Knowing the power of these bursts and their average rate throughout the Universe, it is possible to calculate the likely rate of events in our "neighborhood". In this case, any burst on our side of the Milky Way galaxy is likely to be dangerous, if it is pointed at the Earth. Several groups have independently estimated that one or more GRBs is likely to have "blasted" the Earth from a distance of 6,000 light years, or even less, within the last 500 million years or so. Some evidence to support this rate has come from the observation of a GRB remnant in our own galaxy. It appears to be young, but not pointed at our solar system.
Possible effects on the Earth include sky-darkening levels of "smog", nitric acid rain, and especially destruction of the ozone layer, which protects us from the Sun's ultraviolet radiation. Brian Thomas has been computing just how severe these effects are likely to have been given various possible GRB "events" affecting our planet.
We are also examining the fossil record for signs of GRB effects. Given the severity of the effects, we think it is reasonable that some of the dieoffs known as "mass extinctions" may have been induced by GRBs. The end Ordovician extinction, the second-largest in the fossil record, seems to be a good candidate. Here is an image of Ordovician life provided by William Berry, University of California Museum of Paleontology.
We have written a paper (click here for pdf file) on the possible connection of a GRB event with the end Ordovician extinction. We hypothesize that this extinction is linked to the effects of a gamma-ray burst, resulting in a damaged ozone layer and a decline in global temperatures, both as side effects of changes in atmospheric chemistry. Patterns from the fossil record suggest that organisms whose lifestyle left them little shielded from the Sun's UV radiation may have been most susceptible to extinction. Our work may have some practical value in understanding the dangers we face today from our adverse impact on the ozone layer. There were news stories about our work in Nature, New Scientist, and CNN.com.
We have explored the details of how the X-rays from such an event would impact the Earth's atmosphere. We have simulated the evolution of the chemistry and the resultant loss of ozone protection from solar UVB. It is possible to combine this information with known DNA damage at various wavelengths to construct a time-dependent map of the DNA damage. You can view a movie of the DNA damage from a burst occuring at noon over the equator in March. In the film, you first see a "normal" year prior to the burst at day 0. White is coded to the global annual average rate of DNA damage, blue below that, and increasingly darker red to higher damage rates. The effects are extremely intense for a few months and persist for years. To view the movie in gif format click here . A screen refresh will rerun the movie. To view in mpeg (better image, but may cause problems in some viewers) click here. Anything darker than pink corresponds to likely widespread lethal UV for organisms living near the surface of the water. Bursts at other latitudes, times of year, times of day have somewhat different effects, but because they get much more sunlight than polar regions, equatorial and mid-latitudes have the biggest integrated damage. In April 2005, NASA issued a press release on this extinction hypothesis which contains additional narrative and animations.
The following papers (linked in a variety of ways) contain details of the atmospheric computations, with a focus on ozone depletion and UVB transmission:
Astrophysical Journal, 2005
Astrophysical Journal, 2007
In addition to damage from solar UV due to ozone depletion, two other effects may be biologically important. Nitrogen dioxide, a brown gas, may cut off sunlight and lower temperatures. We are interested in the possibility that this may in particular have triggered the end-Ordovician glaciation. In the years after the burst, the atmosphere is cleaned by rainout, largely in the form of dilute nitric acid--which may paradoxically act as a fertilizer for plants. We have shown results (in Geophysical Research Letters) of computations of these effects for our fiducial gamma ray burst.
Most recently, we have compared the patterns of extinction in the Ordovician with those that emerge in our simulations, and concluded that any such burst would have to have been over the Earth's south pole. This interesting result agrees with the idea of a burst coming from within our galaxy, because the south pole is always close to the galactic disk.