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.