Astrophysical and Astrobiological Implications of Gamma-ray Burst Properties

Combining results from Schmidt (1999) for the local cosmic rate and mean peak luminosity of γ-ray bursts (GRBs) with results on the history of the cosmic star formation rate, we provide estimates for the local GRB rate per unit blue luminosity in galaxies. For a moderate increase in SFR with redshift, we find a GRB rate per unit B luminosity of 2.4× 10h70 L −1 ⊙ yr . The corresponding mean γ-ray luminosity density in the Milky Way is 1.6× 10 erg s pc and the total rate is 5.5× 10h70 yr . These values are used to examine a number of phenomena with the following conclusions: 1) The ratio of supernova rate to isotropic equivalent GRB rate is large: (>∼ 6000 SN Ibc per GRB, >∼ 30,000 SN II per GRB). With no correction for collimation it is difficult to maintain that more than a small fraction of neutron star or black hole-forming events produce GRBs. GRBs could arise in a large fraction of black hole or magnetar-forming events only with collimation in the range ∆Ω/4π ∼ 0.01 − 0.001 and a steep enough slope of the IMF; 2) Without substantial collimation, the GRB rate is small; with collimation, the energy input is small. The net effect is that it is impossible to use these events to account for the majority of large HI holes observed in our own and other galaxies; 3) Modeling the GRB events in the Milky Way as a spatial Poisson process and allowing for modest enhancement in the star formation rate due to birth in a spiral arm, we find that the probability that the solar system was exposed to a fluence large enough to melt the chondrules during the first 10 yr of solar system history is negligibly small, independent of collimation effects. This is especially true considering that there is strong evidence that the chondrules were melted more than once; 4) We calculate the probability that surfaces of planets and satellites have been subjected to irradiation from GRBs at fluence levels exceeding those required for DNA alterations during a given period of time. Downscattering to energies at which photoelectric absorption occurs results in a transmission factor for ionizing radiation that is an exponential function of the atmospheric column density. Even for very opaque atmospheres, a significant fraction of the GRB energy is transmitted as UV lines due to excitation