sup> W Hz<jats:sup>−1</jats:sup>. Possible scenarios for the origin of the variability are an active galactic nucleus (AGN) in the host galaxy, an afterglow caused by the interaction between an off-axis jet and circumstellar medium, and a wind nebula powered by a newly born magnetar. Comparisons with models show that the radio light curve can be reproduced by both the afterglow model and magnetar wind nebula model. Considering the flat radio spectrum at 1–15 GHz and an upper limit at 0.6 GHz obtained in previous studies, plausible scenarios are a low-luminosity flat-spectrum AGN or a magnetar wind nebula with a shallow injection spectral index.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The peak frequencies of the two broad humps evident in the spectral energy density of blazars (SED) are time dependent and vary a lot between different blazars. However, their ratio in most blazars appears to be almost universal and equal to <jats:italic>m</jats:italic> <jats:sub> <jats:italic>e</jats:italic> </jats:sub> <jats:italic>c</jats:italic> <jats:sup>2</jats:sup>/4(1 + <jats:italic>z</jats:italic>)<jats:italic>ϵ</jats:italic> <jats:sub> <jats:italic>p</jats:italic> </jats:sub> to a good approximation, where <jats:italic>m</jats:italic> <jats:sub> <jats:italic>e</jats:italic> </jats:sub> is the electron mass, <jats:italic>ϵ</jats:italic> <jats:sub> <jats:italic>p</jats:italic> </jats:sub> is the peak energy of the cosmic microwave background radiation, and <jats:italic>z</jats:italic> is the redshift of the blazar. We discuss a possible origin of such a universal ratio in blazars: gamma-ray bursts (GRBs). We point out a possible connection between the knee in the energy spectrum of cosmic-ray electrons and the maximal peak energies of the two broad humps in the SED of high energy peaked blazars and GRBs. We also point out that a universal peak ratio in double hump blazars that belong to different classes in the BL Lac sequence may simply reflect different viewing angles of otherwise similar blazars.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>In 2017, the Event Horizon Telescope (EHT) Collaboration succeeded in capturing the first direct image of the center of the M87 galaxy. The asymmetric ring morphology and size are consistent with theoretical expectations for a weakly accreting supermassive black hole of mass ∼6.5 × 10<jats:sup>9</jats:sup> <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub>. The EHTC also partnered with several international facilities in space and on the ground, to arrange an extensive, quasi-simultaneous multi-wavelength campaign. This Letter presents the results and analysis of this campaign, as well as the multi-wavelength data as a legacy data repository. We captured M87 in a historically low state, and the core flux dominates over HST-1 at high energies, making it possible to combine core flux constraints with the more spatially precise very long baseline interferometry data. We present the most complete simultaneous multi-wavelength spectrum of the active nucleus to date, and discuss the complexity and caveats of combining data from different spatial scales into one broadband spectrum. We apply two heuristic, isotropic leptonic single-zone models to provide insight into the basic source properties, but conclude that a structured jet is necessary to explain M87’s spectrum. We can exclude that the simultaneous <jats:italic>γ</jats:italic>-ray emission is produced via inverse Compton emission in the same region producing the EHT mm-band emission, and further conclude that the <jats:italic>γ</jats:italic>-rays can only be produced in the inner jets (inward of HST-1) if there are strongly particle-dominated regions. Direct synchrotron emission from accelerated protons and secondaries cannot yet be excluded.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Accretion disks and coronae around massive black holes have been studied extensively, and they are known to be coupled. Over a period of 30 yr, however, the X-ray (coronal) flux of Mrk 817 increased by a factor of 40 while its UV (disk) flux remained relatively steady. Recent high-cadence monitoring finds that the X-ray and UV continua in Mrk 817 are also decoupled on timescales of weeks and months. These findings could require mechanical beaming of the innermost accretion flow, and/or an absorber that shields the disk and/or broad line region (BLR) from the X-ray corona. Herein, we report on a 135 ks observation of Mrk 817 obtained with NuSTAR, complemented by simultaneous X-ray coverage via the Neil Gehrels Swift Observatory. The X-ray data strongly prefer a standard relativistic disk reflection model over plausible alternatives. Comparable fits with related models constrain the spin to lie in the range of 0.5 ≤ <jats:italic>a</jats:italic> ≤ 1, and the viewing angle to lie between 10° ≤ <jats:italic>θ</jats:italic> ≤ 22° (including 1<jats:italic>σ</jats:italic> statistical errors and small systematic errors related to differences between the models). The spectra also reveal strong evidence of moderately ionized absorption, similar to but likely less extreme than obscuring events in NGC 5548 and NGC 3783. Archival Swift data suggest that the absorption may be variable. Particularly if the column density of this absorber is higher along the plane of the disk, it may intermittently mask or prevent coupling between the central engine, disk, and BLR in Mrk 817.