Gamma-ray Emission from an Accretion Flow around a Kerr Black Hole

Kazutaka Oka1 and Tadahiro Manmoto2

1 Department of earth and planetary sciences, Kobe University
2 Department of physics, Chiba University

E-mail contact: kazutaka@kobe-u.ac.jp

We investigate the dependence of gamma-ray spectrum emitted from ADAFs on the spin parameter of the central Kerr black holes, in order to examine whether the spectrum can be used as a probe to investigate the spin parameter of black holes. In the vicinity of a black hole, the ion temperature of an ADAF is so high that gamma-rays are produced through the decay of neutral pions created by proton-proton collisions. The temperature and the density of ions in ADAFs vary according to the spin parameter, so does the intensity of the emitted gamma-rays.

Since the energy distribution of the ion particles in the ADAF is not known, we consider two types of proton energy distributions: a thermal distribution and a power-law distribution (nonthermal model). In the thermal model, we find that the changes in the spin parameter from -0.95 to 0.95 can enhance the gamma-ray intensity by orders of magnitude. Thus, if the proton gas in an ADAF has the thermal distribution, the gamma-ray spectrum can be used as a probe to investigate the spin parameter of the central black hole. In the nonthermal model, on the other hand, the gamma-ray intensity is much less sensitive to the changes in the spin parameter than in the thermal model. We apply our model to the Galactic Center.