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As far as I can gather (and based on an article in this month's Astronomy magazine, and this webpage), a Gamma Ray Burst (GRB) can be short or long duration, and seems to be caused by a rare kind of powerful supernova - a 'hypernova'.

The impression I get is that the gamma ray blast is focussed along a particular axis of the exploding star. We see the GRBs because we happen to be situated along that axis (and they don't wipe out life on Earth because they're billions of lightyears away).

But what I can't find any info on is just how wide that 'beam' of gamma rays from a GRB a billion or two lightyears away is when it reaches us. Is it an AU across (so we're really lucky to even see it)? A lightyear? 10 lightyears wide? More? Or is it so thin that we could be in a different spot on Earth and miss it (unlikely I think)?

The Astronomy article implies that a close GRB that was only 6,500 ly away could have been responsible for the mass extinction at the end of the Ordovician period on Earth. In a mere 10 seconds, the gamma rays 'reacted' with the N2 and O2 in our atmosphere and created nitrogen oxide smogs that spread over the planet and UV from the blast penetrated to the ground, severely depleting the ozone layer and causing major climate and biological damage. 15 years later the smogs had gone and the initial damage repaired, though global cooling could be a result (and one thing I always wonder is that if 50% of the biomass on the Earth gets wiped out in such a short space of time, then won't that much decaying organic matter cause problems too?)

If we were to find habitable planets in the space near our solar system - and if this scenario turned out to be true - would we see similar mass extinctions at the same time there because they were hit by the GRB too? Or is this something that would just affect one system, or even one planet in a system? Again it all depends on the width of the beam - so does anyone have any clue how wide the GRB beam could actually be?

EDIT: I found this article that says that the width of the beam could be 'a few degrees across' but how does that translate to a width measured in actual distance units?

A quibble:
My understanding is that the longer bursts are from hypernovae, but the short ones probably are caused by neutron-star mergers. See http://web.mit.edu/newsoffice/2005/gamma-ray.html for a recent announcement of the results of relevant research.

One article I read claims the beams probably are a few degrees in diameter.

[Edit]Calculating the diameter of a beam when it intersects the earth is simple trigonomety: tan(theta)*(distance to supernova) = diameter

e.g. tan (2 degrees) = 0.017 * 6500LY = 110LY

It's not very small.

[/edit]

Yeah, I was more interested in the longer ones - sorry, should have made that clearer.

Calculating the diameter of a beam when it intersects the earth is simple trigonomety: tan(theta)*(distance to supernova) = diameter

So a hypernova that is 1 billion ly from Earth with a beam with angular radius of 1 degree would have a radius of 17.5 million lightyears at Earth?! That'd cover pretty much anywhere in the milky way or local group! Though it wouldn't wipe out life anywhere since it's too far away.

If it's 6,500 ly from Earth as in the Ordovician extinction example, then it'd be 113.5 ly in radius. Pretty damn huge. if it happened today then a lot of nearby star systems would be affected.

Would the radiation be uniformly distributed throughout the width of the beam? Or would it be less damaging if we were at the edges?

Hang on, have I done something wrong here? Because that value from the first scenario doesn't make sense.

Say the beam from the aforementioned GRB a billion ly from Earth is two degrees in diameter as seen in Earth's sky, and Earth was smack in the middle of the beam cross-section.

If the beam really was 35 million ly in diameter when it reached Earth, then it'd surely cover the entire sky (in one direction anyway) seen from the ground and not just 2 degrees, wouldn't it?!

They mean it's a few degrees wide as in seen from the source.
Just as the 'beam' from the sun is 360 degrees 'wide' as seen from the sun.
Still the sun doesn't cover the whole sky. But, as the earth moves round it, it's still being lit when it's on the opposite side.

Or do I completely misinterpret your question?

julesstoop wrote:They mean it's a few degrees wide as in seen from the source.
Just as the 'beam' from the sun is 360 degrees 'wide' as seen from the sun.
Still the sun doesn't cover the whole sky. But, as the earth moves round it, it's still being lit when it's on the opposite side.

Or do I completely misinterpret your question?

No, I think you're right - I think it's the width of the beam at the source, not in the sky. Was just having a brain-fart is all .

The rednova website I linked to says:

However, an analysis of the afterglows of 17 GRBs that was published in the fall of 2001 did place limits on the width of the beam, stating that it was probably only a few degrees across. With such a narrow beam, the energy of a GRB could be provided by a supernova only slightly more powerful than average. Incidentally, theorists say that if the Earth were hit by a gamma-ray beam from a burster only a few hundred light years away, it would incinerate the surface of our planet.

The narrowness of the beam also suggested that only one in 500 GRBs are seen from Earth, meaning they are a fairly common phenomenon in the Universe, probably occurring about once every minute. This means that astronomers might be able to observe "orphan afterglows", exactly like those following a GRB, but not associated with a gamma-ray burst.

What I'm kinda curious about though is how they know that the beam is only a few degrees across.

Malenfant wrote:What I'm kinda curious about though is how they know that the beam is only a few degrees across.

Nobody knows for sure. The nature of these bursts is still uncertain and the subject of research. My guess is that the bursts would not all have the same angular width.