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The detection of cyclotron resonance scattering features (CRSFs) is the only way to directly and reliably measure the magnetic field near the surface of a neutron star (NS). The broad energy coverage and large collection area of Insight-HXMT in the hard X-ray band allowed us to detect the CRSF with the highest energy known to date, reaching about 146 keV during the 2017 outburst of the first galactic pulsing ultraluminous X-ray source (pULX) Swift J0243.6+6124. During this outburst, the CRSF was only prominent close to the peak luminosity of ∼2 × 1039 erg s−1, the highest to date in any of the Galactic pulsars. The CRSF is most significant in the spin-phase region corresponding to the main pulse of the pulse profile, and its centroid energy evolves with phase from 120 to 146 keV. We identify this feature as the fundamental CRSF because no spectral feature exists at 60–70 keV. This is the first unambiguous detection of an electron CRSF from an ULX. We also estimate a surface magnetic field of ∼1.6 × 1013 G for Swift J0243.6+6124. Considering that the dipole magnetic field strengths, inferred from several independent estimates of magnetosphere radius, are at least an order of magnitude lower than our measurement, we argue that the detection of the highest-energy CRSF reported here unambiguously proves the presence of multipole field components close to the surface of the neutron star. Such a scenario has previously been suggested for several pulsating ULXs, including Swift J0243.6+6124, and our result represents the first direct confirmation of this scenario.

We present a methodology for modeling the joint ionizing impact due to a “simple X-ray population” (SXP) and its corresponding simple stellar population (SSP), where “simple” refers to a single age and metallicity population. We construct composite spectral energy distributions (SEDs) including contributions from ultraluminous X-ray sources and stars, with physically meaningful and consistent consideration of the relative contributions of each component as a function of instantaneous burst age and stellar metallicity. These composite SEDs are used as input for photoionization modeling with Cloudy, from which we produce a grid for the time- and metallicity-dependent nebular emission from these composite populations. We make the results from the photoionization simulations publicly available. We find that the addition of the SXP prolongs the high-energy ionizing output from the population—and correspondingly increases the intensity of nebular lines such as He ii λ1640,4686, [Ne v] λ3426,14.3 μm, and [O iv] 25.9 μm by factors of at least two relative to models without an SXP spectral component. This effect is most pronounced for instantaneous bursts of star formation on timescales >10 Myr and at low metallicities (∼0.1 Z ⊙), due to the imposed time- and metallicity-dependent behavior of the SXP relative to the SSP. We propose nebular emission line diagnostics accessible with JWST suitable for inferring the presence of a composite SXP + SSP, and we discuss how the ionization signatures compare to models for sources such as intermediate-mass black holes.

We summarize results from a survey of radiation-dominated black hole accretion flows across a wide range of mass accretion rates, as well as two values of black hole spin and initial magnetic field geometry. These models apply an algorithm targeting direct solutions to the radiation transport equation in full general relativity and have been enabled by access to modern exascale computing systems. Super-Eddington accretion flows form geometrically thick radiation-pressure-supported disks that drive powerful equatorial outflows. A narrow funnel-shaped photosphere in the inner region results in very low radiative efficiencies in this regime. The structure of near- and sub-Eddington accretion depends on whether there is net vertical magnetic flux at the midplane of the disk. With net flux, the disk forms a thin, dense layer at the midplane surrounded by a magnetically dominated corona, whereas without net flux, the disk remains magnetically dominated everywhere. Although none of our models achieve the magnetically arrested disk regime, those with net vertical flux and a rapidly spinning black hole still produce powerful relativistic jets. Our calculations adopt simple opacity models (with scalings appropriate to stellar-mass black hole accretion). We discuss the application of our results to observations of X-ray binaries and ultraluminous X-ray sources such as Cyg X-3 and SS433. We also speculate on the application of our super-Eddington models to the interpretation of little red dots recently discovered by JWST.

Ultraluminous X-ray sources (ULXs) are our best laboratories for studying extreme super-Eddington accretion. Most studies of these objects are of relatively persistent sources; however, there is growing evidence to suggest a large fraction of these sources are transient. Here we present a sample of five newly reported transient ULXs in the galaxies NGC 4945, NGC 7793, and M81 serendipitously discovered in Swift/XRT observations. Swift monitoring of these sources have provided well-sampled lightcurves, allowing for us to model the lightcurves with the disk-instability model of Hameury & Lasota, which implies durations of 60–400 days and that the mass-accretion rate through the disk is close to or greater than the Eddington rate. Of the three source regions with prior Hubble Space Telescope imaging, color–magnitude diagrams of the potential stellar counterparts show varying ages of the possible stellar counterparts. Our estimation of the rates of these sources in these three galaxies is 0.4–1.3 yr−1. We find that, while persistent ULXs dominate the high end of galaxy luminosity functions, the number of systems that produce ULX luminosities are likely dominated by transient sources.

