Investigating past activity of the Fermi Bubbles using cosmic-ray signatures in paleodetectors.

The Fermi Bubbles, two large lobes of plasma extruding from the Galactic Center, are theorized to be remnants of a past AGN-like activity of the Milky Way some Myr ago. AGNs are promising candidate sources of cosmic rays, so it is possible that the flux of these particles in the past was significantly higher. This flux could have left traces in natural mineral formations in the Earth's crust. In fact, interactions with secondary cosmic rays can leave visible tracks that can be preserved up to the Gyr timescale. These so-called "Paleo-Detectors" have been proposed also for other astroparticle physics applications, such as atmospheric neutrino flux evolution and supernova neutrino detection. The age of the Fermi Bubbles is serendipitously coincident to the Mediterranean draining event known as the "Messinian salinity crisis" which produced evaporites, mostly halites. These minerals were at first exposed to the cosmic-ray flux and then covered during the sudden reflooding of the Mediterranean basin $5.3 {Myr}$ ago; the cosmic-ray flux information remained then frozen due to the shielding of the body of water, possibly allowing us to extract them and compare them to the present day flux.