Gamma-ray bursts (GRBs), the most powerful type of explosion known in the Universe, come in two different flavours, long and short ones. Over the past few years, international efforts have shown that long gamma-ray bursts are linked with the ultimate explosion of massive stars.

Very recently, the observations by different teams - including the GRACE and MISTICI collaborations that use ESO's telescopes - of the afterglows of two short gamma-ray bursts provided the first conclusive evidence that this class of objects originates most likely from the collision of compact objects, neutron stars or black holes.

On July 24, 2005, the NASA/PPARC/ASI Swift satellite detected another short gamma-ray burst, GRB 050724. Subsequent observations, including some with the ESO Very Large Telescope, allowed astronomers to precisely pinpoint the position of the object, lying about 13,000 light-years away from the centre of an elliptical galaxy that is located 3,000 million light-years away (redshift 0.258).

"From its characteristics, we infer that this galaxy contains only very old stars," says Guido Chincarini (INAF-Brera and Milan University, Italy), co-author of the paper presenting the results [1]. "This is similar to the host galaxy of the previous short GRB which could be precisely localised, GRB 050509B, and very different from host galaxies of long bursts."

These observations thereby confirm that the parent populations and consequently the mechanisms for short and long GRBs are different in significant ways. The most likely scenario for short GRBs is now the merger of two compact objects.

The observations also show this short burst has released between 100 and 1000 less energy than typical long GRBs. "The burst itself was followed after about 200-300 seconds by another, less-energetic flare," says Sergio Campana (INAF-Brera), co-author of the paper.

"It is unlikely that this can be produced by the merger of two neutron stars. We therefore conclude that the most probable scenario for the origin of this burst is the collision of a neutron star with a black hole [2]."

[1]: The results are being published in the December 15 issue of the journal Nature ("Unravelling the origin of short gamma-ray bursts", by Scott Barthelmy, Guido Chincarini, Dave Burrows, et al.). [2]: In the case of a merger between a neutron star and a black hole, the neutron star may be only partially disrupted in the initial plunge. The remainder may orbit the black hole, transferring mass at closest approach ("periastron"), until the neutron star mass is reduced to less than 2 tenths of the mass of the sun, where it expands to disruption. The activity may thus extend over few tens of seconds, unlike the case of a merging between two neutron stars.

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