Low-luminosity AGNs

Abstract

Context. We propose that low-luminosity AGNs (LLAGNs), or some of them, are sources extracting their energy from the black hole rotation by the Blandford-Znajek mechanism. Aims. It is shown that almost all energy of the black hole rotation is converted to relativistic protons in a jet. Owing to the high magnetic-field magnitude near the black hole, required for the Blandford-Znajek mechanism, electrons are not strongly accelerated because of their high synchrotron losses. Conversely, protons gain energies on the order of (10 4 −10 5 )mpc 2 when crossing the light cylinder surface. Protons are also accelerated in a disk by 2D turbulent motion of the disk matter. Methods. We calculate the luminosity of the synchrotron radiation by fast protons in the disk, the frequencies of this radiation being in the infrared band, and the luminosities corresponding to LLAGNs. We measure the very high energy (VHE) radiation luminosities from the disk and the jet, finding that VHE radiation is produced by collisions of accelerated protons with surrounding matter. Results. We predict a correlation between the infrared luminosity LIR and the VHE luminosity LVHE of the disk, LVHE ∝ L 1/2 IR M 3/2 , where M is the mass of a black hole. Two low-luminosity sources Sgr A* and M 87, for which luminosities LIR and LVHE are known, appear to follow this scheme. Conclusions. The discovery of new bright VHE sources from LLAGNs could confirm our hypotheses that they are energy sources powered by the Blandford-Znajek mechanism.