short hard GRB caught - mystery solved?

For a long time, the nature of Gamma Ray Bursts - very high energy, very high power flashes of gamma-rays from very large distances - has been one of the big open questions in astronomy. First detected by Vela nuclear bomb test monitoring satellites more than 30 years ago, they were a favourite of theorists, until the Compton Gamma Ray Observatory in the early 90s basically proved all the theories wrong, and strongly suggested they were at cosmological distances.

The next big breakthrough came in 1997 when BeppoSAX localised the x-ray counterparts to a few GRBs well enough that optical searches could be done (gamma ray images have very poor resolution and singling out optical transient in 100 square degree images was impossible back then). And the first counterpart to the long GRBS (multi-second duration) was established. They are typically at redshifts of few; they are seen within or very near small galaxies undergoing intense star formation, and are now thought to be due to shock in strongly beamed, moderately relativistic jets that come from "hypernovae" - failed supernovae that form black holes. The statistics are consistent with only a small fraction of black holes forming GRBs, possibly the most rapidly rotating ones only.

But, there is a separate, harder to pin down class of GRBs, the short-hard group. Typically tens of millisecond duration, and until now, no identified x-ray or optical counterparts.

The Swift Satellite is the current top GRB hunter, optimised to rapidly catch the short GRBs.
And it has now done so. GRB050509b was caught this morning, about 30 msec duration, localised with faint x-ray counterpart and now there's a good possible optical counterpart.

Woo Hoo!.

It is about 35 kpc offset from an elliptical galaxy of redshift of only 0.22 or so.
This is consistent with a scenario where the short GRBs specifically are low mass black holes forming in neutron star-neutron star mergers (a la the Hulse Taylor pulsar or PSR 0737 (aka Burgay's pulsar).
These mergers can significantly lag behind star formation processes, and may therefore be seen in galaxies with no recent star formation; and since NS-NS pairs have significant peculiar velocities, they may be seen with a large offset from the host galaxy, as predicted by Bloom, Sigurdsson & Pols and elaborated upon extensively in Bloom's thesis.


Here is the raw info:


TITLE: GCN GRB OBSERVATION REPORT
NUMBER: 3381
SUBJECT: Swift Detection of GRB050509b: A short duration burst
DATE: 05/05/09 05:03:23 GMT
FROM: Louis M Barbier at NASA/GSFC/Swift

C. Hurkett, E. Rol, (U Leicester), L. Barbier (GSFC), S. Barthelmy (GSFC),
A. Blustin (MSSL), D. Burrows (PSU), J. Cummings (GSFC/NRC),
N. Gehrels (GSFC), J. Kennea (PSU), C. Markwardt (GSFC/UMD), S. Holland
(GSFC/USRA) on behalf of the Swift Team;

At 04:00:19.23 UT, the Swift Burst Alert Telescope (BAT) triggered and
located on-board GRB050509b (trigger #118749). The BAT on-board
calculated location is RA, dec 189.056, +29.000 (12h 36m 13s, +29d 00' 01'')
(J2000) with an uncertainty of 4 arcmin (radius, 3-sigma, including
estimated systematic uncertainty). The BAT light curve showed a single short
spike with a duration of less than 128 milliseconds. The peak
count rate measured by BAT was about 2100 counts/sec in the 15 -
350 keV band.

TITLE: GCN GRB OBSERVATION REPORT
NUMBER: 3383
SUBJECT: GRB 050509b: Swift XRT Position
DATE: 05/05/09 06:29:23 GMT
FROM: Jamie A. Kennea at PSU/Swift-XRT

J. A. Kennea, D. N. Burrows, J. Nousek (PSU), C. P. Hurkett, E. Rol, J.
Osborne, A. Wells (U. Leicester), N. Gehrels (NASA/GSFC), G. Chincarini
(INAF-OAB) and P. Giommi (ASDC) report on behalf of the Swift XRT team:

The Swift BAT instrument detected GRB 050509b at 04:00:19 UT on 9th May
2005 (GCN Circ 3381). The observatory executed an automated slew to the
BAT position and the XRT began taking data at 04:01:12 UT. The XRT was in
Auto state but was not able to centroid on the afterglow due to low source
intensity. From downlinked data we find a faint uncatalogued X-ray source
located at:

RA(J2000) = 12:36:13.6,
Dec(J2000) = +28:58:58.6


TITLE: GCN GRB OBSERVATION REPORT
NUMBER: 3388
SUBJECT: GRB 050509b: Optical Counterpart Candidate
DATE: 05/05/09 08:44:13 GMT
FROM: Josh Bloom at Harvard/CFA

J. Bloom (UCB), C. Blake (Harvard), J. X. Prochaska (UCSC), J. Hennawi
(UCB), M. Gladders (Carnegie), B. Koester (U Michigan) and H. W. Chen
(MIT) report:

We have continued to observe GRB 050509b (GCN #3383) with the imager
on the 3.5m WIYN telescope. Under improving conditions, a 600s observation
in the sloan r band beggining at 06:05 UT revealed a apparent point source
within the XRT error circle (GCN #3383) and within the detectable light of
the nearby galaxy (GCN #3386).

A finding chart may be found at:
http://pairitel.org/grb050509b-candidate.gif