This paper V. V. Hambaryan, R. Neuhaeuser: A Galactic short gamma-ray burst as cause for the 14C peak in AD 774/5 http://arxiv.org/abs/1211.2584 claims in 1 year either 774 or 775 AD, the concentration of carbon 14 increased by an amount equivalent to 14C production in 10 typical years. As detected in tree rings on 2 continents. Also there was a spike in 10Be at the same time (to within time measurement errors). This was the only such spike during the last 3K years. They explain this by postulating there was a gamma ray burst which fried the Earth for 2 seconds with 7*10^17 joules of gamma ray energy. (This is equivalent to the energy of the sunlight hitting the Earth during 4 seconds, but in gamma ray rather than light region of spectrum.) This was presumably caused by e.g. merger of 2 neutron stars at a distance of 3K to 13K lightyears. They think if it had been much closer than that it would have caused notable extinction (of life) events... over, I presume, exactly half of the Earth(?)... which did not happen. I have a problem with this. If you believe them and believe such events happen at least once every 5000 years within 3K to 13K lightyears distance, then it would follow they ought to happen within 1K lightyears at least once every 11M years. Causing big fat extinction events over exactly half the Earth, at that frequency. Which haven't been noticed (have they)?