2013MNRAS.432.1576N

We present a model of γ-ray emission through neutral pion production and decay in two-temperature accretion flows around supermassive black holes. We refine previous studies of such a hadronic γ-ray emission by taking into account (1) relativistic effects in the photon transfer and (2) absorption of γ-ray photons in the radiation field of the flow. We use a fully general relativistic description of both the radiative and hydrodynamic processes, which allows us to study the dependence on the black hole spin. The spin value strongly affects the γ-ray emissivity within ∼ 10 gravitational radii. The central regions of flows with the total luminosities L ≲ 10- 3 of the Eddington luminosity (LEdd) are mostly transparent to photons with energies below 10 GeV, permitting investigation of the effects of space-time metric. For such L, an observational upper limit on the γ-ray (0.1-10 GeV) to X-ray (2-10 keV) luminosity ratio of L0.1-10GeV/L2-10keV ≪ 0.1 can rule out rapid rotation of the black hole; on the other hand, a measurement of L0.1-10GeV/L2-10keV ∼ 0.1 cannot be regarded as the evidence of rapid rotation, as such a ratio can also result from a flat radial profile of γ-ray emissivity (which would occur for non-thermal acceleration of protons in the whole body of the flow). At L ≳ 10- 2LEdd, the γ-ray emission from the innermost region is strongly absorbed and the observed γ-rays do not carry information on the value of a. We note that if the X-ray emission observed in Centaurus A comes from an accretion flow, the hadronic γ-ray emission from the flow should contribute significantly to the MeV/GeV emission observed from the core of this object, unless it contains a slowly rotating black hole and protons in the flow are thermal.