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From a Reddit post

GRB 080319B was a gamma-ray burst (GRB) detected by the Swift satellite at 06:12 UTC on March 19, 2008. The burst set a new record for the farthest object that was observable with the naked eye: it had a peak visual apparent magnitude of 5.7 and remained visible to human eyes for approximately 30 seconds. The magnitude was brighter than 9.0 for approximately 60 seconds. If viewed from 1 AU away, it would have had a peak apparent magnitude of −67.57 (21 quadrillion times brighter than the Sun seen from Earth). Wiki

Someone asked what would happen if that GRB happened from the distance of Proxima Centauri, what effect would that have had on us?

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    $\begingroup$ Question is fine and well-received, two well-received answers support this as well. The vote to close is unproductive, clutters the review queue and makes unnecessary work for others $\endgroup$ – uhoh Mar 21 at 23:19
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    $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$ – called2voyage Mar 24 at 12:58
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Faster than light's answer is fine if the gamma ray burst is "pointing" towards us. The emission from rapidly moving material in a GRB is thought to be beamed in the direction of that motion, along the rotation axis of the progenitor object in fact.

The opening angle of the beam may only be a few degrees (e.g. Frail et al. 2001), which would mean that for random orientations of the GRB source there would only be about a 1 in 500 chance of the solar system being "in the beam". Thus the highly destructive conclusion of Faster than Light's answer is a remote possibility.

As the fireball phase of the GRB develops then it is thought that the beam opening angle will widen over days and weeks, but of course the source brightness also fades drastically on these timescales too. It should also be noted that most of the highly destructive X-ray and Gamma ray emission arises during the tightly beamed phase within minutes or hours of the burst.

Thus although I'm not saying we're not "screwed", I am saying that the issue is more complex than scaling the apparent magnitude of a GRB and comparing it to the Sun. I suspect that in most cases the effects would be similar to the explosion of a supernova at the distance of Proxima Cen.

The conclusions I can garner from such as https://physics.stackexchange.com/questions/26578/near-earth-supernova?rq=1 , suggest that would still likely remove the ozone layer for some time and therefore could have serious and deadly consequences for life on Earth.

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    $\begingroup$ 'The opening angle of the beam may only be a few tenths of a degree (e.g. Frail et al. 2001), which would mean that for random orientations of the GRB source there would only be about a 1 in 500 chance of the solar system being "in the beam".' I find the number 1/500 in the paper, but wouldn't the corresponding angle be $\arccos(1-1/500) \approx 3.6^\circ$? $\endgroup$ – JiK Mar 20 at 21:54
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    $\begingroup$ @JiK I misread the x-axis of the plot. $\endgroup$ – ProfRob Mar 20 at 22:42
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Short answer: We're screwed.

Long answer:

Using the inverse square law, the apparent magnitude of the GRB would be about -36.51, which is about 8091 times brighter than the Sun. That's amazing, but no one would be there to witness it.

Earth would be completely roasted, with all life destroyed by the deadly radiation. The ozone layer would also be destroyed, scattered into molecular oxygen, further reducing the probability that life can redevelop on Earth.

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    $\begingroup$ Yes, that's right, I'm too sleepy and not able to do maths in my head, consider my comment deleted. $\endgroup$ – James K Mar 19 at 22:32
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    $\begingroup$ If it was only for a minute, only half of the Earth would be fried. I guess the GRB would be pretty hot for 24 hours, though. $\endgroup$ – Keith McClary Mar 20 at 1:32
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    $\begingroup$ What about the life around deep ocean hydrothermal vents? Would a couple of miles of water be enough of a shield for those life forms to survive? $\endgroup$ – Mike Scott Mar 20 at 7:45
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    $\begingroup$ The ozone layer is not "destroyed" as much as it is "depleted". It's constantly renewed by sunlight. As long as there were no long term changes it would just form again. It's not like helium; (when it's gone, it's gone). commons.wikimedia.org/wiki/File:Ozone_cycle.svg Of course the atmosphere might be trashed by fires and perhaps a "nuclear winter" type scenario would unfold, but I don't think a temporary drop in ozone is going to be a major problem. $\endgroup$ – uhoh Mar 20 at 10:53
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    $\begingroup$ @uhoh I think you might have an interesting (addendum) answer there! $\endgroup$ – Tom Sol Mar 21 at 16:08

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