Diffuse gamma-ray emission from the Galactic center and implications of
  its past activities
  It has been indicated that low-luminosity active galactic nuclei (LLAGNs) are
accelerating high-energy cosmic-ray (CR) protons in their radiatively
inefficient accretion flows (RIAFs). If this is the case, Sagittarius A* (Sgr
A*) should also be generating CR protons, because Sgr A* is a LLAGN. Based on
this scenario, we calculate a production rate of CR protons in Sgr A* and their
diffusion in the central molecular zone (CMZ) around Sgr A*. The CR protons
diffusing in the CMZ create gamma-rays through pp interaction. We show that the
gamma-ray luminosity and spectrum are consistent with observations if Sgr A*
was active in the past.
