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GRB question

The Warthog

By The Warthog
in The Astro Lounge

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Astroman

Astroman

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An amateur astronomer on another board made this statement, that I hae ma doots about:

...1 GRB ANYWHERE in the Milkway, and all life on earth, ALL life, is gone. Permanently...

Can this be true?

I'm not so sure it would annihilate us. It would depend a lot on the distance to the burst and intensity. Our atmosphere preotects us from detecting gamma rays from the surface quite well. We have detected changes in our magnetosphere due to magnetar outbursts, which are another animal, but still quite powerful, yet they've had no effect on surface life at all. If Sirius somehow exploded and caused a GRB, it could be very, very bad for us, but I doubt total elimination of life would result.

I'll do some more checking and see what others say. Just to be sure.

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The Warthog

The Warthog

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OK, so a GRB could eliminate all life on Earth, or at least damage a lot of it. On the plus side, these things are beamed, right? So, the beam may not be pointing our way, even if there is one in the parts of our galaxy that we can see? On the minus side, they are detected in other galaxies on a daily basis, so they aren't truly rare, and the ones we are seeing may just be the ones that are pointed at us.

So, how many life-bearing planets have been aborted, and can you work that into the equations for extra-terrestrial life? I guess if we are about to be annihilated by a nearby GRB, we won't know until the wavefront gets here, and then we won't know much. We'll be saying, "Oh, look, a GR...." If there's stuff out there that can do that to us, what are the real chances for life in the universe?

I think I'll get a better umbrella. You never know.

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Astroman

Astroman

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You bring up a good point, WH. Actually, a few good ones.

First, these things are detected on a daily basis. It seems the ones we can detect are beamed directly at us. This does two things. It reduces the amount of energy sent our way, and it shows us that we are missing more GRB's than we are seeing.

Second, we've been protected from GRB's radiation for a couple or three billion years, since life emerged. So far, all of the mass extinctions in the fossil record we know about point to mechanisms other than massive irradiation from space.

So, taken one and two above, I'd say the chances of life elsewhere in the universe are pretty good, and the chances of being toasted by GRB's is rare, even if, or maybe because, GRB's are not.

I'm still not convinced a GRB in our galaxy would harm us much, simply because there's nothing we've seen that's close enough that would be capable of producing one. Haven't heard back from my contacts yet, though. One of the magnetars that did measurably impact our magnetosphere did so on a very limited basis. Something less than a full on CME released by the Sun, but measured at the source, (which was on the order of 20,000 ly) released ~3x1031 ergs/cm2. It damaged one satellite that I know of, but no other surface effects were measured.

No harm in another umbrella though, WH. 8)

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OXO

OXO

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One of the magnetars that did measurably impact our magnetosphere did so on a very limited basis. Something less than a full on CME released by the Sun, but measured at the source, (which was on the order of 20,000 ly) released ~3x1031 ergs/cm2. It damaged one satellite that I know of, but no other surface effects were measured.

If it managed to damage a Satellite then surely theres the risk of something much larger hitting us and getting through the earths natural defence's? But like you rightly say Astroman we have been hit with all sorts in the life history of the planet and were still here my point is if something so large was gonna hit us we wouldnt know till the lights suddenly went out :). As to a GRB in our milkyway i was with the understanding there was one suspected canidate star in the Orions Arm part of our Galaxy? if thats the case then kiss ya beer goodbye :shock: maybe i heard or read wrongly :lol:.

James :)(Umbrella at the ready)

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Astroman

Astroman

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Of course, you're right, James. Just about anything's possible. But the closest actual GRB recorded was over 1.5 billion ly away and was a quite weak one, as these things go. Sure, if a GRB went off in our galaxy it could cause some damage, but sterilizing the Earth is a very, very remote possibility and not a sure thing at all.

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The Warthog

The Warthog

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Even if all life was sterilized, it's not the end, life would find a way back from component molecules as has been demonstrated in the lab. Would take a while though...

That's a great comfort to those of us who might be sterilized, Gordon. 8)

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Astroman

Astroman

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Even if all life was sterilized, it's not the end, life would find a way back from component molecules as has been demonstrated in the lab. Would take a while though...

That's a great comfort to those of us who might be sterilized, Gordon. 8)

...and to those of us who definitely are, WH. :shock:

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lunator

lunator

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I was thinking about this topic today - not much else going in and a thought occurred to me that I hope AM might be able to shed some light on.

If we see these GRB's in the gamma ray part of the spectrum now and they are very distant objects then they must be redshifted significantly so the actual burst must be at a much higher level in the G ray spectrum?

I know GRB's are measured in eV's but is that what we measure them as now or what they were when they happend?

Cheers

Ian

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Ambermile

Ambermile

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gamma rays are part of the spectrum and therefore can't be "shifted". If they were to be then they'd move out of the gamma ray band and be something else. You are mixing things up here in that a spectrum phase can shift along a spectrum section and therefore display C to some extent... but an axiom of the spectrum must remain just that, a band *of* the spectrum.

arthur

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lunator

lunator

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Thanks Arthur

I just want to check my understanding and make myself clear.

I know the GRB's are in the gamma part of the spectrum but due to the expansion all light is red-shifted.

The distant (young) galaxies emitted most od their light in UV but due to expansion we see it in the IR part of the spectrum and GRB's must obey the same laws as they are just a form of EM radiation. So the gamma rays we measure must have been so high that even with redsight they still are strong enought to stay in the gamma part of the spectrum.

By spectrum phase do you mean the groups of spectal lines thay are used to recognise specfic elements?

Cheers

Ian

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Astroman

Astroman

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You're both right.  The amount of redshift of the gamma rays is significant, but they start out at such high frequencies that they remain in the gamma ray band.  The redshift due to the universe expansion from UV to IR is narrow, about 400nm, when compared to the width of the gamma ray band.  Check out this diagram:

emspec.gif

To shift from UV all the way to IR, the object has to be close to 10 billion ly away.  Most shifts are much less.  Also, the eV Arthur refers to, just for reference, is Electron Volts.  The H-Alpha band we use to view the Sun's prominences is about 14 eV, while a decent gamma ray energy will be 5,000 eV and up to hundreds of millions of eV.  That's quite a range to shift and still be in gamma.

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Astroman

Astroman

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If I could add, the only way we can gauge the distance to these events is by the redshifts of spectral absorption lines in the lower bands, ie visible, UV and x-ray, by intervening material. Matter in the host galaxy are taken as the source, but the distance can be off by several percent, since we won't know if the event occurred on the far side or near side. Gamma rays are not absorbed by known elements, so we don't see the shift in them. We measure the energies in them in different ways, by "flux" and fluence". These are complex terms, but boil down to how the detector actually reacts to the incoming gamma rays. The more interaction, the higher the energy, or fluence and the longer the interaction, the higher the flux.

Cheeres. :lol:

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