Question regarding Gamma Ray bursts?

[cosmicalstorm]

I was reading the chapter on Gamma Ray bursts in Existential Risks by M.Circovic, N.Bostrom.
If one occurred in our galaxy it would have the potential to deposit something like 1 Kt of TNT/ Km² of the atmosphere, if we were so unlucky as to be in the way of of the Gamma-laser shooting out. Needless to say this would be catastrophic for our planets biosphere.

But if a GRB occurred somewhere a bit further away, lets say in the Andromeda galaxy or in one of the dwarf galaxies orbiting the Milky Way and the energy beam passed our solar system. Would it be possible that we got hit by a beam that would do less damage, lets say it would deposit between 1-100 tons of TNT/Km² or something along those lines. What would the effects on the planet be in the hemisphere that got hit, what would the general climate effects be of such a "lesser beam"?

Maybe there would be a massive storm coming out of the blue skies with no forewarning on the side that got hit? Ten years later we might have a massive spike of cancers and so on?
Could it happen at all, perhaps the beam would thin out so rapidly if it occurred in the out-galactic environment that it would have negligible effects on the planet?

[Executor32]

GRB 070201 originated in M31 and nothing of note happened, so it'd likely have to be in one of the dwarf galaxies (probably closer, within the Milky Way) to ruin your day.

[Simon_Jester]

The beam intensity drops off with the square of the distance- think of it as a giant flashlight of death.

Andromeda is about 100 times farther away than a typical star in our galaxy, so the intensity drops off by a factor of ten thousand. So instead of a gamma ray flux of ~4 MJ per square meter, you get something like 400 J per square meter. Now, it's hard for me to estimate what kind of radiation dose that translates into, especially after you factor in the Earth's atmosphere which makes a huge difference.

[Napoleon the Clown]

From another galaxy? It'd just be a bright flash of gamma-rays and do nothing to the planet at all. Just because the things emit a fuckton of energy in a short time span doesn't mean they've got a nigh-infinite "fuck shit up" track. If it were 1 kiloparsec away? Yeah, very bad news, mass extinction. That's only 3260 light-years away, which in astronomical terms is next door. Another ARM of the Milky Way and we'd be fine. Space is big.

Besides, a GRB has to be pointed right for us to even see it in the first place. Generally meaning it was pointed more or less right at us. And considering how many we've detected since they were first discovered in the sixties... We'd have been long since screwed if a GRB in another galaxy posed any threat to us. Solar flares are much more likely to give us problems. There aren't any stars within a thousand parsecs of us that we suspect to be even capable of a GRB, much less are capable and have a pole pointed directly at us. It's a neat "What If?" but we'll never so much as get our hair mussed by a GRB.

Short version: A GRB pointed perfectly at us from Andromeda would simply make astronomers go "Oh, pretty." Nothing else would happen.

[Irbis]

It's a neat "What If?" but we'll never so much as get our hair mussed by a GRB.

Historical evidence might not necessarily agree with that assessment. Seen any trilobites around recently?

[Sarevok]

just speculation. No concrete evidence GRBs caused extinction of trilobites.

[Simon_Jester]

That said, it's plausible and given the sheer number of supernovae that have occurred in the Milky Way some time in the last several hundred million years, it's hardly surprising if the Earth did get gamma-bursted in there at some point.

[Surlethe]

How many GRBs can be expected to have occurred in the MW during, say, 300 million years? Say we average 100 GRBs/day = 4E2 GRBs/yr through the universe and there are 2E11 galaxies in the observable universe. In any given year, there are on average 2E-8 GRBs in the MW (throwing in an order of magnitude because the MW is larger than average galaxies). That means over 3E8 years, there should have been 6 GRBs in the MW. IIRC GRBs are believed to release their energy in jets about 10 degrees wide, so one average GRBs have zapped about 120 degrees of the MW's disk. I take this to mean it's unlikely that the Earth has been hit.

Sound reasonable? I'm not feeling so good about my "angular analysis."

[Simon_Jester]

Er, you miscalculated frequency. If you posit a hundred GRBs a day, that's about thirty-six thousand a year. Divide by number of galaxies and you get 1.825*10^-7 GRBs per galaxy per year, or about 5.4 million years between bursts. Which makes it a bit more likely that Earth's gotten zapped at least once between now and the Cambrian Explosion.

Calculating based on the past 300 million years alone won't necessarily give the whole picture; my working hypothesis is that a nearby GRB would cause a detectable extinction event if there were multicellular life to leave fossils and vanish mysteriously, but would be much harder to detect if it happened at a time when there was only single-celled life on Earth.

Me, I was working from the number of known supernovae; they happen in the Milky Way on the order of once every hundred years, using the formal definition of "on the order of" since I don't know the exact number.

[Surlethe]

Oh right, I was off by another order of magnitude. 4E3 per year, not 4E2 per year. So there have been something like 60 GRBs in the MW since the advent of multicellular life on Earth. I'm still concerned about the angular coverage of the jets, though; the luminous MW is a disk, which means that if the jets are randomly distributed, they'll probably be pointing out of the face of the disk rather than cutting through it.

[Simon_Jester]

Eh. Put it this way- my take on it is not "with high probability the Earth got zapped by a GRB." It's "GRBs are common enough that it would hardly be a big surprise for that to happen."

[Surlethe]

Oh yeah, I wouldn't be that surprised. A little surprised, but not really super-surprised. Sort of like the surprise you hear when a relative gets in a car accident: it's surprising, but there are lots of car accidents.