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1. The role of magnetospheric current sheets in pair enrichment and ultra-high energy proton acceleration in M87*

   https://doi.org/10.1088/1475-7516/2024/12/009  

   Stathopoulos, SI; Petropoulou, M; Sironi, L; Giannios, D (December 2024, Journal of Cosmology and Astroparticle Physics)

   Abstract Recent advances in numerical simulations of magnetically arrested accretion onto supermassive black holes have shed light on the formation and dynamics of magnetospheric current sheets near the black hole horizon. By considering the pair magnetizationσ_ein the upstream region and the mass accretion rateṁ(in units of the Eddington mass accretion rate) as free parameters we estimate the strength of the magnetic field and develop analytical models, motivated by recent three-dimensional particle-in-cell simulations, to describe the populations of relativistic electrons and positrons (pairs) in the reconnection region.Applying our model to M87*, we numerically compute the non-thermal photon spectra for various values ofσ_e. We show that pairs that are accelerated up to the synchrotron radiation-limited energy while meandering across both sides of the current sheet, can produce MeV flares with luminosity of ∼ 10⁴¹ erg s^-1— independent ofσ_e— for a black hole accreting atṁ=10^-5. Pairs that are trapped in the transient current sheet can produce X-ray counterparts to the MeV flares, lasting about a day for current sheets with length of a few gravitational radii. We also show that the upstream plasma can be enriched due to photon-photon pair creation, and derive a new equilibrium magnetization ofσ_e∼ 10³-10⁴forṁ= 10^-6- 10^-5. Additionally, we explore the potential of magnetospheric current sheets to accelerate protons to ultra-high energies, finding that while acceleration to such energies is limited by various loss mechanisms, such as synchrotron and photopion losses from the non-thermal emission from pairs, maximal proton energies in the range of a few EeV are attainable in magnetospheric sheets forming around supermassive sub-Eddington accreting black holes. 

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2. Diffuse neutrino flux from relativistic reconnection in AGN coronae

   https://doi.org/10.1051/0004-6361/202558489  

   Karavola, D; Petropoulou, M; Fiorillo, D_F G; Georgakakis, A; Comisso, L; Sironi, L (May 2026, Astronomy & Astrophysics)

   Context.IceCube observations point to active galactic nuclei (AGNs) as promising contributors to the observed astrophysical neutrino flux. Close to the central black hole, protons can be accelerated through magnetic reconnection to very high energies and subsequently interact with abundant X-ray photons in the source, leading to neutrino production. Aims.We investigate whether the diffuse neutrino flux observed by IceCube can originate, via proton acceleration, in reconnection-powered coronae of non-jetted AGNs. Methods.We created a library of neutrino spectral templates over a large grid of values for the three key model parameters: the proton plasma magnetization of the coronaσ_p, the X-ray coronal luminosity, and the black hole mass. Synchrotron cooling of pions and muons plays a significant role due to the large coronal magnetic fields. To infer the diffuse neutrino flux, we coupled the single-source model with a mock AGN catalog consistent with the observed X-ray and mid-infrared AGN samples at redshiftsz = 0 − 4. Results.The coronal emission satisfactorily explains the most recent IceCube measurements of the diffuse neutrino flux up to energies of ∼1 PeV, provided that ∼10% of the AGN coronae haveσ_p ∼ 10⁵, while the rest are distributed over a range of lower magnetizations. Coronal emission is suppressed at higher energies by pion and muon cooling so that another population is required, with jetted AGNs being strong candidates. 

