Sagittarius A* Rivaled the Sun in the Ancient X-ray Sky

Sagittarius A*, lying the Galactic Center $8$ kpc from Earth, hosts the closest supermassive black hole known to us. It is now inactive, but there are evidences indicating that about six million years ago it underwent a powerful outburst where the luminosity could have approached the Eddington limit. Motivated by the fact that in extragalaxies the supermassive black holes with similar masses and near-Eddington luminosities are usually strong X-ray emitters, we calculate here the X-ray luminosity of Sagittarius A*, assuming that the outburst was due to accretion of gas or tidal disruption of stars, both scenarios having been considered to trigger the previous outburst. We show that in both cases Sagittarius A* could precipitate on Earth an X-ray ($h\nu>2$ keV) irradiance comparable to that from the current quiescent sun. The irradiance in harder energy band $20~{\rm keV}<h\nu