Flashes shed light on cosmic clashes

ESO’s telescopes see afterglows of elusive short bursts

An international team of astronomers led by Danish astronomer Jens Hjorth [1] has for the first time observed the visible light from a short gamma-ray burst (GRB). Using the 1.5m Danish telescope at La Silla (Chile), they showed that these short, intense bursts of gamma-ray emission most likely originate from the violent collision of two merging neutron stars. The same team has also used ESO’s Very Large Telescope to constrain the birthplace of the first ever short burst whose position could be pinpointed with high precision, GRB 050509B. The results are being published in the October 6 issue of the journal Nature.

Gamma-ray bursts, the most powerful type of explosion known in the Universe, have been a mystery for three decades. They come in two different flavours, long and short ones. Over the past few years, international efforts have convincingly shown that long gamma-ray bursts 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 them with the energetic explosion of a massive star as it collapses into a black hole, came from the discovery of their long-lived X-ray and optical afterglows,” says Jens Hjorth (Dark Cosmology Centre, Niels Bohr Institute, University of Copenhagen, Denmark). “Short duration GRBs have however evaded optical detection for more than 30 years,” he adds.

Things changed recently. In the night of July 9 to 10, 2005, the NASA HETE-2 satellite detected a burst of only 70-millisecond duration and, based on the detection of X-rays, was able to determine its position in the sky. Thirty-three hours after, Jens Hjorth and his team obtained images of this region of the sky using the Danish 1.5m telescope at ESO La Silla. The images showed the presence of a fading source, sitting on the edge of a galaxy.

“We have thus discovered the first optical afterglow of a short gamma-ray burst”, says co-author Kristian Pedersen, also from the Dark Cosmology Centre of the University of Copenhagen.

The burst, named GRB 050709, resides 11,000 light-years from the centre of a star-forming dwarf galaxy that is about 2,400 million light-years away and is quite young – about 400 million years old. From observations conducted until 20 days after the burst, the astronomers can rule out the occurrence of an energetic hypernova as found in most long GRBs. This supports the hypothesis that short GRBs are the consequence of the merging of two very compact stars.

The same conclusion comes forward from the study of another event, GRB 050509B. This 40-millisecond burst was detected on May 9 by the NASA/ASI/PPARC Swift satellite, which could, for the first time, determine its position. Images obtained with the FORS instruments on ESO’s Very Large Telescope allowed the astronomers to study the vicinity of the burst.

The GRB was found to sit very close to a luminous, non-star forming elliptical galaxy lying 2,700 million light-years away and belonging to a cluster of galaxies.

“It is striking that the two short bursts that have finally been localised appear in quite different environments”, says Jesper Sollerman, a member of the team from Stockholm Observatory (Sweden) and Dark Cosmology Centre (Denmark). “The most important aspect of these discoveries is probably that we have finally shown that the short bursts are indeed cosmic explosions from far away in the Universe”, he adds.

Because elliptical galaxies are generally devoid of very massive stars but rich in tight binary systems containing compact stars, the association of the burst with this kind of galaxy gives the merging hypothesis another boost.

Whilst Hjorth and his colleagues still caution not to jump too quickly to definitive conclusions, astronomers cannot but marvel at the new chapter in astronomy that has just been opened.