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<jats:title>Abstract</jats:title> <jats:p>X-ray line fluorescence is ubiquitous around powerful accretion sources, namely, active galactic nuclei and X-ray binaries. The brightest and best-studied line is the Fe K<jats:italic>α</jats:italic> line at <jats:italic>λ</jats:italic> = 1.937 Å(6.4 keV). This paper presents a survey of all well-measured Chandra/HETG grating spectra featuring several K<jats:italic>α</jats:italic> fluorescence lines from elements between Mg and Ni. Despite the variety of sources and physical conditions, we identify a common trend that dictates the K<jats:italic>α</jats:italic> line intensity ratios between elements. For the most part, the line intensities are well described by a simple, plane-parallel approximation of a near-neutral, solar-abundance, high column density (<jats:italic>N</jats:italic> <jats:sub>H</jats:sub> &gt; 10<jats:sup>24</jats:sup> cm<jats:sup>−2</jats:sup>) medium. This approximation gives canonical photon-intensity line ratios for the K<jats:italic>α</jats:italic> fluorescence of all elements, e.g., 0.104:0.069:1.0:0.043 for Si:S:Fe:Ni, respectively. Deviations from these ratios are shown to be primarily due to excess column density along the line of sight beyond the Galactic column. Therefore, measured fluorescence line ratios provide an independent estimate of <jats:italic>N</jats:italic> <jats:sub>H</jats:sub> and insight into the environment of accretion sources. Residual discrepancies with the canonical ratios could be due to a variety of effects such as a fluorescing medium with <jats:italic>N</jats:italic> <jats:sub>H</jats:sub> &lt; 10<jats:sup>24</jats:sup> cm<jats:sup>−2</jats:sup>, a non-neutral medium, variations in the illuminating spectrum, nonsolar abundances, or an irregular source geometry. However, evidently and perhaps surprisingly, these are uncommon, and their effect remains minor.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The 774 AD solar proton event (SPE) detected in cosmogenic nuclides had an inferred &gt;1 GV (&gt;430 MeV) fluence estimated to have been ∼30–70 times larger than that of the 1956 February 23 ground level event (GLE). The 1956 GLE was itself ∼2.5 times larger at &gt;430 MeV than the episode of strong GLE activity from 1989 August–October. We use an inferred soft X-ray (SXR) class of X20 ± 10 for the 1956 February 23 eruptive flare as a bridge to the source flare for the 774 SPE. A correlation of the &gt;200 MeV proton fluences of hard-spectra post-1975 GLEs with the SXR peak fluxes of their associated flares yields an SXR flare class of X285 ± 140 (bolometric energy of ∼(1.9 ± 0.7) × 10<jats:sup>33</jats:sup> erg) for the 774 flare. This estimate is within theoretical determinations of the largest flare the Sun could produce based on the largest spot group yet observed. Assuming a single eruptive flare source for the 774 SPE, the above estimate indicates that the Sun can produce a threshold-level 10<jats:sup>33</jats:sup> erg superflare. If the 774 event originated in two closely timed, equal-fluence SPEs, the inferred flare size drops to X180 ± 90 (∼(1.4 ± 0.5) × 10<jats:sup>33</jats:sup> erg). We speculate on favorable solar conditions that can lead to enhanced shock acceleration of high-energy protons in eruptive flares.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We statistically study the relationship between the Ly<jats:italic>α</jats:italic> and 1–8 Å soft X-ray (SXR) emissions from 658 M- and X-class solar flares observed by the Geostationary Operational Environmental Satellite during 2006–2016. Based on the peak times of the two wave band emissions, we divide the flares into three types. Type I (III) has an earlier (a later) peak time in the Ly<jats:italic>α</jats:italic> emission than that in the SXR emission, while type II has nearly the same peak time (within the time resolution of 10 s) between the Ly<jats:italic>α</jats:italic> and SXR emissions. In these 658 flares, we find that there are 505 (76.8%) type I flares, 10 (1.5%) type II flares, and 143 (21.7%) type III flares, and that the three types appear to have no dependence on the flare duration, flare location, or solar cycle. Besides the main peak, the Ly<jats:italic>α</jats:italic> emission of the three type flares also shows sub-peaks which can appear in the impulsive or gradual phase of the flare. It is found that the main-peak (for type I) and subpeak (for type III) emissions of Ly<jats:italic>α</jats:italic> that appear in the impulsive phase follow the Neupert effect in general. This indicates that such Ly<jats:italic>α</jats:italic> emissions are related to the nonthermal electron beam heating. While the main-peak (for type III) and subpeak (for type I) emissions of Ly<jats:italic>α</jats:italic> that appear in the gradual phase are supposed to be primarily contributed by the thermal plasma that cools down.