Biological Effects of Gamma-Ray Bursts: distances for severe damage on the biota

We present in this work a unified, quantitative synthesis of analytical and numerical calculations of the effects that could be caused on Earth by a Gamma-Ray Burst (GRB), considering atmospheric and biological implications. The main effects of the illumination by a GRB are classified in four distinct ones and analyzed separately, namely: direct gamma Flash, UV Flash, Ozone Layer Depletion and Cosmic Rays. The effectiveness of each of these effects is compared and distances for significant biological damage are given for each one. We find that the first three effects have potential to cause global environmental changes and biospheric damages, even if the source is located at Galactic distances or even farther (up to 150 kpc, about five times the Galactic diameter of 30 kpc). Instead, cosmic rays would only be a serious threat for close sources (on the order of a few pc). As a concrete application from a well-recorded event, the effects on the biosphere of an event identical to the giant flare of SGR1806-20 on Dec 27, 2004 have been calculated. In spite of not beeing a classical GRB, most of the parameters of this recent flare are quite well-known and have been used as a calibration for our study. We find that a giant flare impinging on Earth is not a threat for life in all practical situations, mainly because it is not as energetic, in spite of being much more frequent than GRBs, unless the source happens to be extremely close.