High-energy Gamma Rays from Magnetically Arrested Disks in Nearby Radio Galaxies

Abstract

Abstract The origins of the GeV gamma rays from nearby radio galaxies are unknown. Hadronic emission from magnetically arrested disks (MADs) around central black holes is proposed as a possible scenario. Particles are accelerated in a MAD by magnetic reconnection and stochastic turbulence acceleration. We pick out the 15 brightest radio galaxies in the GeV band from The Fourth Catalog of Active Galactic Nuclei Detected by the Fermi Large Area Telescope, Data Release 2 and apply the MAD model. We find that we can explain the data in the GeV bands by the MAD model if the accretion rate is lower than 0.1% of the Eddington rate. For a higher accretion rate, GeV gamma rays are absorbed by two-photon interaction due to copious low-energy photons. If we assume another proposed prescription of the electron-heating rate by magnetic reconnection, the MAD model fails to reproduce the GeV data for the majority of our sample. This indicates that the electron-heating rate is crucial. We also apply the MAD model to Sagittarius A* (Sgr A*) and find that GeV gamma rays observed at the Galactic center do not come from the MAD of Sgr A*. We estimate the cosmic ray (CR) intensity from Sgr A*, but it is too low to explain the high-energy CR intensity on Earth.