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3. A Luminous X-Ray Active Galactic Nucleus in the Dwarf–Dwarf Galaxy Merger RGG 66

   https://doi.org/10.3847/1538-4357/ad712d  

   Kimbrell, Seth J; Reines, Amy E (October 2024, The Astrophysical Journal)

   Abstract We present the discovery of a luminous X-ray active galactic nucleus (AGN) in the dwarf galaxy merger RGG 66. The black hole is predicted to have a mass ofM_BH∼ 10^5.4M_⊙and to be radiating close to its Eddington limit (L_bol/L_Edd∼ 0.75). The AGN in RGG 66 is notable both for its presence in a late-stage dwarf–dwarf merger and for its luminosity ofL_{2–10 keV}= 10^42.2erg s⁻¹, which is among the most powerful AGNs known in nearby dwarf galaxies. The X-ray spectrum has a best-fit photon index of Γ = 2.4 and an intrinsic absorption ofN_H∼ 10²¹cm⁻². These results come from a follow-up Chandra X-ray Observatory study of four irregular/disturbed dwarf galaxies with evidence for hosting AGNs based on optical spectroscopy. The remaining three dwarf galaxies do not have detectable X-ray sources with upper limits ofL_{2–10 keV}≲ 10⁴⁰erg s⁻¹. Taken at face value, our results on RGG 66 suggest that mergers may trigger the most luminous of AGNs in the dwarf galaxy regime, just as they are suspected to do in more massive galaxy mergers. 

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4. Probing a Cosmogenic Origin of Astrophysical Neutrinos and Cosmic Rays Using Gamma-Ray Observations of TXS 0506+056

   https://doi.org/10.3847/1538-4357/ae1d72  

   Acharyya, A; Archer, A; Bangale, P; Bartkoske, J T; Benbow, W; Buckley, J H; Chen, Y; Christiansen, J L; Duerr, A; Errando, M; et al (December 2025, The Astrophysical Journal)

   Abstract In 2017 September, a high-energy neutrino event detected by the IceCube Neutrino Observatory (IceCube-170922A) was associated, at the 3σlevel, with a gamma-ray flare from the blazar TXS 0506+056. Cosmic rays that are accelerated in astrophysical sources can escape from their jets and interact with background radiation fields. Interactions with the extragalactic background light can produce pions and hence neutrinos, while interactions with the cosmic microwave background predominantly drive inverse Compton scattering, contributing to electromagnetic cascades in intergalactic space. The resulting secondary gamma-ray emission can be detected with high-energy gamma-ray telescopes. Here, we report on a new search for such cosmogenic cascade emission from the blazar TXS 0506+056, using a combined data set from the Fermi Large Area Telescope and VERITAS. We compare the gamma-ray spectrum and neutrino observations with the predictions of cosmic-ray-induced cascades in intergalactic space. The observed gamma-ray spectrum is modeled as a combination of the primary spectrum and the cascade spectrum. We apply a Monte Carlo simulation with a Δχ²-based likelihood analysis to jointly determine the best-fit parameters of a proton emission spectrum describing the data and derive constraints on the proton escape luminosity. Assuming an LP primary photon spectrum, we find consistency with a proton injection spectral index ofα_p ≃ 2.0 and a cutoff energy ofE_p,max ≃ 1.3 × 10¹⁶eV and constrain the isotropic proton escape luminosity to 1 × 10⁴⁴erg s⁻¹≲L_p,esc ≲ 3 × 10⁴⁵erg s⁻¹at the 90% confidence level. 

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5. An X-Ray Census of Fast Radio Burst Host Galaxies: Constraints on Active Galactic Nuclei and X-Ray Counterparts

   https://doi.org/10.3847/1538-4357/acf843  

   Eftekhari, T; Fong, W; Gordon, A C; Sridhar, N; Kilpatrick, C D; Bhandari, S; Deller, A T; Dong_董, Y_雨 欣; Rouco_Escorial, A; Heintz, K E; et al (November 2023, The Astrophysical Journal)

   Abstract 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 × 10⁴²erg s⁻¹) X-ray source at the nucleus of FRB 20190608B’s host, for which we infer an SMBH mass ofM_BH∼ 10⁸M_⊙and an Eddington ratioL_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 thanL_X≳ 10⁴⁰erg 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. 

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