Supershells and the closest GRB remnants

Abstract

Following recent suggestions of a gamma-ray burst origin of large ISM holes (supershells) detected in the galaxy, we discuss the effects of the recently discovered shell GSH 238+00+09 (and other nearby extremely energetic supershells) on the earth's atmosphere. We argue that the large flux of gammas from these specific events would have provoked major catastrophes to living organisms. The lack of evidence for the latter can then be used to rule out local isotropic nearby GRBs in the last tens of Myr and another origin for large supershells is likely. Alternatively, this argument may be taken as independent evidence for beaming of the gamma rays, although statistics are not yet sufficient to constrain the beaming angle better than $\sim$ tens of degrees. The effects of less-energetic events of the type of SN1998bw are also considered. It is shown that more than a dozen of them should have occurred within $40 (E/10^{48} erg)^{1/2} pc$ of the earth in the last 500 Myr with similar effects. Beaming of the latter would also alleviate the conflict, although another likely possibility (an actual rate much smaller than $10^{-4} yr^{-1}$) may be a solution, at the same time avoiding an excessive number of supershells active in the galaxy.