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.

--A "Gray" is a radiation dose unit of 1 joule per kg of receiving-matter. Viewing "the Earth's atmosphere" as the receiving matter, and figuring 1 kg of atmosphere per cm^2 of Earth-surface, and noting only half the atmosphere was irradiated, I compute that the dose received during this event was 0.27 Grays. For humans the lethal radiation dose (LD50) is reckoned to be 5 Grays. So a dose event 18 times larger would have (?) wiped out a lot of humanity in 1 hemisphere... corresponding to the gamma-ray-source being sqrt(18)=4.26 times closer, which presumably would happen 78=4.26^3 times less frequently. However... that calculation assumes we are not protected by the atmosphere above us serving as a shield. In fact the atmosphere shields out gamma cosmic rays by a large factor, so large that cosmic gamma rays are usually detected from high altitude balloons and gamma-ray bursters were never even detected on ground and instead were detected with satellites. Gamma rays' fall off depth amounts to the depth of atmosphere that amounts to a few grams air per cm^2... and in fact the whole atmosphere is 1000 grams/cm^2... which means gamma rays essentially cannot penetrate to the ground at all, they would be damped by such an enormous factor. So I conclude that gamma ray bursters would NOT cause life-extinctions on Earth, and the reasoning in the Hambaryan-Neuhauser paper which says "burster could not have been closer otherwise would have caused extinction event" is simply wrong reasoning. They do not do that reasoning themselves, instead citing the rather cryptic "Melott A.L., et al., 2004, JBioA 3, 56" and "Thomas B.C., Melott, A.L., Jackman, C.H., et al., 2005, ApJ 634, 509". The latter has title Gamma-Ray Bursts and the Earth: Exploration of Atmospheric, Biological, Climatic and Biogeochemical Effects and is available here: http://arxiv.org/abs/astroph/0505472 I also examined http://arxiv.org/pdf/astro-ph/0309415v3.pdf by those same authors. I found these highly unconvincing that gamma ray bursts would cause major biological havoc. They rest on the idea the gamma rays would deplete ozone and increase nitrogen oxides in the atmosphere, and those would cause problems. This in turn is based on extremely unconvincing and vaguely described modeling which they admit they themselves do not understand. I would think both ozone and NOx would be affected by UV from the sun by an amount which in just 1 day of sun, would dwarf the gamma ray effects. So I'm very skeptical. So I do not believe this part of the Hambaryan-Neuhauser paper. And that is a good thing too, because if we did believe it, it would yield a clear contradiction with reality, i.e predicting lots of extinctions which apparently did not happen.