Terrestrial Consequences of Spectral and Temporal Variability in Ionizing Photon Events

Gamma-Ray Bursts (GRBs) directed at Earth from within a few kpc may havedamaged the biosphere, primarily though changes in atmospheric chemistry whichadmit greatly increased Solar UV. However, GRBs are highly variable in spectrumand duration. Recent observations indicate that short (~0.1 s) burst GRBs,which have harder spectra, may be sufficiently abundant at low redshift thatthey may offer an additional significant effect. A much longer timescale isassociated with shock breakout luminosity observed in the soft X-ray (~10^3 s)and UV (~10^5 s) emission, and radioactive decay gamma-ray line radiationemitted during the light curve phase of supernovae (~10^7 s). Here wegeneralize our atmospheric computations to include a broad range of peak photonenergies and investigate the effect of burst duration while holding totalfluence and other parameters constant. The results can be used to estimate theprobable impact of various kinds of ionizing events (such as short GRBs, X-rayflashes, supernovae) upon the terrestrial atmosphere. We find that the ultimateintensity of atmospheric effects varies only slightly with burst duration from10^-1 s to 10^8 s. Therefore, the effect of many astrophysical events causingatmospheric ionization can be approximated without including time development.Detailed modeling requires specification of the season and latitude of theevent. Harder photon spectra produce greater atmospheric effects for spectrawith peaks up to about 20 MeV, because of greater penetration into thestratosphere.Comment: 30 pages, to be published in ApJ. Replaced for conformity with published version, including correction of minor typos and updated reference