A Galactic short gamma-ray burst as cause for the 14C peak in AD 774/5

Abstract

In the last 3000 yr, one significant and rapid increase in the concentration of 14C in tree rings was observed; it corresponds to a gamma-ray energy input of 7x10^24 erg at Earth within up to one year in AD 774/5 (Miyake et al. 2012). A normal supernova and a solar or stellar flare are unlikely as cause (Miyake et al. 2012), so that the source remained unknown. Here, we show that a short gamma-ray burst (GRB) in our Galaxy is consistent with all observables: Such an event is sufficiently short and provides the necessary energy in the relevant spectral range of $\gamma$-rays. Its spectral hardness is consistent with the differential production rates of 14C and 10Be as observed. The absence of reports about a historic sighting of a supernova in AD 774/5 or a present-day supernova remnant are also consistent with a short GRB. We estimate the distance towards this short GRB to be ~ 1 to 4 kpc - sufficiently far away, so that no extinction event on Earth was triggered. This is the first evidence for a short GRB in our Galaxy.