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We present a model of gamma-ray emission through neutral pion production anddecay in two-temperature accretion flows around supermassive black holes. Werefine previous studies of such a hadronic gamma-ray emission by taking intoaccount (1) relativistic effects in the photon transfer and (2) absorption ofgamma-ray photons in the radiation field of the flow. We use a fully generalrelativistic description of both the radiative and hydrodynamic processes,which allows us to study the dependence on the black hole spin. The spin valuestrongly affects the gamma-ray emissivity within ~10 gravitational radii. Thecentral regions of flows with the total luminosities L < 0.001 of the Eddingtonluminosity are mostly transparent to photons with energies below 10 GeV,permitting investigation of the effects of space-time metric. For such L, anobservational upper limit on the gamma-ray (0.1-10 GeV) to X-ray (2-10 keV)luminosity ratio of L_{0.1-10 GeV}/L_{2-10 keV} << 0.1 can rule out rapidrotation of the black hole; on the other hand, a measurement of L_{0.1-10GeV}/L_{2-10 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 gamma-rayemissivity (which would occur for nonthermal acceleration of protons in thewhole body of the flow). At L > 0.01 of the Eddington luminosity, the gamma-rayemission from the innermost region is strongly absorbed and the observedgamma-rays do not carry information on the value of the black hole spin. Wenote that if the X-ray emission observed in Centaurus A comes from an accretionflow, the hadronic gamma-ray emission from the flow should contributesignificantly 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 arethermal.

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