Soft gamma rays from low accreting supermassive black holes and connection to energetic neutrinos

Abstract

The Universe is filled with a diffuse background of MeV gamma-rays and PeV neutrinos, whose origins are unknown. Here, we propose a scenario that can account for both backgrounds simultaneously. Low-luminosity active galactic nuclei have hot accretion flows where thermal electrons naturally emit soft gamma rays via Comptonization of their synchrotron photons. Protons there can be accelerated via turbulence or reconnection, producing high-energy neutrinos via hadronic interactions. We demonstrate that our model can reproduce the gamma-ray and neutrino data. Combined with a contribution by hot coronae in luminous active galactic nuclei, these accretion flows can explain the keV – MeV photon and TeV – PeV neutrino backgrounds. This scenario can account for the MeV background without non-thermal electrons, suggesting a higher transition energy from the thermal to nonthermal Universe than expected. Our model is consistent with X-ray data of nearby objects, and testable by future MeV gamma-ray and high-energy neutrino detectors.

High-Energy and Ultra-High-Energy Neutrinos

“…There has been significant progress in numerical simulations of particle acceleration in shocks and accretion flows [328][329][330], and in spark gaps [331,332]. Non-jetted AGN [333][334][335][336], TDEs [337][338][339], choked jets [340][341][342][343], and interacting supernovae [344,345] are actively discussed as promising targets for high-energy neutrino detection.…”

High-Energy Neutrinos from Active Galactic Nuclei

“…In this model, the same acceleration mechanism is expected to operate in RIAFs. 56,63,82 By applying similar physical parameters to RIAFs for particle acceleration (although the plasma β in RIAFs is expected to be larger), higher-energy neutrino data above 100 TeV can simultaneously be explained (see Figure 11). In this sense, AGN with different luminosities from LL AGN to luminous AGN including Seyfert galaxies and quasars can explain the all-sky neutrino intensity from a few TeV to a few PeV energies.…”

“…Such a scenario was suggested by Stecker and Salamon 113 and Inoue et al 114 More recently, LL AGN have also been suggested as the sources of the MeV γ-ray background. 56 In this scenario, the MeV γ-ray background is attributed to Compton emission by thermal electrons in the hot RIAF plasma. To identify the sources and discriminate among different possibilities, both auto-correlation and cross-correlation searches are necessary, which will be feasible with future MeV γ-ray telescopes.…”

“…AGN scenario to explain the all-sky neutrino intensity from 10 TeV to a few PeV energies, measured in IceCube, together with the X-ray and MeV γ-ray backgrounds. 50,56 The GeV-TeV γrays may be explained by blazars and radio galaxies, as well as star-forming galaxies (not shown). This provides an example of the multimessenger connection between the TeV-PeV neutrino and the keV-MeV photon backgrounds.…”

“…Stacking searches are also powerful for LL AGN. Kimura et al 56,82 studied the detectability of nearby LL AGN as the sources of high-energy cosmic neutrinos, and promising Notably, in any of the models described above, neutrino production is efficient and the system is calorimetric (i.e., f pp/pγ 1). One typically expects L ν ∝ L X given that the cosmic-ray loading factor ξ CR is similar.…”