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1. The emergence of diffused gamma-ray burst afterglows from the discs of active galactic nuclei

   https://doi.org/10.1093/mnras/stac1968  

   Wang, Yi-Han; Lazzati, Davide; Perna, Rosalba (July 2022, Monthly Notices of the Royal Astronomical Society)

   ABSTRACT The discs of active galactic nuclei (AGNs) have emerged as rich environments for the production and capture of stars and the compact objects that they leave behind. These stars produce long gamma-ray bursts (GRBs) at their deaths, while frequent interactions among compact objects form binary neutron stars and neutron star–black hole binaries, leading to short GRBs upon their merger. Predicting the properties of these transients as they emerge from the dense environments of AGN discs is key to their proper identification and to better constrain the star and compact object population in AGN discs. Some of these transients would appear unusual because they take place in much higher densities than the interstellar medium. Others, which are the subject of this paper, would additionally be modified by radiation diffusion, since they are generated within optically thick regions of the accretion discs. Here, we compute the GRB afterglow light curves for diffused GRB sources for a representative variety of central black hole masses and disc locations. We find that the radiation from radio to ultraviolet and soft X-rays can be strongly suppressed by synchrotron self-absorption in the dense medium of the AGN disc. In addition, photon diffusion can significantly delay the emergence of the emission peak, turning a beamed, fast transient into a slow, isotropic, and dimmer one. These would appear as broad-band correlated AGN variability with a dominance at the higher frequencies. Their properties can constrain both the stellar populations within AGN discs and the disc structure. 

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2. Mapping the Outcomes of Stellar Evolution in the Disks of Active Galactic Nuclei

   https://doi.org/10.3847/1538-4357/ada896  

   Fabj, Gaia; Dittmann, Alexander J; Cantiello, Matteo; Perna, Rosalba; Samsing, Johan (February 2025, The Astrophysical Journal)

   Abstract The disks of active galactic nuclei (AGNs) are expected to be populated by numerous stars, either formed in the outer regions of the disk via gravitational instability or captured from the nearby nuclear star cluster. Regardless of their formation mechanism, these stars experience altered evolutionary paths, mostly shaped by the accretion of dense disk material. In this study, through the comparison of different timescales, we chart the evolutionary outcomes of these AGN stars as a function of disk radius and across a range of supermassive black hole masses, spanning from 10⁶to 10⁹M_⊙, for two popular AGN disk models. We find that in the outer regions of the disk, stars evolve similarly to those in the interstellar medium, but in the inner and denser regions, accretion quickly turns low-mass stars into massive stars, and their fate depends on just how quickly they accrete. If accretion occurs at a faster rate than nuclear burning, they can reach a quasi-steady “immortal” state. If stars accrete faster than they can thermally adjust, runaway accretion occurs, potentially preventing a quasi-steady state and altering the disk structure. During the AGN lifetime, in the regions of the disk that produce massive stars, supernovae (SNe) and gamma-ray bursts (GRBs) may occur within the disk over a wide range of optical depths and ambient densities. Subsequently, in the final phase of the AGN, as the disk becomes depleted, formerly immortal stars will be unable to replenish their fuel, leading to additional SNe and GRBs. 

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3. Continuum Reverberation in Active Galactic Nuclei Disks Only with Sufficient X-Ray Luminosity and Low Albedo

   https://doi.org/10.3847/1538-4357/adc25b  

   Secunda, Amy; Jiang_姜, Yan-Fei 燕飞; Greene, Jenny E (April 2025, The Astrophysical Journal)

   Abstract Disk continuum reverberation mapping is one of the primary ways we learn about active galactic nuclei (AGN) accretion disks. Reverberation mapping assumes that time-varying X-rays incident on the accretion disk drive variability in UV–optical light curves emitted by AGN disks and uses lags between X-ray and UV–optical variability on the light-crossing timescale to measure the radial temperature profile and extent of AGN disks. However, recent reverberation mapping campaigns have revealed oddities in some sources, such as weakly correlated X-ray and UV light curves, longer than anticipated lags, and evidence of intrinsic variability from disk fluctuations. To understand how X-ray reverberation works with realistic accretion disk structures, we perform 3D multifrequency radiation magnetohydrodynamic simulations of X-ray reprocessing by the UV-emitting region of an AGN disk using sophisticated opacity models that include line opacities for both the X-ray and UV radiation. We find there are two important factors that determine whether X-ray irradiation and UV emission will be well-correlated: the ratio of X-ray to UV luminosity and significant absorption. When these factors are met, the reprocessing of X-rays into UV is nearly instantaneous, as is often assumed, although linear reprocessing models are insufficient to fully capture X-ray reprocessing in our simulations. Nevertheless, we can still easily recover mock lags in our light curves using software that assumes linear reprocessing. Finally, the X-rays in our simulation heat the disk, increasing temperatures by a factor of 2–5 in the optically thin region, which could help explain the discrepancy between measured and anticipated lags. 

