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

TL;DR

This study explores whether magnetically arrested disks around black holes can produce the observed GeV gamma-rays in nearby radio galaxies, finding the model explains the data at low accretion rates but not at higher ones.

Contribution

It demonstrates that MAD models can account for GeV gamma-ray emissions in radio galaxies at low accretion rates, highlighting the importance of electron heating rates.

Findings

MAD model explains GeV data at accretion rates below 0.1% Eddington

Gamma-ray absorption occurs at higher accretion rates due to photon interactions

MAD model does not account for gamma-rays from Sgr A* or cosmic rays on Earth

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 (BHs) is proposed as a possible scenario. Particles are accelerated in the MAD by magnetic reconnection and stochastic turbulence acceleration. We pick out the fifteen brightest radio galaxies in the GeV band from the Fermi 4LAC-DR2 catalog 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 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 intensity from Sgr A*, but it is too low to explain the high-energy cosmic ray intensity on Earth.