Occurrence of potentially hazardous GRBs launched in globular clusters
  Nearby, Galactic gamma-ray bursts (GRBs) may affect the terrestrial biota if their radiation is beamed towards the Earth. Compact stellar binary mergers are possible central engines of short GRBs and their rate could be boosted in globular clusters. Globular cluster typically follow well defined orbits around the galactic center. Therefore their position relative to the solar system can be calculated back in time. This fact is used to demonstrate that globular cluster - solar system encounters define possible points in time when a nearby GRB could have exploded. Additionally, potential terrestrial signatures in the geological record connected to such an event are discussed. Assuming rates of GRBs launched in globular cluster found from the redshift distribution of short burst and adopting the current globular cluster space-density around the solar system it is found that the expected minimal distance d_min for such a GRB in the last Gyr is in the range d_min ~ 1 - 3.5 kpc. From the average gamma-ray luminosity of a short GRB significant depletion of the terrestrial ozone-layer is expected if such an event explodes at a distance of ~1 kpc. In the last Gyr a few globular cluster passages are expected within a distance of d_min from the solar system and a GRB should have exploded during one of these passages. Globular cluster - solar system encounters and events of mass extinction in the history of life can be correlated to investigate the impact of a nearby GRB on the terrestrial biota. To explore such a correlation reliable globular cluster positions relative to the solar system have to be calculated for the time span of the fossil record of the last 600 Myr. The upcoming GAIA mission will be crucial to determine the possible time intervals of the occurrence of nearby GRBs launched in globular clusters. 
Wilfried F. Domainko
astro-ph.EP astro-ph.HE