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Flashes Shed Light on Cosmic Clashes
ESO's Telescopes See Afterglows of Elusive Short Bursts
06 Ottobre 2005
An international team of astronomers led by Danish astronomer Jens Hjorth  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.
It all started like in a James Bond movie. During the 1960s, in the midst of the Cold War, the United States launched a series of satellites sensitive to gamma radiation  to monitor compliance with the Nuclear Test Ban Treaty.
No explosions were detected in the Earth's atmosphere. Instead, mysterious flashes of gamma-rays were seen, that appeared to come from outside the Solar System!
This information was of course a military secret and it wasn't until 1973 that the discovery of these enigmatic explosions could be announced. Unfortunately, the early gamma-ray detectors couldn't localise the source of gamma-rays on the sky very accurately, leading to a long-standing controversy on their origin.
Within the last few years, however, it has become possible to locate the sites of some of these events (e.g. with the Compton Gamma-Ray Observatory or the Beppo-Sax satellite) and since the beginning of 1997, astronomers have identified tens of optical sources in the sky that are associated with gamma-ray bursts.
They have been found to be situated at extremely large (i.e., "cosmological") distances. The farthest has recently been found at a redshift of 6.3 , indicating it is seen as when the Universe was less than 900 million years old. This implies that the energy release during a gamma-ray burst within a few seconds is larger than that of the Sun during its entire life time (about 10,000 million years). Apart from the Big Bang itself, "Gamma-ray bursts" or GRBs are in fact by far the most powerful events that are known in the Universe.
The long and the short
The wealth of observations on GRBs has revealed that they come in two different flavours: the long (lasting more than 2 seconds) and the short ones. The difference between the two is not only in the duration: short bursts also consist of higher energy photons than the long ones. One may thus infer that the physical origins of the two are different.
Over the past few years, a large international effort has convincingly shown that long gamma-ray bursts are linked with the ultimate explosion of massive stars ("hypernovae"), in particular, with the first detection of an otpical afterglow of a gamma-ray burst by Jan van Paradijs. A key proof was provided with the help of ESO telescopes in 2003 (eso0318). On March 29, 2003, NASA's High Energy Transient Explorer (HETE-II) satellite detected a very bright gamma-ray burst. Following identification of the "optical afterglow" by a 40-inch telescope at the Siding Spring Observatory (Australia), a high-dispersion spectrum obtained with the UVES spectrograph at the 8.2-m VLT KUEYEN telescope at the ESO Paranal Observatory (Chile) allowed to measure its distance to about 2,650 million light-years. This was the nearest normal GRB ever detected and, using two other powerful instruments at the ESO Very Large Telescope (VLT), the FORS1 and FORS2 multi-mode instruments, astronomers obtained, over a period of one month, spectra of the fading object.
The astronomers observed the gradual "emergence" with time of a supernova-type spectrum, revealing the extremely violent explosion of a star. With velocities well in excess of 30,000 km/sec (i.e., over 10% of the velocity of light), the ejected material was moving at record speed, testifying to the enormous power of the explosion. This set of data provided irrefutable evidence of a direct connection between the GRB and the "hypernova" explosion of a very massive, highly evolved star.
An historical breakthrough
"The breakthrough in our understanding of long-duration GRBs, 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.
Unlike long GRBs, there was indeed no detection of an afterglow neither in X-rays nor in the optical for short GRBs. It was thus not possible to know in which environment they formed nor to study their light-curve or spectrum to characterise them. That is, until very recently.
On May 9, 2005, the NASA/ASI/PPARC Swift satellite detected a 40-millisecond duration gamma-ray burst. Further observations with the X-ray detector on board the satellite detected an X-ray afterglow of a short burst for the first time. Thanks to this, its position could be determined with an accuracy better than 10 arcseconds, allowing astronomers  to point ESO's Very Large Telescope towards it and to take images with FORS2.
