  Cosmic radiation backgrounds are a constraint on life, and their distribution
will affect the Galactic Habitable Zone. Life on Earth has developed in the
context of these backgrounds, and characterizing event rates will elaborate the
important influences. This in turn can be a base for comparison with other
potential life-bearing planets. In this review we estimate the intensities and
rates of occurrence of many kinds of strong radiation bursts by astrophysical
entities ranging from gamma-ray bursts at cosmological distances to the Sun
itself. Many of these present potential hazards to the biosphere: 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. We enumerate
the known sources of radiation and characterize their intensities at the 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. Given their
precursors, short bursts may come with little or no warning.