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4. Discovery of a Pulsar Wind Nebula Candidate Associated with the Galactic PeVatron 1LHAASO J0343+5254u

   https://doi.org/10.3847/1538-4357/adb7e0  

   DiKerby, Stephen; Zhang, Shuo; Ergin, Tülün; Tsuji, Naomi; Mori, Kaya; Acero, Fabio; Safi-Harb, Samar; Takekawa, Shunya; Woo, Jooyun (April 2025, The Astrophysical Journal)

   Abstract The astronomical origins of the most energetic galactic cosmic rays and gamma rays are still uncertain. X-ray follow-up of candidate “PeVatrons”—systems producing cosmic rays with energies exceeding 1 PeV—can constrain their spatial origin, identify likely counterparts, and test particle emission models. Using ∼120 ks of XMM-Newton observations, we report the discovery of a candidate pulsar wind nebula, a possible counterpart for the LHAASO PeVatron J0343+5254u. This extended source has a power-law X-ray spectrum with spectral index Γ_X = 1.9—softer at greater distance from the center—and asymmetric spatial extension out to $\approx 2\prime$. We conduct leptonic modeling of the X-ray and gamma-ray radiation from this complex system, showing that a fully leptonic model with elevated IR photon fields can explain the multiwavelength emission from this source, similar to other very high-energy pulsar wind nebulas; excess gamma-ray emissivity not explained by a leptonic model may be due to hadronic interactions in nearby molecular cloud regions, which might also produce detectable astroparticle flux. 

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5. Zooming In On The Multi-Phase Structure of Magnetically-Dominated Quasar Disks: Radiation From Torus to ISCO Across Accretion Rates

   https://doi.org/10.33232/001c.137296  

   Hopkins, Philip F; Su, Kung-Yi; Murray, Norman; Steinwandel, Ulrich P; Kaaz, Nicholas; Ponnada, Sam B; Bardati, Jaeden; Piotrowska, Joanna M; Wang, Hai-Yang; Shi, Yanlong; et al (January 2025, The Open Journal of Astrophysics)

   Recent radiation-thermochemical-magnetohydrodynamic simulations resolved formation of quasar accretion disks from cosmological scales down to ~300 gravitational radii $R_g$, arguing they were ‘hyper-magnetized’ (plasma $\beta \ll 1$supported by toroidal magnetic fields) and distinct from traditional $\alpha$-disks. We extend these, refining to $\approx 3R_g$around a BH with multi-channel radiation and thermochemistry, and exploring a factor of 1000 range of accretion rates ( $\dot{m}\sim 0.01-20$). At smaller scales, we see the disks maintain steady accretion, thermalize and self-ionize, and radiation pressure grows in importance, but large deviations from local thermodynamic equilibrium and single-phase equations of state are always present. Trans-Alfvenic and highly-supersonic turbulence persists in all cases, and leads to efficient vertical mixing, so radiation pressure saturates at levels comparable to fluctuating magnetic and turbulent pressures even for $\dot{m}\gg 1$. The disks also become radiatively inefficient in the inner regions at high $\dot{m}$. The midplane magnetic field remains primarily toroidal at large radii, but at super-Eddington $\dot{m}$we see occasional transitions to a poloidal-field dominated state associated with outflows and flares. Large-scale magnetocentrifugal and continuum radiation-pressure-driven outflows are weak at $\dot{m}<1$, but can be strong at $\dot{m}\gtrsim 1$. In all cases there is a scattering photosphere above the disk extending to $\gtrsim 1000R_g$at large $\dot{m}$, and the disk is thick and flared owing to magnetic support (with $H/R$nearly independent of $\dot{m}$), so the outer disk is strongly illuminated by the inner disk and most of the inner disk continuum scatters or is reprocessed at larger scales, giving apparent emission region sizes as large as . 

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