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Optical depth variations in the Galactic neutral interstellar medium (ISM) with spatial scales from hundreds to thousands of astronomical units have been observed through H <jats:sc>i</jats:sc> absorption against pulsars and continuum sources, while extremely small structures with spatial scales of tens of astronomical units remain largely unexplored. The nature and formation of such tiny-scale atomic structures (TSAS) need to be better understood. We report a tentative detection of TSAS with a signal-to-noise ratio of 3.2 toward PSR B1557−50 in the second epoch of two Parkes sessions just 0.36 yr apart, which are the closest-spaced spectral observations toward this pulsar. One absorption component showing marginal variations has been identified. Based on the pulsar’s proper motion of 14 mas yr<jats:sup>−1</jats:sup> and the component’s kinematic distance of 3.3 kpc, the corresponding characteristic spatial scale is 17 au, which is among the smallest sizes of known TSAS. Assuming a similar line-of-sight (LOS) depth, the tentative TSAS cloud detected here is overdense and overpressured relative to the cold neutral medium (CNM), and can radiatively cool fast enough to be in thermal equilibrium with the ambient environment. We find that turbulence is not sufficient to confine the overpressured TSAS. We explore the LOS elongation that would be required for the tentative TSAS to be at the canonical CNM pressure, and find that it is ∼5000—much larger than filaments observed in the ISM. We see some evidence of line width and temperature variations in the CNM components observed at the two epochs, as predicted by models of TSAS-like cloud formation colliding warm neutral medium flows.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Compact objects are expected to exist in the accretion disks of supermassive black holes (SMBHs) in active galactic nuclei (AGNs), and in the presence of such a dense environment (∼10<jats:sup>14</jats:sup> cm<jats:sup>−3</jats:sup>), they will form a new kind of stellar population denoted as accretion-modified stars (AMSs). This hypothesis is supported by recent LIGO/Virgo detection of the mergers of very high-mass stellar binary black holes (BHs). We show that the AMSs will be trapped by the SMBH disk within a typical AGN lifetime. In the context of SMBH disks, the rates of Bondi accretion onto BHs are ∼10<jats:sup>9</jats:sup> <jats:italic>L</jats:italic> <jats:sub>Edd</jats:sub>/<jats:italic>c</jats:italic> <jats:sup>2</jats:sup>, where <jats:italic>L</jats:italic> <jats:sub>Edd</jats:sub> is the Eddington luminosity and <jats:italic>c</jats:italic> is the speed of light. Outflows developed from the hyper-Eddington accretion strongly impact the Bondi sphere and induce episodic accretion. We show that the hyper-Eddington accretion will be halted after an accretion interval of <jats:italic>t</jats:italic> <jats:sub>a</jats:sub> ∼ 10<jats:sup>5</jats:sup> <jats:italic>m</jats:italic> <jats:sub>1</jats:sub> s, where <jats:italic>m</jats:italic> <jats:sub>1</jats:sub> = <jats:italic>m</jats:italic> <jats:sub>•</jats:sub>/10<jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub> is the BH mass. The kinetic energy of the outflows accumulated during <jats:italic>t</jats:italic> <jats:sub>a</jats:sub> is equivalent to 10 supernovae driving an explosion of the Bondi sphere and developing blast waves. We demonstrate that a synchrotron flare from relativistic electrons accelerated by the blast waves peaks in the soft X-ray band (∼0.1 keV), significantly contributing to the radio, optical, UV, and soft X-ray emission of typical radio-quiet quasars. External inverse Compton scattering of the electrons peaks around 40 GeV and is detectable through Fermi-LAT. The flare, decaying with <jats:italic>t</jats:italic> <jats:sup>−6/5</jats:sup> with a few months, will appear as a slowly varying transient. The flares, occurring at a rate of a few per year in radio-quiet quasars, provide a new mechanism for explaining AGN variability.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>As a powerful source of gravitational waves (GW), a supermassive black hole (SMBH) merger may be accompanied by a relativistic jet that leads to detectable electromagnetic (EM) emission. We model the propagation of post-merger jets inside a pre-merger wind bubble formed by disk winds, and calculate multiwavelength EM spectra from the forward shock region. We show that the nonthermal EM signals from SMBH mergers are detectable up to the detection horizon of future GW facilities such as the Laser Interferometer Space Antenna (LISA). Calculations based on our model predict slowly fading transients with time delays from days to months after the coalescence, leading to implications for EM follow-up observations after the GW detection.</jats:p>