Gamma Ray Bursts

[Lt. Rico]

I just saw a show on, I think it was NOVA, about research being done on Gamma Ray Bursts. While no one seems quite ready to say for sure, one of the more widely accepted theories seems to be that these are very very large stars that are suddenly and catastrophically collapsing into black holes, releasing a enormous burst of gamma ray energy in the process. The show indicated that all of these bursts, of which they detect almost one a day now, are located very far away in distant galaxies, and yet they are so powerful that they still outshine any other gamma ray source in the sky when they occur. It was suggested that the energy release from such an event, if it were to occur within our own galaxy, even several hundred light years away, would be enough to effectively broil and sterilize the surface of the earth.

What I am curious about, is why these events seem relatively common outside our own galaxy and yet never occur closer to home. Not that I'm complaining, mind you. But it does make me a little nervous- a star exlploding so violently that the gamma radiaiton could fry us even from several 100 light years distance?

Jim Puckett

[Tom Mazanec]

Eta Carinae is the nearest "pre-GRB"...it would broil us if it were pointed at us but its axis is about 45 degrees off. The reason you see one a day is that you are looking at the whole universe, and there are "billions and billions" of galaxies out there.

[John Dlugosz]

Lets see... a hundred billion galaxies produce one per day, so the probability of one in our galaxy is 1 to a hundred billion against, per day, which comes to an average time between them as a quarter-billion years. Our galaxy is much larger than the thousand light-year radius, so there is about a 150 in 1 chance that an event in our galaxy will be near us.

So you can expect a GRB close to us about once every 40 billion years. That we've not had one in the last 4 billion is not surprising.

[TravisM]

Yup, and you have to remember, that these huge stars they're talking about are probably the precursors to galactic cores i.e.: super massive black holes. There was one here, in our galaxy, or the space that would eventually become our galaxy. The black hole is still at the center, it's just sleepy.

[peter eldergill]

Are the astronomers detecting all of them (I mean the ones that are detectable)? Are they watching all the skies at once? I'm just curious as to how they are detected at all if they are so far away, should the bursts be spread out so much that you wouldn't eve know where to look? Does anyone even understand my question ?

later

Pete

[The Bad Astronomer]

Try here: http://swift.sonoma.edu. 8)

[peter eldergill]

So....can they estimate one per day by using satatistical analysis?

Later

Pete

[Jorge]

very interesting website!

sinds GRB's are far far far away, if we detected one, what is the estamte age(i mean how long ago did it happen?)

[DoctorDon]

since GRB's are far far far away, if we detected one, what is the estamte age(i mean how long ago did it happen?)

Age estimates vary. For one thing, they're far enough away that the exact model you are using for the expansion of the universe affects how far away you think they are. There have been 38 GRBs for which a redshift has been measured, and they mostly range between 0.1 and 4.3, which corresponds to look-back times between 1 and 12 billion years ago. People have tried to use those 38 bursts to calibrate the many thousands that have been observed with no redshift, and those methods indicate we might be seeing GRBs as far out as redshift of 10 (13 billion years ago). This is using models where the universe is 13.7 billion years old. (See http://www.astro.ucla.edu/~wright/CosmoCalc.html for calculations of ages for various cosmologies.)

I work on the ROTSE project, trying to observe GRBs from the ground. Our web site is www.rotse.net, and I've been writing an introductory page about GRBs for folks who might want to learn about them. That's at http://www.rotse.net/summary/
It's very much a work in progress, so I welcome comments and suggestions if you have ideas for how it could be improved.

If you could see the whole sky all at once, you would see about 1 GRB every day. At the moment, we don't have any instruments out there that can do this, since they dumped the Compton Gamma-Ray Observatory into the pacific in 2000. Right now, it's more like a few GRBs a month, but if Swift works as promised, we should be getting about one a week starting soon.

If these bursts are associated with the collapse of the most massive stars, one reason we only see them far away is that big stars live fast and die young. It's possible that stars big enough to form GRBs mostly formed at a much earlier phase in the universe, so any such stars near us would have exploded billions of years ago, and the light from those bursts would be billions of light years away from us by now. The presence of heavier elements than Helium tends to enable stars to form with smaller masses, so the first generation of stars (before those heavier elements were formed) would be expected to be much bigger than our current generation of stars. However, it is also true that the farther out in the universe you look, the bigger volume you are searching, so even if bursts are still happening in the universe now, it's much less likely we would see them nearby, because the local universe is so small compared to the volume at large distances.

Yours,

Don

[Jorge]

thanx for the reply that cleared it up,
i also wanted to take a look at your website but i keep getting
server not found errors

[Ulster]

Also, don't they focus their energy in tight beams? That would greatly reduce the odds of getting fried by a burst in the neighborhood.

[Brady Yoon]

Also, don't they focus their energy in tight beams? That would greatly reduce the odds of getting fried by a burst in the neighborhood.

Yes, Eta Carinae, as someone said earlier, is a good example. It is a potential gamma ray burster, but it isn't aligned correctly.

[Whitedwarf]

While no one seems quite ready to say for sure, one of the more widely accepted theories seems to be that these are very very large stars that are suddenly and catastrophically collapsing into black holes, releasing a enormous burst of gamma ray energy in the process.
Jim Puckett

That is one possible source. Another would be particles exiting a black hole. This requires far less release of energy. It would be very random and the GR would be directed as a beam in the direction of the particles motion. It would be "braking" radiation.

[Tom Mazanec]

DIASPORA was a good SF novel about a close GRB caused by a merger of two neutron stars (an inward spiralling binary...they actually exist, BTW, and one could be close to us...not bloody likely, though).