The burst, named GRB 050509B, was found to sit very close to a luminous, non-star forming elliptical galaxy lying 2,700 million light-years away (redshift 0.225) and belonging to a cluster of galaxies. Based on the unlikeness of a chance alignment between GRB 050509B and such a galaxy, it is argued that this is the host galaxy of the burst. This, the astronomers explain, makes it difficult for the hypernova model to be invoked. Indeed, it is highly improbable to find a core-collapse supernova in this galaxy.
On the other hand, the other prevailing model, the merging of two neutron stars  in a binary, seems more likely. Such a galaxy indeed is known to host many tight binaries with compact stars.
To be sure that the hypernova model could be ruled out, Jens Hjorth and his team performed further observations - until three weeks after the burst - with the FORS1 and FORS2 instruments. With these observations, the astronomers are confident that even the faintest supernovae would have been detected. But none were found.
And as sometime happens, a few months later, the astronomers were given the chance to study another afterglow of a short burst. And this time, in the optical.
In the night of July 9 to 10, 2005, the NASA HETE-2 satellite detected a 70-millisecond duration burst and was able, based on the detection of X-rays, to precise its position. Thirty-three hours after, Jens Hjorth and his team obtained images of this region of the sky using the Danish 1.5m telescope at La Silla. The images showed the presence of a fading source, sitting on the edge of a galaxy, most probably the host galaxy of the burst.
"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 2,000 million light-years away and is quite young - about 400 million years old. From the 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 gives further credit to the hypothesis that short GRBs are the consequence of the merging of two very compact stars.
"It is striking that the two short bursts that could finally be localised appear in quite different environments", says Jesper Sollerman, a member of the team from Stockholm Observatory (Sweden).
"But with a sample of only two events," stresses Jens Hjorth, "it would be prudent not to draw definitive conclusions at this stage about the progenitors of short gamma-ray bursts."
Hjorth and his colleagues may be cautious, yet astronomers cannot but marvel at the new chapter in astronomy that has just been opened.
 Gamma radiation and X-rays are electromagnetic radiation like visible light, radio waves, and ultraviolet light. These electromagnetic radiations differ only in the amount of energy they have. Gamma rays and X-rays are the most energetic of these.
 In astronomy, the redshift denotes the fraction by which the lines in the spectrum of an object are shifted towards longer wavelengths. The observed redshift of a remote galaxy provides an estimate of its distance.
 The team behind the discovery of the optical afterglow of GRB050709 is led by Jens Hjorth (Dark Cosmology Centre - DARK, Niels Bohr Institute, University of Copenhagen), 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. Joergensen (DARK), Daniel Kubas (ESO, Santiago), Javier Gorosabel (Instituto de Astrofisica de Andalucia), Pàll Jakobsson (DARK), Jesper Sollerman (DARK and Department of Astronomy, Stockholm University), Kristian Pedersen (DARK), and Chryssa Kouveliotou (NASA/Marshall Space Flight Center). The team is part of the Gamma-Ray burst Afterglow Collaboration at ESO (GRACE) carrying out gamma-ray burst afterglow studies.
 A neutron star is like one big atom with a diameter of 10-20 kilometres, and weighing about as much as the Sun. Thus, a pinhead of neutron star material (1 millimetre across) weighs almost 1 million tons, or about as much as the largest oil carrier ever built, fully loaded.
Some of the results described in this ESO Press Release will appear in the October 6, 2005 issue of the journal Nature ("The optical afterglow of the short gamma-ray burst GRB 050709", by J. Hjorth et al. and "A short gamma-ray burst apparently associated with an elliptical galaxy" by N. Gehrels et al.). Other results are either in press or published: "GRB 050059B: Constraints on short gamma-ray burst models" by J. Hjorth et al. (Astrophysical Journal Letters vol. 630, p. 117) and "The host galaxy cluster of the short gamma-ray burst GBR 050509B" by K. Pedersen et al., to appear in Astrophysical Journal Letters. The press release issued by the Dark Cosmology Centre is available at http://www.nbi.ku.dk/. The NASA Swift homepage - from which the historical information was taken - is at http://swift.sonoma.edu/about_swift/grbs.html.
The associated press release is here.
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