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  • On the Extremely X-Ray Variable Active Galactic Nuclei in the XMM-LSS Field
    We present a systematic investigation of extremely X-ray variable active galactic nuclei (AGNs) in the ≈5.3 deg2 XMM-SERVS XMM-LSS region. Eight variable AGNs are identified with rest-frame 2 keV flux density variability amplitudes around 6–12. We comprehensively analyze the X-ray and multiwavelength data to probe the origin of their extreme X-ray variability. It is found that their extreme X-ray variability can be ascribed to changing accretion state or changing obscuration from dust-free absorbers. For five AGNs, their X-ray variability is attributed to changing accretion state, supported by contemporaneous multiwavelength variability and the absence of X-ray absorption in the low-state spectra. With new Multiple Mirror Telescope (MMT) spectra for four of these sources, we confirm one changing-look AGN. One MMT AGN lacks multiepoch spectroscopic observations, while the other two AGNs do not exhibit changing-look behavior, likely because the MMT observations did not capture their high states. The X-ray variability of the other three AGNs is explained by changing obscuration, and they show only mild long-term optical/IR variability. The absorbers of these sources are likely clumpy accretion-disk winds, with variable column densities and covering factors along the lines of sight.

  • Core Revelations: The Star Formation and Active Galactic Nucleus Connection at the Heart of NGC 7469
    We investigate the star formation–active galactic nucleus (AGN) connection in the Seyfert 1 NGC 7469 using James Webb Space Telescope mid-infrared spectroscopic integral field unit (IFU) data. We use the IFU data to generate maps of different emission lines present in the spectrum, such as the star formation (SF) tracer [Ne ii] 12.81 μm and the AGN tracer [Ne v] 14.32 μm. We can separate the AGN- and SF-dominated regions using spatially resolved mid-infrared diagnostic diagrams, and we further investigate the ionization sources powering each region by constructing photoionization models. We find that the previously detected eastern wind populates an intermediary region of the diagrams, between our star-forming and AGN points. Although it is possible that the star-forming ring may inherently not be uniform, this wind also coincides with a reduction in the [Ne ii] emission in the ring, which suggests that the ionization cone intersects the ring in this direction.

  • Subphotospheric Emission from Short Gamma-Ray Bursts. II. Signatures of Nonthermal Dissipation in the Multi-messenger Signals
    Building on a general relativistic magnetohydrodynamic simulation of a short gamma-ray burst (sGRB) jet with initial magnetization σ0 = 150, propagating through the dynamical ejecta from a binary neutron star merger, we identify regions of energy dissipation driven by magnetic reconnection and collisionless subshocks within different scenarios. We solve the transport equations for photons, electrons, protons, neutrinos, and intermediate particles up to the photosphere, accounting for all relevant radiative processes, including electron and proton acceleration, and investigate the potential impact of magnetic reconnection occurring in different regions along the jet. We find the photon spectra undergo nonthermal modifications below the photosphere, observable in both on-axis and off-axis emission directions, as well as across different scenarios of energy dissipation and subsequent particle acceleration. Interestingly, the spectral index of the photon energy distribution can vary at most by ∼20% across all different dissipation scenarios. Depending on the dissipation mechanism at play, neutrino signatures may accompany the photon signal, pointing to efficient proton acceleration and shedding light on jet physics. Although our findings are based on one jet simulation, they point to a potential universal origin of the nonthermal features of the Band spectrum observed in sGRBs.

  • Radiation Transport Simulations of Quasiperiodic Eruptions from Star–Disk Collisions
    Periodic collisions between a star on an inclined orbit around a supermassive black hole and its accretion disk offer a promising explanation for X-ray “quasiperiodic eruptions” (QPEs). Each passage through the disk midplane shocks and compresses gas ahead of the star, which subsequently re-expands above the disk as a quasi-spherical cloud. We present spherically symmetric Monte Carlo radiation transport simulations that follow the production of photons behind the radiation-mediated shock, Comptonization by hot electrons, and the eventual escape of the radiation through the expanding debris. Such 1D calculations are approximately justified for thin disks (scale-height h ≲ few × R⋆), through which the star of radius R⋆ passes more quickly than the shocked gas can flow around the star. For collision speeds vcoll ≳ 0.15c and disk surface densities Σ ∼ 103 g cm−2 characteristic of those encountered by stellar orbits consistent with QPE recurrence times, the predicted transient light curves exhibit peak luminosities ≳1042 erg s−1 and Comptonized quasi-thermal (Wien-like) spectra that peak at energies hν ∼ 100 eV, which is broadly consistent with QPE properties. For these conditions, gas and radiation are out of equilibrium, rendering the emission temperature harder than the blackbody value due to inefficient photon production behind the radiation-mediated shock. The predicted eruptions execute counterclockwise loops in hardness–luminosity space, qualitatively similar to QPE observations. Reproducing the observed eruption properties (duration, luminosity, temperature) requires a large radius R⋆ ≳ 10 R⊙, which may point to inflation of the star’s atmosphere from repeated collisions.

  • Seoul National University AGN Monitoring Project. V. Velocity-resolved Hβ Reverberation Mapping and Evidence of Kinematics Evolution
    We present velocity-resolved reverberation lags of Hβ for 20 active galactic nuclei (AGNs) from the Seoul National University AGN Monitoring Project. We detect unambiguous velocity-resolved structures in 12 AGNs, among which eight objects exhibit symmetric structures, two objects show inflow-like characteristics, and two objects display outflow-like signatures. For two AGNs, we successfully measure the velocity-resolved lags in different years, revealing evidence of evolving broad-line region (BLR) kinematics. By combining our sample with the literature velocity-resolved lags, we find that the symmetric velocity-resolved lags are the most common (40%) type among this sample. The frequency of inflow kinematics is also notable (20%), while outflow kinematics are less common (11%). Our sample significantly expands the previous velocity-resolved reverberation mapping sample in the high-luminosity regime, enabling us to constrain BLR kinematics across a large dynamic range of luminosity.