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We report significant hard X-ray excesses in the energy range 2–8 keV for two nearby isolated neutron stars: RX J1856.6−3754 and RX J0420.0−5022. These neutron stars have previously been observed in soft X-rays to have nearly thermal spectra at temperatures ∼100 eV, which are thought to arise from the warm neutron star surfaces. We find nontrivial hard X-ray spectra well above the thermal surface predictions with archival data from the XMM-Newton and Chandra X-ray telescopes. We analyze possible systematic effects that could generate such spurious signals, such as nearby X-ray point sources and pileup of soft X-rays, but we find that the hard X-ray excesses are robust to these systematics to the extent that is possible to test. We also investigate possible sources of hard X-ray emission from the neutron stars and find no satisfactory explanation with known mechanisms, suggesting that a novel source of X-ray emission is at play. We do not find high-significance hard X-ray excesses from the other five Magnificent Seven isolated neutron stars.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We present a large high-resolution study of the distribution and evolution of C <jats:sc>iv</jats:sc> absorbers, including the weakest population with equivalent widths <jats:inline-formula> <jats:tex-math> <?CDATA ${W}_{r}\lt 0.3\,\mathring{\rm A} $?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabbe0bieqn1.gif" xlink:type="simple" /> </jats:inline-formula>. By searching 369 high-resolution, high signal-to-noise ratio spectra of quasars at <jats:inline-formula> <jats:tex-math> <?CDATA $1.1\leqslant {z}_{\mathrm{em}}\leqslant 5.3$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabbe0bieqn2.gif" xlink:type="simple" /> </jats:inline-formula> from Keck/HIRES and VLT/UVES, we find 1268 C <jats:sc>iv</jats:sc> absorbers with <jats:inline-formula> <jats:tex-math> <?CDATA ${W}_{r}\geqslant 0.05$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabbe0bieqn3.gif" xlink:type="simple" /> </jats:inline-formula> Å (our ∼50% completeness limit) at redshifts <jats:inline-formula> <jats:tex-math> <?CDATA $1.0\leqslant z\leqslant 4.75$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabbe0bieqn4.gif" xlink:type="simple" /> </jats:inline-formula>. A Schechter function describes the observed equivalent width distribution with a transition from power-law to exponential decline at <jats:inline-formula> <jats:tex-math> <?CDATA ${W}_{r}\gtrsim 0.5\,\mathring{\rm A} $?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabbe0bieqn5.gif" xlink:type="simple" /> </jats:inline-formula>. The power-law slope <jats:italic>α</jats:italic> rises by ∼7%, and the transition equivalent width <jats:italic>W</jats:italic> <jats:sub>⋆</jats:sub> falls by ∼20% from <jats:inline-formula> <jats:tex-math> <?CDATA $\langle z\rangle =1.7$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabbe0bieqn6.gif" xlink:type="simple" /> </jats:inline-formula> to 3.6. We find that the comoving redshift path density, <jats:inline-formula> <jats:tex-math> <?CDATA ${dN}/{dX}$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabbe0bieqn7.gif" xlink:type="simple" /> </jats:inline-formula>, of <jats:inline-formula> <jats:tex-math> <?CDATA ${W}_{r}\geqslant 0.05$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabbe0bieqn8.gif" xlink:type="simple" /> </jats:inline-formula> Å absorbers rises by ∼1.8 times from <jats:italic>z</jats:italic> ≃  4.0 to 1.3, while the <jats:inline-formula> <jats:tex-math> <?CDATA ${W}_{r}\geqslant 0.6$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabbe0bieqn9.gif" xlink:type="simple" /> </jats:inline-formula> Å <jats:inline-formula> <jats:tex-math> <?CDATA ${dN}/{dX}$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabbe0bieqn10.gif" xlink:type="simple" /> </jats:inline-formula> rises by a factor of ∼8.5. We quantify the observed evolution by a model in which <jats:inline-formula> <jats:tex-math> <?CDATA ${dN}/{dX}$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabbe0bieqn11.gif" xlink:type="simple" /> </jats:inline-formula> decreases linearly with increasing redshift. The model suggests that populations with larger <jats:inline-formula> <jats:tex-math> <?CDATA ${W}_{r}$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabbe0bieqn12.gif" xlink:type="simple" /> </jats:inline-formula> thresholds evolve faster with redshift and appear later in the universe. The cosmological <jats:sc>Technicolor Dawn</jats:sc> simulations at <jats:italic>z</jats:italic> = 3–5 overproduce the observed abundance of absorbers with <jats:inline-formula> <jats:tex-math> <?CDATA ${W}_{r}\leqslant 0.3\,\mathring{\rm A} $?