  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.
