Our supermassive black hole rivaled the Sun in the ancient X-ray sky

Sagittarius A* (SgrA*) lying in the Galactic Centre 8 kpc from Earth, hosts the closest supermassive black hole known to us. It is now inactive, but there is evidence 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 SgrA*. For that, we assume that the outburst was due to accretion of gas or the tidal disruption of a star. We show that these cases could precipitate on Earth a hard X-ray (i.e. hν>2 keV) flux comparable to that from the current quiescent sun. The flux in harder energy band 20 keV<hν<100 keV, however, surpasses that from an X-class solar flare, and the irradiation timescale is also much longer, ranging from weeks to 105 years depending on the outburst scenario. In the solar system gas giants will suffer the biggest impact in their atmospheres. Lower-mass planets such as Earth receive a level of radiation that might have played a role in the evolution of their primitive atmospheres, so that a detailed study of the consequences deserves further investigation. Planetary systems closer to SgrA* receive higher irradiance levels, making them more likely uninhabitable.