Terrestrial Consequences of Spectral and Temporal Variability in Ionizing Photon Events

Abstract

Gamma-ray bursts (GRBs) directed at Earth from within a few kiloparsecs may have damaged the biosphere, primarily through changes in atmospheric chemistry that admit greatly increased solar UV. However, GRBs are highly variable in spectrum and duration. Recent observations indicate that short (similar to 0.1 s) burst GRBs, which have harder spectra, may be sufficiently abundant at low redshift that they may offer an additional significant effect. A much longer timescale is associated with shock breakout luminosity observed in the soft X-ray (similar to 10(3) s) and UV (similar to 10(5) s) emission and radioactive decay gamma-ray line radiation emitted during the light-curve phase of supernovae (similar to 10(7) s). Here, we generalize our atmospheric computations to include a broad range of peak photon energies and investigate the effect of burst duration while holding total fluence and other parameters constant. The results can be used to estimate the probable impact of various kinds of ionizing events ( such as short GRBs, X-ray flashes, and supernovae) on the Earth's atmosphere. We find that the ultimate intensity of atmospheric effects varies only slightly with burst duration from 10(-1) to 10(8) s. Therefore, the effect of many astrophysical events causing atmospheric ionization can be approximated without including time development. Detailed modeling requires specification of the season and latitude of the event. Harder photon spectra produce greater atmospheric effects for spectra with peaks up to about 20 MeV because of greater penetration into the stratosphere.