Biological radiation dose from secondary particles in a Milky Way gamma ray burst

Gamma ray bursts (GBRs) are a class of highly energetic explosions emitting radiation in a very short timescale of a few seconds and with a very narrow opening angle. Although, all GRBs observed so far are extragalactic in origin, there is a high probability of a GRB of galactic origin beaming towards the Earth in the past ~ 0.5 Gyr. We define the level of catastrophic damage to the biosphere as approximation 100 kJ per square meter, based on Thomas (2005 a,b). Using results in Melott and Thomas (2011), we estimate the probability of the Earth receiving this fluence from a gamma ray burst of any type, as 87% during the last 500 Myr. Such an intense burst of gamma rays would ionize the atmosphere and deplete the ozone layer. With depleted ozone, there will be an increased flux of solar UVB on the Earth’s surface with potentially harmful biological effects. In addition to the atmospheric damage, secondary particles produced by gamma ray-induced showers will reach the surface. Amongst all secondary particles, muons dominate the ground-level secondary particle flux (99% of the total number of particles) and are potentially of biological significance. Using the Monte Carlo simulation code CORSIKA, we modeled the air showers produced by gamma ray primaries up to 100 GeV. We found that the number of muons produced by the electromagnetic component of hypothetical galactic GRBs significantly increases the total muon flux. However, since the muon production efficiency is extremely low for photon energies below 100 GeV, and because GRBs radiate strongly for only a very short time, we find that the biological radiation dose from secondary muons is negligible. The main mechanism of biological damage from GRBs is through solar UVB irradiation from the loss of ozone in the upper atmosphere.