Cosmic Bang Baffles Boffins
Last Updated: 12:01am GMT 25/12/2006
Massive explosion has astronomers stumped, says, says Roger Highfield
A star has died in a fashion never seen before, revealing the biggest bursts of energy seen in the cosmos are more varied than astronomers had thought.
The new kind of stellar demise - dubbed a hybrid gamma-ray burst - is reported today in a series of articles in the journal Nature and probably signals the birth of a new black hole, so named because they are so dense and massive that even light cannot escape.
But it remains mysterious what kind of stellar object or objects exploded or merged to create the black hole or, perhaps, something even more bizarre. Stars die when they have exhausted the fuel in their centres.
Small stars like the sun then just cool down forever. Larger stars explode as a supernova, leaving behind a neutron star or black hole, and in the process can produce a bright beam of gamma rays called a gamma-ray burst.
Gamma-ray bursts can also be produced by merging two neutron stars. The two methods differ in the length of the burst - long or short respectively. The hybrid burst, called GRB 060614 after the date it was detected by Nasa's Swift satellite in June, was 1.6 billion light years away in a small spiral galaxy of the constellation Indus.
The intensve burst of gamma rays lasted for 102 seconds, placing it soundly in long-burst territory. But the burst lacked the hallmark of a supernova, or star explosion, commonly seen shortly after long bursts, which scatters material into the cosmos. One scientist likened it to seeing the flash of lightning, without a subsequent rumble of thunder.
"Neither of our standard models fits GRB060614. We see no bright optical light after a few days that we would have expected from a supernova, and yet if the burst is due to a merger the new black hole is expected to feed, and hence release gamma-rays, for only a few seconds. This one is a real puzzle!" said Dr Paul O'Brien from the University of Leicester.
"Everyone was happy that there were two kinds of gamma-ray bursts but this throws a spanner in the works" said Patricia Schady of UCL's Mullard Space Science Laboratory. Archived data from the 1990s from the Compton Gamma-ray Observatory possibly reveal other hybrid "long-short" bursts, but no follow-up observations are available to confirm this.
"Gamma-ray bursts are the most powerful known explosions in the universe. Yet they are random and fleeting, never to appear twice, and only in recent years has their nature been revealed with fantastic new data from the
Swift satellite," said Prof Keith Mason, CEO of the UK Particle Physics and Astronomy Research Council. "Now Swift has presented us with a new intriguing burst that will test our understanding of such phenomena." Swift, launched in November 2004, is a NASA mission in partnership with the Italian Space Agency and the Particle Physics and Astronomy Research Council. Since GRB 060614 was detected the event has been studied with over a dozen telescopes, including the Hubble Space Telescope, ESO's Very Large Telescope, the Gemini South Telescope as well as with smaller telescopes such as the Danish 1.5m telescope at La Silla in Chile.