We present the first X-ray census of fast radio burst (FRB) host galaxies to conduct the deepest search for active galactic nuclei (AGN) and X-ray counterparts to date. Our sample includes seven well-localized FRBs with unambiguous host associations and existing deep Chandra observations, including two events for which we present new observations. We find evidence for AGN in two FRB host galaxies based on the presence of X-ray emission coincident with their centers, including the detection of a luminous (L X ≈ 5 × 1042 erg s−1) X-ray source at the nucleus of FRB 20190608B’s host, for which we infer an SMBH mass of M BH ∼ 108 M ⊙ and an Eddington ratio L bol/L Edd ≈ 0.02, characteristic of geometrically thin disks in Seyfert galaxies. We also report nebular emission-line fluxes for 24 highly secure FRB hosts (including 10 hosts for the first time), and assess their placement on a BPT diagram, finding that FRB hosts trace the underlying galaxy population. We further find that the hosts of repeating FRBs are not confined to the star-forming locus, contrary to previous findings. Finally, we place constraints on associated X-ray counterparts to FRBs in the context of ultraluminous X-ray sources (ULXs), and find that existing X-ray limits for FRBs rule out ULXs brighter than L X ≳ 1040 erg s−1. Leveraging the CHIME/FRB catalog and existing ULX catalogs, we search for spatially coincident ULX–FRB pairs. We identify a total of 28 ULXs spatially coincident with the localization regions for 17 FRBs, but find that the DM-inferred redshifts for the FRBs are inconsistent with the ULX redshifts, disfavoring an association between these specific ULX–FRB pairs.

We present a new five-epoch Chandra X-ray Observatory monitoring survey of the nearby spiral galaxy M33 which probes X-ray variability with time sampling between two weeks and four months. We characterize the X-ray variability of 55 bright point sources outside of the nucleus, many of which are expected to be high-mass X-ray binaries (HMXBs). We detect eight new candidate transients not detected in previous X-ray catalogs of M33 and discuss their possible nature. The final catalog includes 26 known HMXB candidates identified in the literature. We extend the baseline of the X-ray light curves up to 21 yr by including archival X-ray observations of these sources. We compare the detection and nondetection epochs of the sources to suites of simulated source duty cycles and infer that most of our detected sources have duty cycles >30%. We find only four sources whose detection patterns are consistent with having duty cycles below 30%. This large fraction of sources with high duty cycles is unexpected for a population of HMXBs; thus more frequent X-ray monitoring will likely reveal many more low duty cycle HMXBs in M33.

We report on the intriguing properties of a variable X-ray source projected at the outskirts of the elliptical galaxy NGC 6099 (d ≈ 139 Mpc). If truly located near NGC 6099, this is a hyperluminous X-ray source that reached an X-ray luminosity LX ≈ a few times 1042 erg s−1 in 2012 February (XMM-Newton data), about 50–100 times brighter than in 2009 May (Chandra) and 2023 August (XMM-Newton). The X-ray spectrum was soft at all three epochs, with a thermal component at kT ≈ 0.2 keV and a power-law photon index >3. Such properties make it a strong candidate for an intermediate-mass black hole (IMBH). We also discovered a point-like, blue optical counterpart (mg,Vega ≈ 24.7 mag, Mg,Vega ≈ −11.2 mag), from images taken by the Canada–France–Hawaii Telescope and later confirmed with Hubble Space Telescope observations. The optical continuum can be modeled as stellar emission from a compact star cluster or an X-ray-irradiated accretion disk, consistent with the IMBH scenario. We discuss alternative explanations for the nature of this system. A possible scenario is tidal stripping of an orbiting star, with repeated X-ray outbursts every few years. An alternative possibility is that the thermal X-ray emission seen in 2009 was from shocked gas in the self-intersecting tidal stream during the rising phase of a tidal disruption event, while the 2012 and 2023 emissions were from the fully formed accretion disk.

