A general relativistic external Compton-scattering model for TeV emission from M87

Abstract

M87 is the first detected non-blazar extragalactic Tera-Electron-Volt (TeV) source with rapid variation and very flat spectrum in the TeV band. To explain the two-peaks in the spectral energy distribution (SED) of the nucleus of M87 which is similar to those of blazars, the most commonly adopted models are the synchrotron self-Compton scattering (SSC) models and the external inverse Compton scattering (EIC) models. Considering that there is no correlated variation in the soft band (from radio to X-ray) matching the TeV variation, and the TeV sources should not suffer from the gamma-gamma absorption due to the flat TeV spectrum, the EIC models are advantageous in modeling the TeV emission from M87. In this paper, we propose a self-consistent EIC model to explain the flat TeV spectrum of M87 within the framework of fully general relativity, where the background soft photons are from the advection-dominated accretion flow (ADAF) around the central black hole, and the high energy electrons are from the mini-jets which are powered by the magnetic reconnection in the main jet (Giannios et al. 2010). In our model, both the TeV flares observed in the years of 2005 and 2008 could be well explained: the gamma-gamma absorption for TeV photons is very low, even inside the region very close to the black hole 20Rg~50Rg; at the same region, the average EIC cooling time (~ 10^2-10^3s) is short, which is consistent with the observed time scale of TeV variation. Furthermore, we also discuss the possibility that the accompanying X-ray flare in 2008 is due to the direct synchrotron radiation of the mini-jets.