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabbe0bieqn13.gif" xlink:type="simple" /> </jats:inline-formula> while yielding better agreement at higher <jats:inline-formula> <jats:tex-math> <?CDATA ${W}_{r}$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabbe0bieqn14.gif" xlink:type="simple" /> </jats:inline-formula>. Our empirical linear model successfully describes C <jats:sc>iv</jats:sc> evolution in the simulations and the observed evolution of <jats:inline-formula> <jats:tex-math> <?CDATA ${W}_{r}\geqslant 0.6$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabbe0bieqn15.gif" xlink:type="simple" /> </jats:inline-formula> Å C <jats:sc>iv</jats:sc> for the past <jats:inline-formula> <jats:tex-math> <?CDATA $\sim 12\,\mathrm{Gyr}$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabbe0bieqn16.gif" xlink:type="simple" /> </jats:inline-formula>. Combining our measurements with the literature gives us a picture of C <jats:sc>iv</jats:sc> absorbing structures becoming more numerous and/or larger in physical size over the last <jats:inline-formula> <jats:tex-math> <?CDATA $\approx 13\,\mathrm{Gyr}$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabbe0bieqn17.gif" xlink:type="simple" /> </jats:inline-formula> of cosmic time (<jats:italic>z</jats:italic> ∼ 6–0).</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Theoretical modeling of massive stars predicts a gap in the black hole (BH) mass function above ∼40–50 <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub> for BHs formed through single star evolution, arising from (pulsational) pair-instability supernovae (PISNe). However, in dense star clusters, dynamical channels may exist that allow construction of BHs with masses in excess of those allowed from single star evolution. The detection of BHs in this so-called “upper-mass gap” would provide strong evidence for the dynamical processing of BHs prior to their eventual merger. Here, we explore in detail the formation of BHs with masses within or above the pair-instability gap through collisions of young massive stars in dense star clusters. We run a suite of 68 independent cluster simulations, exploring a variety of physical assumptions pertaining to growth through stellar collisions, including primordial cluster mass segregation and the efficiency of envelope stripping during collisions. We find that as many as ∼20% of all BH progenitors undergo one or more collisions prior to stellar collapse and up to ∼1% of all BHs reside within or above the pair-instability gap through the effects of these collisions. We show that these BHs readily go on to merge with other BHs in the cluster, creating a population of massive BH mergers at a rate that may compete with the “multiple-generation” merger channel described in other analyses. This has clear relevance for the formation of very massive BH binaries as recently detected by the Laser Interferometer Gravitational-Wave Observatory/Virgo in GW190521. Finally, we describe how stellar collisions in clusters may provide a unique pathway to PISNe and briefly discuss the expected rate of these events and other electromagnetic transients.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The core–cusp problem is one of the controversial issues in the standard paradigm of Λ cold dark matter (ΛCDM) theory. However, under the assumption of conventional spherical symmetry, the strong degeneracy among model parameters makes it unclear whether dwarf spheroidal (dSph) galaxies indeed have cored dark matter density profiles at their centers. In this work, we revisit this problem using nonspherical mass models, which have the advantage of being able to alleviate the degeneracy. Applying our mass models to the currently available kinematic data of the eight classical dSphs, we find that within finite uncertainties, most of these dSphs favor cusped central profiles rather than cored ones. In particular, Draco has a cusped dark matter halo with high probability even considering a prior bias. We also find the diversity of the inner slopes in their dark matter halos. To clarify the origin of this diversity, we investigate the relation between the inner dark matter density slope and stellar-to-halo mass ratio for the sample dSphs and find that this relation is generally in agreement with the predictions from recent ΛCDM and hydrodynamical simulations. We also find that the simulated subhalos have an anticorrelation between the dark matter density at 150 pc and pericenter distance, which is consistent with the observed one. We estimate their astrophysical factors for dark matter indirect searches and circular velocity profiles associated with huge uncertainties. To more precisely estimate their dark matter profiles, wide-field spectroscopic surveys for the dSphs are essential.