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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 a High-Energy Astronomer
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