An international team of astronomers led by Danish astronomer JensHjorth  has for the first time observed the visible light from ashort gamma-ray burst (GRB). Using the 1.5m Danish telescope at LaSilla (Chile), they showed that these short, intense bursts ofgamma-ray emission most likely originate from the violent collision oftwo merging neutron stars. The same team has also used ESO's Very LargeTelescope to constrain the birthplace of the first ever short burstwhose position could be pinpointed with high precision, GRB 050509B.The results are being published in the October 6 issue of the journalNature.
Gamma-ray bursts, the most powerful type of explosion known in theUniverse, have been a mystery for three decades. They come in twodifferent flavours, long and short ones. Over the past few years,international efforts have convincingly shown that long gamma-raybursts are linked with the ultimate explosion of massive stars(hypernovae; see e.g. ESO PR 16/03).
"The breakthrough in our understanding of long-duration GRBs(those lasting more than about 2 seconds), which ultimately linked themwith the energetic explosion of a massive star as it collapses into ablack hole, came from the discovery of their long-lived X-ray andoptical afterglows," says Jens Hjorth (Dark Cosmology Centre, NielsBohr Institute, University of Copenhagen, Denmark). "Short durationGRBs have however evaded optical detection for more than 30 years," headds.
Things changed recently. In the night of July 9 to 10, 2005,the NASA HETE-2 satellite detected a burst of only 70-millisecondduration and, based on the detection of X-rays, was able to determineits position in the sky. Thirty-three hours after, Jens Hjorth and histeam obtained images of this region of the sky using the Danish 1.5mtelescope at ESO La Silla. The images showed the presence of a fadingsource, sitting on the edge of a galaxy.
"We have thus discovered the first optical afterglow of ashort gamma-ray burst", says co-author Kristian Pedersen, also from theDark Cosmology Centre of the University of Copenhagen.
The burst, named GRB 050709, resides 11,000 light-years fromthe centre of a star-forming dwarf galaxy that is about 2,400 millionlight-years away and is quite young -- about 400 million years old.From observations conducted until 20 days after the burst, theastronomers can rule out the occurrence of an energetic hypernova asfound in most long GRBs. This supports the hypothesis that short GRBsare the consequence of the merging of two very compact stars.
The same conclusion comes forward from the study of anotherevent, GRB 050509B. This 40-millisecond burst was detected on May 9 bythe NASA/ASI/PPARC Swift satellite, which could, for the first time,determine its position. Images obtained with the FORS instruments onESO's Very Large Telescope allowed the astronomers to study thevicinity of the burst.
The GRB was found to sit very close to a luminous, non-starforming elliptical galaxy lying 2,700 million light-years away andbelonging to a cluster of galaxies.
"It is striking that the two short bursts that have finallybeen localised appear in quite different environments", says JesperSollerman, a member of the team from Stockholm Observatory (Sweden) andDark Cosmology Centre (Denmark). "The most important aspect of thesediscoveries is probably that we have finally shown that the shortbursts are indeed cosmic explosions from far away in the Universe", headds.
Because elliptical galaxies are generally devoid of verymassive stars but rich in tight binary systems containing compactstars, the association of the burst with this kind of galaxy gives themerging hypothesis another boost.
Whilst Hjorth and his colleagues still caution not to jump tooquickly to definitive conclusions, astronomers cannot but marvel at thenew chapter in astronomy that has just been opened.
Some of the results described in this ESOPress Release will appear in the October 6, 2005 issue of the journalNature ("The optical afterglow of the short gamma-ray burst GRB 050709"by J. Hjorth et al. and "A short gamma-ray burst apparently associatedwith an elliptical galaxy" by N. Gehrels et al.). Other results areeither in press or published: "GRB 050059B: Constraints on shortgamma-ray burst models" by J. Hjorth et al. (Astrophysical JournalLetters vol. 630, p. 117) and "The host galaxy cluster of the shortgamma-ray burst GBR 050509B" by K. Pedersen et al., to appear inAstrophysical Journal Letters.
 The team behind the discovery of the optical afterglow of GRB050709is lead by Jens Hjorth, (Dark Cosmology Centre, Niels Bohr Institute,University of Copenhagen (DARK)), and includes Darach Watson (DARK),Johan P.U. Fynbo (DARK), Paul A. Price (Institute for Astronomy,University of Hawaii), Brian L. Jensen (DARK), Uffe G. Jørgensen(DARK), Daniel Kubas (ESO, Santiago), Javier Gorosabel (Instituto deAstrofisica de Andalucia), Páll Jakobsson (DARK), Jesper Sollerman(DARK and Department of Astronomy, Stockholm University), KristianPedersen (DARK), and Chryssa Kouveliotou (NASA/Marshall Space FlightCenter).
The team is part of the Gamma-Ray burst Afterglow Collaborationat ESO (GRACE) carrying out gamma-ray burst afterglow studies with ESOtelescopes.
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