  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. 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 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 hypothetical galactic GRBs significantly increases the total
muon flux. However, since the muon production efficiency is extremely low for
photon energy 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.
