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. h ν>2 keV) flux comparable to that from the current quiescent sun. The flux in harder energy band 20 keV< h ν<100 keV, however, surpasses that from an X-class solar flare, and the irradiation timescale is also much longer, ranging from weeks to 10 5 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.