6 February 1998


Cambridge astronomers have found evidence that mysterious gamma-ray bursts detected in space are coming from the most distant known objects in the universe. They say that the dimmest bursts now being observed may have been given off when the universe was only one twentieth of its present age, much earlier than was previously thought. According to a team headed by Dr Ralph Wijers at the University of Cambridge's Institue of Astronomy, it has taken almost the whole time the universe has existed - about 15 billion years - for these bursts of radiation to reach us. If their redshifts could be measured, they would be about 6. So far, the largest measured redshifts - values around 4.9 - belong to quasars.

Dr Wijers and his collaborators reached their conclusions by comparing the observed strengths of large numbers of gamma- ray bursts with predictions based on the theory that the bursts are linked to the death-throes of massive stars. Their results are published in the 11th February issue of the Monthly Notices of the Royal Astronomical Society. The other members of the team are Joshua S. Bloom (now at Caltech, USA), Dr Jasjeet S. Bagla, and Dr Priya Natarajan (now at CITA, University of Toronto, Canada).

Gamma-ray bursts are flashes of gamma rays lasting on average 10 seconds. They were first noticed in the 1960s. The BATSE ('Burst and Transient Source Experiment') instrument on board NASA's Compton Gamma Ray Observatory satellite finds about one a day and has counted thousands altogether. Until recently, astronomers did not even know whether these enigmatic burst are coming from objects on the outskirts of our own Galaxy or from distant galaxies. But a breakthrough was achieved in 1997, when the Italian-Dutch BeppoSAX satellite pinpointed the location of a gamma-ray burst very accurately for the first time. Jan van Paradijs of the University of Amsterdam and collaborators immediately looked at the spot with powerful optical telescopes, the William Herschel and Isaac Newton telescopes on La Palma in the Canary Islands. They discovered a fading star-like object at the location of the burst. A few months later another such 'afterglow' was found, and this time Caltech astronomers were able to use the Keck Telescope in Hawaii to take a spectrum of it. This unambiguously showed that it was among distant galaxies.

The energy spewed out by gamma-ray bursts is phenomenal: despite being very remote, they would appear as some of the brightest objects in the sky during their 10 seconds of glory if we could see the gamma rays. The energy released by a gamma-ray burst is greater than all the energy produced by the Sun in its 10- billion-year life. The only other thing that produces anything like so much energy in a short time is a dying massive star, which explodes and is observed as a so-called supernova. Our current best guess for what causes gamma-ray bursts is something similar to the collapse of a massive star. Specific suggestions, first made by Bohdan Paczynski of Princeton University, include the merger of two neutron stars or the collapse of a very massive star into a black hole.

These ideas about what could generate a stupendous gamma-ray burst provided the key the Cambridge team needed as they thought about ways of deducing where the bursts come from. They noted that massive stars live only a short time, so they die almost where and when they are born - by astronomical standards of time and distance. Recently, astronomers have established how many stars have been born at various times since the universe began - what they call the star-formation history of the universe. If gamma-ray bursts are due to deaths of massive stars, then the formation history of gamma-ray bursts should be broadly the same as that of stars.

Taking up this idea, Dr Wijers and his colleagues went on to calculate how many gamma-ray bursts we should be seeing now from different eras in the universe's history. Because of the time taken by the gamma rays to travel through space at the speed of light, looking farther into space is the same as probing farther back in time. So the team calculated how many bursts should be seen now at each brightness level if the theory is correct, and compared the answer with the actual observations. The calculated numbers agreed with the observed ones, thereby confirming the hypothesis.

The brightness of bursts as we see them depends both on the distance of the objects sending them out, and on the total energy they give off. To their amazement, the Cambridge team found that the energy output of the bursts had to be 20 times greater than had been thought previously, so they are coming from much farther away than previously thought. The researchers concluded that the faintest gamma-ray bursts ever seen by BATSE may well be from distances corresponding to a redshift greater than 6.

Dr Wijers's team is encouraged by supporting evidence. Tomonori Totani of the University of Tokyo investigated the same issue independently in a slightly different way, and came to much the same conclusion. There is also direct evidence that gamma-ray bursts are related to stellar deaths and occur in regions where lots of stars have just formed. The first gamma- ray burst for which a visible afterglow was spotted occurred on 28 February 1997. It lies at the edge of a distant blue galaxy, typically a location of abundant star formation. In the bursts of 8 May and 28 August 1997, signatures were seen of dense gas that is typically associated with star forming regions.

Apart from the importance to the study of gamma-ray bursts themselves, this discovery will influence our understanding of the early history of the universe, when the first stars and galaxies were forming. When new gamma-ray satellites are built that are sensitive enough to see even fainter bursts, we may be able to explore what the universe was like at even earlier times. "We do not know at what time the first stars and galaxies in the universe formed but, if our result is correct, we might be able to detect gamma-ray bursts from the deaths of the first stars that ever formed, and perhaps find out when they formed", says Jasjeet Bagla of the Cambridge team.

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