Tuesday, July 26, 2005

GRB050724 - short, hard and conclusive?

Swift scores again - it detected another short (less than 1 sec), hard gamma ray burst on sunday. See GCN for info as it comes in

X-ray localization was good - and RXTE caught the x-ray afterglow as well.

There are 3 candidate hosts or counterparts in the field. The "Energizer Bunny" team out in California pounced again, and have spectra (as do others). Two are stars. One (object D) is a low (z ~ 0.25 - tentative) Elliptical galaxy! With a variable radio source, maybe associated with the GRB (to be confirmed)

And, the most recent circular hints at a near-infrared counterpart in the outskirts of the galaxy.

If that is the case, it pretty much clinches the case for short, hard GRBs being neutron star mergers, as conjectured on many occasions, including in this paper.
Its location offset from, but near an elliptical galaxy is consistent with the binary neutron star merger scenario, something like 20-30% of NS-NS mergers in the local universe might be expected to take place in ellipticals, with the binaries offset from the stellar light due to their high peculiar natal velocities, but since the ellipticals tend to be large and have deep potentials, some NS-NS binaries are retained within the galaxies, rather than escape into intergalactic space.

The NS-NS mergers which take place either inside the galaxy, or in the centers of clusters of galaxies, are preferentially localised by their x-ray afterglow, because they are still embedded in moderate density warm gas - the NS-NS binaries that went too far out in to intergalactic space are embedded in gas so tenuous that the shock is late and weak, so the intensity of x-ray and optical emission is too low for detection with current instruments.

This all fits into a very neat elegant package, and confirms our previous conjectures and speculations.

Maybe, could still be surprises out there. But it looks real good. Real, real good.
Now we just have to figure the microphysics of how the energy gets channeled into a relativistic outflow...


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