ASTROPHYSICS AND EXTINCTIONS: NEW RATE ESTIMATES FOR POTENTIALLY LETHAL IONIZING RADIATION EVENTS

It has been recognized for some time that astrophysical sources of high-energy radiation (including gamma-rays, X-rays and charged particles known as cosmic rays) may play a role in extinctions on Earth. Several studies have demonstrated the ability of events such as supernovae, gamma-ray bursts and extreme Solar flares to deplete stratospheric ozone, leading to enhanced surface-level solar ultraviolet radiation, which is damaging to organisms. On timescales long compared with human history, the probability of an event intense enough to disrupt life on the land surface or in the oceans becomes large and therefore knowledge of the rates and intensities of such events is important in efforts to connect to extinctions in the fossil record. We enumerate the known sources of radiation and characterize their intensities at Earth and rates or upper limits on these quantities. When possible, we estimate a ‘‘lethal interval,’’ our best estimate of how often a major extinction-level event is probable given the current state of knowledge; we base these estimates on computed or expected depletion of stratospheric ozone. In general, moderate-level events are dominated by the Sun, but the far more severe infrequent events are probably dominated by gamma-ray bursts and supernovae. We note for the first time that so-called ‘‘short-hard’’ gamma-ray bursts are a substantial threat, comparable in magnitude to supernovae and greater than that of the higher-luminosity long bursts considered in most past work.