Gamma Ray Bursters Can Fry Us

Found this online, worthy of future investigation. The proximity of a supernova required to do us harm is reasonably well-known, but apparently a Gamma Ray Burster can be much more distant (thousands of light years) and still fry us:

An Australian astronomer, Ray Norris, notes that a supernova would be likely to depopulate all planets within 50 light years. A Gamma Ray Burster is even more formidable, releasing energy on the order of five magnitudes the order of energy released by a supernova. It can exterminate life over many thousands of light years. Norris calculates that supernovas and GRBs should activate Earth's "reset" button every 200 million years but as far as Norris can tell this has not happened for about twenty times that period. Perhaps our luck will continue to hold, perhaps not.

Found here:
http://abob.libs.uga.edu/bobk/ccc/cc051002.html

At Wikipedia it says:

The scientists calculated that gamma-ray radiation from a relatively nearby star explosion, hitting the Earth for only ten seconds, could deplete up to half of the atmosphere's protective ozone layer. Recovery could take at least five years.