After foiling astronomers for years, the enigmatic gamma ray bursts are finally
starting to shed their veil of mystery, thanks to a prodigious world-wide
observational effort. In my January 3 article, I elaborated on our dreadful
lack of understanding of gamma ray bursts, and I discussed hypotheses for their
nature ranging from comet collisions within the solar system, to neutron
star-neutron star collisions in distant galaxies. Within the last months, the
Dutch and Italian satellite Beppo-SAX, along with a multi-national
collaboration of optical and radio telescopes, has all but confirmed that the
bursts are at cosmological (not solar system) distances, a view most notably
advocated over the years by Bohdan Paczynski of Princeton.
Beppo-SAX is a simple but clever instrument specifically designed to locate
gamma ray bursts to a very precise angular position on the sky (with errorboxes
of order arcminutes). First, a 40 degree wide gamma-ray detector waits for a
burst within its field of view. When a burst goes off, high energy X-ray
detectors look at the same part of the sky the burst came from. The X-ray
detectors are able to pinpoint the burst in the sky, a feat impossible with
gamma rays which are simply too energetic to be reflected into a focus by any
conceivable material. Once Beppo-SAX determines the position, the Dutch and
Italian team issues a world-wide telegram to the astronomical community, so
that all possible telescopes can monitor the event in optical and radio light.
Last month, just such an event occurred, and detailed optical observations with
many telescopes followed the burst over several days. The Keck telescope group
reported that the optical counterpart to the gamma-ray burst had absorption
features in its spectrum at a redshift of 0.8, a distance of order 10 billion
light-years. Now the absorption could come from a cloud of gas closer to us
than the gamma ray source, or within the source itself, but either way, the
source has to occur at a redshift of at least 0.8, laying to rest a 15-year old
(hotly contested) controversy over the distance of the gamma-ray bursts.
Now that we know the distance, we have to figure out what in the world makes so
much energy so fast that we can see it from the other side of the universe.
The prevailing theory is some sort of expanding fireball, presumably resulting
from the collision of massive objects, such as neutron stars; however other
progenitors have been suggested, and by no means ruled out. The most
rudimentary predictions of the basic fireball model, advanced by Eli Waxman
(Institute for Advanced Study) and Martin Rees (Cambridge), have been confirmed
