Monday, May 09, 2005

Short GRBs - what's the fuzz?

Ok, having lapsed into exuberant mode, I should probably explain myself.


and here

GRBs are short flashes of high energy gamma rays. Very high energy radiation, effectively blocked by the atmosphere and only see from space. The energy budget is comparable to that of the most powerful supernovae, and even very distant ones have enough energy flux to affect the upper layers of the Earth's atmosphere.
Very close GRBs (inside our own galaxy, and near us, which occur ~ every 500 million years) may be bright enough to cause mass extinctions!

Before we knew what they were, we knew there were two types: a "short hard" type, lasting less than a second; and a long type, typically lasting many seconds. So the possibility there might be two physical mechanisms has long been speculated upon.

When BeppoSAX found optical counterparts to GRBs, they were exclusively the long GRBs.
We are now pretty sure that these form from very young, massive stars in small blue galaxies undergoing strong episodes of star formation - some of these GRBs are "dark" - we are also fairly confident this is because they are hidden behind dense dust clouds which obscure the optical radiation while letting through the gamma and x-ray radiation.

The GRB, according to the Rees-Meszaros-Paczynski scenario, occurs when a rapidly rotating massive (~ 10 solar masses) black hole forms in a "failed supernova" that become re-energised as a "hypernova". In the process, two jets of material, traveling at almost the speed of light, punch out through the poles of the star and "shock" against the clouds of interstellar gas near the star, causing non-thermal gamma-rays to be emitted. This is a fairly well tested scenario; the one weakness is that the details by which (and how) the black hole forms and ejects the energy is not well know - we know the basics of the "engine" but not the detailed microphysics (but there are theories...).

So a GRB is the signature of a black hole forming. But possibly only a sub-set of all black holes; the jet of radiation are strongly and narrowly beamed, so we miss most of them (the radiation beams are not pointed at us). Visible GRBs seem to occur at the rate of ~ 3 per day in the universe. But we may be missing 99+% of the events.
Black holes form at the rate of maybe 100,000 per day in the universe (about one per second). So maybe as few as one black hole in a thousand makes a successful GRB!?

But, we couldn't nail down the short GRBs.

One theory was that they were "clean" mergers of two neutron stars, bound together and spiraling in through emission of gravitational radiation (a process we see happening in the galaxy).
The lower mass neutron star, would disrupt just before contact (fraction of a second) and form a relativistic disk, cooling through neutrino emission. The higher mass neutron star would implode into a black hole, but the inflowing material has too much angular momentum, so something has to go out. This would be relativistic jets of material, which would then shock, as above, to make gamma rays.

But, neutron star pairs take 100s of millions to billions of years to merge. So this happens long after star formation, so we might expect to see it in old galaxies with no ongoing star formation. Like elliptical galaxies.
Further, NS-NS pairs are observed (and theoretically should) have high peculiar velocities. So they are displaced by 10s of kpc from their site of formation by the time they merge, and should be seen in the outskirts of galaxies.
(There is a subtlety - basically the higher the peculiar speed, the shorter the timescale to merger, on average, so the typical displacement is not as broad as one might naively infer).

Anyway, Josh Bloom looked at this extensively for his PhD thesis. And one of Swift's major science goals was to localize short GRBs and test theories of them.

If the variability of the alleged counterpart is confirmed, then this has been done.

Preliminary data is that it looks good and it could well be a NS-NS merger.

Disclaimer: I am not a member of the Swift team and have no association with any group taking data.
I have a little bit of experience chasing optical counterparts, but am not in competition with these groups.
I wrote a couple of theory papers on this a long time ago. I am chuffed but have nothing to gain from this.


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