We use the Athena++ Monte Carlo (MC) radiation transfer module to postprocess simulation snapshots from nonrelativistic Athena++ radiation magnetohydrodynamic (RMHD) simulations. These simulations were run using a gray (frequency-integrated) approach but were also restarted and ran with a multigroup approach that accounts for Compton scattering with a Kompaneets operator. These simulations produced moderately super-Eddington accretion rates onto a 6.62 M ⊙ black hole. Since we only achieve inflow equilibrium out to 20–25 gravitational radii, we focus on the hard X-ray emission. We provide a comparison between the MC and RMHD simulations, showing that the treatment of Compton scattering in the gray RMHD simulations underestimates the gas temperature in the funnel regions above and below the accretion disk. In contrast, the restarted multigroup snapshots provide a treatment for the radiation field that is more consistent with the MC calculations, and result in postprocessed spectra with harder X-ray emission compared to their gray snapshot counterparts. We characterize these MC postprocessed spectra using commonly employed phenomenological spectral fitting models. We also attempt to fit our MC spectra directly to observations of the ultraluminous X-ray source (ULX) NGC 1313 X-1, finding best-fit values that are competitive to phenomenological model fits, indicating that first principle models of super-Eddington accretion may adequately explain the observed hard X-ray spectra in some ULX sources.

Strong high-ionization lines such as He ii of young galaxies are puzzling at high and low redshift. Although recent studies suggest the existence of nonthermal sources, whether their ionizing spectra can consistently explain multiple major emission lines remains a question. Here we derive the general shapes of the ionizing spectra for three local extremely metal-poor galaxies (EMPGs) that show strong He ii λ4686. We parameterize the ionizing spectra composed of a blackbody and power-law radiation mimicking various stellar and nonthermal sources. We use photoionization models for nebulae and determine seven parameters of the ionizing spectra and nebulae by Markov Chain Monte Carlo methods, carefully avoiding systematics of abundance ratios. We obtain the general shapes of ionizing spectra explaining ∼10 major emission lines within observational errors with smooth connections from observed X-ray and optical continua. We find that an ionizing spectrum of one EMPG has a blackbody-dominated shape, while the others have convex downward shapes at >13.6 eV, which indicate a diversity of the ionizing spectrum shapes. We confirm that the convex downward shapes are fundamentally different from ordinary stellar spectrum shapes, and that the spectrum shapes of these galaxies are generally explained by the combination of the stellar and ultraluminous X-ray sources. Comparisons with stellar synthesis models suggest that the diversity of the spectrum shapes arises from differences in the stellar age. If galaxies at z ≳ 6 are similar to the EMPGs, high-energy (>54.4 eV) photons of the nonstellar sources negligibly contribute to cosmic reionization due to relatively weak radiation.

As the nearest confirmed Lyman continuum (LyC) emitter, Haro 11 is an exceptional laboratory for studying LyC escape processes crucial to cosmic reionization. Our new Hubble Space Telescope/Cosmic Origins Spectrograph G130M/1055 observations of its three star-forming knots now reveal that the observed LyC originates in Knots B and C, with 903–912 Å luminosities of 1.9 ± 1.5 × 1040 erg s−1 and 0.9 ± 0.7 × 1040 erg s−1, respectively. We derive local escape fractions f esc,912 = 3.4% ± 2.9% and 5.1% ± 4.3% for Knots B and C, respectively. Our Starburst99 modeling shows dominant populations on the order of ∼1–4 Myr and 1–2 × 107 M ⊙ in each knot, with the youngest population in Knot B. Thus, the knot with the strongest LyC detection has the highest LyC production. However, LyC escape is likely less efficient in Knot B than in Knot C due to higher neutral gas covering. Our results therefore stress the importance of the intrinsic ionizing luminosity, and not just the escape fraction, for LyC detection. Similarly, the Lyα escape fraction does not consistently correlate with LyC flux, nor do narrow Lyα red peaks. High observed Lyα luminosity and low Lyα peak velocity separation, however, do correlate with higher LyC escape. Another insight comes from the undetected Knot A, which drives the Green Pea properties of Haro 11. Its density-bounded conditions suggest highly anisotropic LyC escape. Finally, both of the LyC-leaking Knots, B and C, host ultraluminous X-ray sources (ULXs). While stars strongly dominate over the ULXs in LyC emission, this intriguing coincidence underscores the importance of unveiling the role of accretors in LyC escape and reionization.