Our Supermassive Black Hole Rivaled the Sun in the Ancient X-ray Sky

Abstract

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

Minor changes, accepted for publication

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Cited by in corpus (7)

• Gravitational waves from bodies orbiting the Galactic Center black hole and their detectability by LISA
• Eta-Earth Revisited II: Deriving a Maximum Number of Earth-like Habitats in the Galactic Disk
• The Impact of Tidal Disruption Events on Galactic Habitability
• The impact of AGN outflows on the surface habitability of terrestrial planets in the Milky Way
• Impact of an Active Sgr A* on the Synthesis of Water and Organic Molecules Throughout the Milky Way
• On the relative importance of AGN winds for the evolution of exoplanet atmospheres
• How black hole activity may influence exoplanetary evolution in our Galaxy