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The Question

(Submitted September 10, 1999)

I have studied with fascination the massive gamma ray burst event (GRB990123) that has shocked astronomers with the unimaginable power of the burst that occurred on the 23 of January this year. One press release claimed that if the same event happened a mere 2000 light years away, it would appear twice as bright as the Sun for the brief time of the burst. My question relates to the impact of the gamma rays on earth if such an event occurred within a relatively close proximity to earth, would it compare to the electromagnetic pulse (EMP) effect of a high-altitude nuclear explosion on earths delicate electronic hardware? (How close would such a burst have to be to create such an effect?)

The Answer

If a gamma-ray burst occurred near to us, it would be Bad.

For a description of what a mere supernova could do, see http://stupendous.rit.edu/richmond/answers/snrisks.txt

The gamma-rays from 990123 had 1000 times the energy flux of the optical light, so at 2000 light years the gammas would deposit 2000 times as much energy as the Sun (in addition to twice as much visible light). Furthermore, this gamma-ray energy would interact in the upper atmosphere, producing nitrogen oxides that would rapidly catalyze the destruction of the ozone layer.

And then, a few centuries later, it gets worse, if current models are correct. A storm of cosmic rays would pretty much wipe out everything that wasn't beneath a few hundred meters of rock.

See Sky and Telescope for February 1998 (in most good libraries) for more details.

In answer to your EMP question, I believe that the typical scenarios involve a a big bomb, call it 10 megatons, at a high altitude, call it 1000 km. 10 megatons is 4.2 x 1023 ergs. 990123 produced about 4 x 54 ergs of gamma-rays, so it produced 1 x 1031 times as much, and would be as vicious at 3 x 1015 times the distance. 3 x 1015 x 1000 km is about 300,000 light years.

So by this analysis, from anywhere in our Galaxy, the gamma rays would cause a massive EMP event and smite all of the electronics on that side of the planet. (And maybe on the other side as well, I don't know how EMP propagates over the horizon.)

However, there are probably mitigating effects. A bomb produces a very fast release of gamma-rays (microseconds to milliseconds) causing a fast rise in electric field, the same amount of energy over a shorter time means more power in the pulse. The slower GRB 990123 (lasting about a minute) would probably cause a corresponding decrease in the EMP. There are, however, GRBs with rise times of less than a millisecond.

David Palmer and Samar Safi-Harb
for Ask an Astrophysicist

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