Gamma-ray emission from proton-proton interactions in hot accretion flows

Abstract

We present a model of α-ray emission through neutral pion production and decay in twotemperature 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 L0.1-10 GeV/L2-10 keV « 0.1 can rule out rapid rotation of the black hole; on the other hand, a measurement of L0.1-10 GeV/L2-10 keV keV~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. © 2013 The Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society.