Gamma-Ray Bursts are spectacular bursts of gamma-rays that at a rate of a few per day outshines all of the other sources of cosmic gamma-rays on the sky added together.

It is still not known what produces these bursts, but it was early on realised that GRBs distribute uniformly over the sky and hence can not be related to objects in the Milky Way disk (where most of the stars in our galaxy are found).

Since 1997 we have, mainly thanks to precise afterglow localisations by the Italian/Dutch BeppoSax satelitte, learned that at least one class of Gamma-Ray Bursts, the most common long-duration burts, originate from extragalactic sources, most likely somehow related to the most extreme supernova explosions in distant galaxies. For this reason the occurrence of cosmic Gamma-Ray Bursts may be used to probe the evolution of (massive) star formation in the universe. In more detail, by locating the galaxies that hosted the bursts we can build up a sample of galaxies that is selected based on the occurrence of star formation and which selection is independent on the emission properties of the galaxies. This is crucial as it is currently unknown how much star formation in the universe that is obscured due to dust extinction in galaxies. The study of a sample of galaxies hosting cosmic Gamma-Ray Bursts can most likely give us the answer to this question. Until now our samples of GRB host galaxies still suffer from the bias that it is much easier to locate the precise position of a GRB, and hence its host galaxy, if it has a bright optical afterglow. This may very well exclude very dusty host galaxies from the current samples (although the burst it self may destroy some of the dust in its environment). In the future precise X-ray and/or radio localisations will hopefully lift this possible bias.

Another concern (or interesting issue dependent on your taste) is that there could be other parameters in addition to mass controlling weather a star ends up producing a gamma ray burst. One such parameter could be metallicity. In the collapsar model by MacFadyen and Woosley low metallicity is predicted since this helps a massive star retain the mass and the angular momentum a higher metallicity star would loose due to radiatively driven stellar winds. In the collapsar model a high angular momentum and a large Helium core mass is necessary to make the black hole and the jet that produces the gamma ray burst. Observationally, a preference for low metallicity would also help explain the high fraction of Ly-α emitters among GRB host galaxies.

The galaxy in the centre of the figure below is an example of a Gamma-Ray Burst selected galaxy - the host galaxy of GRB000926 at redshift z=2.038 observed at the Nordic Optical Telescope by the Copenhagen GRB-group. The R-band magnitude of the objects is R=24. The position of the Gamma-Ray Burst in the galaxy is marked by a green cross. The brighter galaxy above and to the left of the host is not related to the GRB. Click on the image to get the paper.

Here is a figure showing the HST image of the same host galaxy taken by the CalTech group (published in ApJ by Castro et al. 2002) together with a Ly-α contour image from the Nordic Optical Telescope (click on the image to get the paper):

Many more such galaxies can be seen at The Survey of the Host Galaxies of Gamma-Ray Bursts web-page. See also Andy Fruchter's page on GRB host galaxies, Jochen Greiner's, and NASA's page on Gamma-Ray Bursts, and the Caltech-NRAO-CARA page on GRB follow-up. Last but not least here is a link to the GRACE-homepage (Gamma-Ray burst Afterglow Collaboration at ESO). Here is a brief history of the discovery of GRBs.

Some of the most important Gamma-Ray and X-ray satellites relevant for GRB studies :
BeppoSax
CGRO/BATSE, BATSE at MSFC, see also HEASARC and the Image Gallery at CGRO
IPN-3
INTEGRAL (Science)
Hete-2
Swift
GLAST
XMM
Chandra


Here is a link to a search for optical transients (independent on GRB triggers) at Bell laboratories. Very interesting.

See also: APOD March 14 2000, APOD Feb 28 2002, APOD April 5 2002, APOD May 17 2002, APOD December 10 2002, APOD March 25 2003, APOD April 3 2003.

Our group has released three press-releases on GRBs : ESO PR on GRB000131, ESA PR on GRB980425, and ESO PR on GRB030329. See also the reponses of the media:
GRB000131: space.com, Spaceflight Now, Sci.Am., BBC, Voyages.
GRB980425: space.com, Spaceflight Now, ScienceNet.
GRB030329: Space Today, Spaceflight Now, Space Daily, Astronews, The chronicle of higher eductation.

In Danish : (rummet.dk and AstroGym)
Gamma Ray Bursts,
Universets klareste Glimt,
Danske Astronomer i front,
Gammaglimt og supernova på samme sted,
Gamma glimt og deres værts galakser I,
Gamma glimt og deres værts galakser II