UHECR production by a compact black hole dynamo: application to Sgr A*

The possibility that the excess cosmic ray (CR) flux near 1018 eV, reported recently by the AGASA group, is due to a compact black hole dynamo associated with the Sgr A* source is considered. Under the assumption that the Galactic center black hole rotates with nearly maximal spin, and that the magnetic field threading the horizon is in rough equipartition with matter accreted by the hole, the spectra and total fluxes of accelerated CRs and their associated curvature emission depend only on accretion rate. For the accretion rate estimated on the basis of observations of stellar winds near Sgr A* (∼10−3 Eddington), the maximum proton energy achievable by this mechanism is of the order of 1018 eV, and the corresponding CR power is ∼1041αCR erg s−1, where αCR≪1 is the CR production efficiency. The corresponding spectrum of curvature photons peaks at around 100 GeV, with a total luminosity comparable to the CR power released. For much lower accretion rates both the CR and gamma-ray fluxes predicted are completely insignificant. Upcoming gamma-ray experiments, such as CANGAROO, HESS and GLAST, can be used to probe the parameters of this system.