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.
$h\nu>2~{\rm keV}$) flux comparable to that from the current quiescent sun. The
flux in harder energy band $20~{\rm keV}<h\nu<100~{\rm keV}$, 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.
