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Gamma Ray bursts (GRBs), short and intense bursts of γ-rays, are the brightest explosions known. The copious flux of γ-ray photons with energies above 100 keV from a galactic GRB could destroy the ozone layer making them potentially damaging to life on Earth ... Given the recent significant progress in quantifying the main ingredients that determine whether GRBs have any effect on Earth: their rate, luminosity function and dependence on metallicity it is therefore timely to reasses this issue...
To estimate the effect of a GRB on life on Earth we need to know what the dangerous radiation doses are. Ruderman [1], who considered at the time the effect of a nearby Supernovae on Earth, realized that the most damaging effect would be the depletion of the Earth protective Ozone layer for a period of months. This would happen via formation of stratospheric nitric oxide that destroys the Ozone. The Ozone depletion would lead to enhancement of UVB solar radiation that, in turn, would be harmful to life. Note that the UVB fluence on the surface of the ocean will destroy surface marine life [as described in detail in Ref. 8] among them plankton, which will deprive (marine) life of their main nutrient. In 1995, after it was realized that GRBs are cosmological and their rate was estimated, Thorsett [2] applied these ideas to Galactic GRBs. A decade later Thomas et al. [7, 8] carried out the most extensive, to date, calculation of the effects of the gamma-ray flux on the Earth atmosphere. They find that a fluence of 10kJ/m² will cause a depletion of -68% of the ozone layer on a time scale of a month. Fluences of 100 kJ/m² and 1000 kJ/m² will cause depletions of - 91% and -98% respectively... Following Thomas et al. [7, 8] we estimate that a fluence of 10 kJ/m² will cause some damage to life, while 1000kJ/m2 will wipe out nearly the whole atmosphere causing a catastrophic life extinction event; we consider F = 100 kJ/m² as our canonical life threatening fluence.
We ... explore the possible threat caused by GRBs to life elsewhere in the Milky Way... Clearly to do so one must assume the lethal radiation dose that will be threatening to life elsewhere. While life can take numerous other forms and could be much more resilient to radiation than on earth, we make here the conservative assumption that life is rather similar to the one on Earth. This common assumption is the basis for searches of Earth like exoplanets as places that harbour life. Under this assumption, we explore what is the likelihood that a nearby GRB results in a dose of 100 as well as 10 and 1000 kJ/m² in various regions of the Milky Way...
The stellar density is significantly larger towards the center of the Galaxy and hence the threat to life on most exoplanets, that reside in this region, is much larger... A lethal GRBs of 100 kJ/m² would be more likely than 95% up to a distance of 2 kpc from the Galactic center in which 25% of the Milky Way stars reside. When considering Fluence= 10 and 1000 kJ/m² we find 12 and 0.5 kpc respectively. In agreement with the specific estimates for Earth, events around the Solar distance from the Galactic could be significant but rare and only at a distance above 10 kpc the threat from GRBs becomes small. Therefore, life can be preserved with certainty only in the outskirts of our Galaxy. In total 90, 40 and 5% of the exoplanets in the MW would be exposed to a fluence of 10, 100, and 1000 kJ/m² from GRBs within a period of 1 Gyr.
(Submitted on 8 Sep 2014 (v1), last revised 13 Nov 2014 (this version, v2))
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