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We prove from the modified gravity (MG) galaxy/halo mock catalogs that the shape of the pairwise kinematic Sunyaev–Zeldovich (kSZ) power spectrum <jats:italic>P</jats:italic> <jats:sub>kSZ</jats:sub> has constraining power on discriminating different gravity theories on cosmological scales. By varying the effective optical depth <jats:italic>τ</jats:italic> <jats:sub>T</jats:sub> as a free parameter, we verify that the <jats:italic>τ</jats:italic> <jats:sub>T</jats:sub>-<jats:italic>f</jats:italic> (the linear growth rate) degeneracy in the linear theory of <jats:italic>P</jats:italic> <jats:sub>kSZ</jats:sub> is broken down by the nonlinear structure growth and the scale dependence of <jats:italic>f</jats:italic> in some MG theories. Equivalently speaking, the shape of <jats:italic>P</jats:italic> <jats:sub>kSZ</jats:sub> alone could be used to tightly constrain the MG theories on cosmological scales. With good knowledge of galaxy density biases, we verify that a combination of the next-generation galaxy spectroscopic redshift and CMB surveys, e.g., BOSS+CMB-S4 or DESI+CMB-S4, could potentially discriminate <jats:italic>f</jats:italic>(<jats:italic>R</jats:italic>) models from the general relativity at the ∼5<jats:italic>σ</jats:italic> level using the shape of the galaxy pairwise kSZ dipole <jats:italic>P</jats:italic> <jats:sub>kSZ,<jats:italic>ℓ</jats:italic> = 1</jats:sub> alone, when <jats:italic>f</jats:italic> <jats:sub> <jats:italic>R</jats:italic>0</jats:sub> = 10<jats:sup>−4</jats:sup>.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The nearby (3.8 Mpc) galaxy NGC 4945 hosts a nuclear starburst and Seyfert type 2 active galactic nucleus (AGN). We use the Atacama Large Millimeter/submillimeter Array (ALMA) to image the 93 GHz (3.2 mm) free–free continuum and hydrogen recombination line emission (H40<jats:italic>α</jats:italic> and H42<jats:italic>α</jats:italic>) at 2.2 pc (0.″12) resolution. Our observations reveal 27 bright, compact sources with FWHM sizes of 1.4–4.0 pc, which we identify as candidate super star clusters. Recombination line emission, tracing the ionizing photon rate of the candidate clusters, is detected in 15 sources, six of which have a significant synchrotron component to the 93 GHz continuum. Adopting an age of ∼5 Myr, the stellar masses implied by the ionizing photon luminosities are <jats:inline-formula> <jats:tex-math> <?CDATA ${\mathrm{log}}_{10}$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabb67dieqn1.gif" xlink:type="simple" /> </jats:inline-formula>(<jats:italic>M</jats:italic> <jats:sub>⋆</jats:sub>/M<jats:sub>⊙</jats:sub>) ≈ 4.7–6.1. We fit a slope to the cluster mass distribution and find <jats:italic>β</jats:italic> = −1.8 ± 0.4. The gas masses associated with these clusters, derived from the dust continuum at 350 GHz, are typically an order of magnitude lower than the stellar mass. These candidate clusters appear to have already converted a large fraction of their dense natal material into stars and, given their small freefall times of ∼0.05 Myr, are surviving an early volatile phase. We identify a pointlike source in 93 GHz continuum emission that is presumed to be the AGN. We do not detect recombination line emission from the AGN and place an upper limit on the ionizing photons that leak into the starburst region of <jats:italic>Q</jats:italic> <jats:sub>0</jats:sub> &lt; 10<jats:sup>52</jats:sup> s<jats:sup>−1</jats:sup>.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The strong equivalence principle (SEP) distinguishes general relativity (GR) from other viable theories of gravity. The SEP demands that the internal dynamics of a self-gravitating system under freefall in an external gravitational field should not depend on the external field strength. We test the SEP by investigating the external field effect (EFE) in Milgromian dynamics (MOND), proposed as an alternative to dark matter in interpreting galactic kinematics. We report a detection of this EFE using galaxies from the Spitzer Photometry and Accurate Rotation Curves (SPARC) sample together with estimates of the large-scale external gravitational field from an all-sky galaxy catalog. Our detection is threefold: (1) the EFE is individually detected at 8<jats:italic>σ</jats:italic> to 11<jats:italic>σ</jats:italic> in “golden” galaxies subjected to exceptionally strong external fields, while it is not detected in exceptionally isolated galaxies, (2) the EFE is statistically detected at more than 4<jats:italic>σ</jats:italic> from a blind test of 153 SPARC rotating galaxies, giving a mean value of the external field consistent with an independent estimate from the galaxies’ environments, and (3) we detect a systematic downward trend in the weak gravity part of the radial acceleration relation at the right acceleration predicted by the EFE of the MOND modified gravity. Tidal effects from neighboring galaxies in the Λ cold dark matter (CDM) context are not strong enough to explain these phenomena. They are not predicted by existing ΛCDM models of galaxy formation and evolution, adding a new small-scale challenge to the ΛCDM paradigm. Our results point to a breakdown of the SEP, supporting modified gravity theories beyond GR.</jats:p>