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Mystery Cosmic Explosions

Scientists have discovered what appears to be a new kind of cosmic explosion, the subject of four articles in this week's issue of Nature. They call the explosion a hybrid gamma-ray burst.

As with other gamma-ray bursts, this hybrid burst is likely signalling the birth of a new black hole. It is unclear, however, what kind of object or objects exploded or merged to create the black hole or, perhaps, something even more bizarre. The hybrid burst exhibits properties of the two known classes of gamma-ray bursts yet possesses features that cannot be explained.

The burst was discovered by NASA's Swift satellite on June 14, 2006, and has since 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.

"We have lots of data on this, dedicated lots of observation time, and we just can't figure out what exploded," said Neil Gehrels of NASA Goddard Space Flight Center in Greenbelt, Md., lead author on one of the Nature reports. "All the data seem to point to a new but perhaps not so uncommon kind of cosmic explosion."

“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 Professor Keith Mason, CEO of the Particle Physics and Astronomy Research Council, “And now Swift has presented us with a new intriguing burst that will test our understanding of such phenomena.”

Gamma-ray bursts fall into two categories, long and short. The long bursts last for more than two seconds and appear to be from the core collapse (supernova) of massive stars forming a black hole. The surrounding star can feed the newly formed black hole for many seconds leading to a long period of energy release. Most of these bursts come from near the edge of the visible universe. The short bursts, under two seconds long and often lasting just a few milliseconds, appear to be caused by the merger of two neutron stars or a neutron star with a black hole, which subsequently creates a new or bigger black hole. The small amount of remaining merger material can only feed the black hole for about a second and hence gives a short period of energy release.

The hybrid burst, called GRB 060614 after the date it was detected, was 1.6 billion light years away in the constellation Indus. The burst 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.

"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.

Certain properties of the burst concerning its brightness and the arrival time of photons of various energies, called the lag-luminosity relationship, suggest that burst behaved more like a short burst (from a merger) than a long burst. Yet no theoretical model of mergers can support a sustained release of gamma-ray energy for 102 seconds.

“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.

“The lack of a supernova from this relatively nearby event was so puzzling, that some astronomers tried to explain it by proposing that the burst was actually at a great distance behind the galaxy it was found in - a cosmic coincidence. However, the images from the UK-built UltraViolet-Optical Telescope on Swift prove conclusively that this is not the case.” Added Dr Mat Page, also of UCL-MSSL.

The burst is perhaps not unprecedented. 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. Another burst in May appears to also have had no supernova associated with it. Named GRB 060505, the burst lasted for 4 seconds but unfortunately did not have the lag-luminosity measured that makes GRB 060614 appear to be a hybrid.

“Two of the six long gamma ray bursts seen at close distances appear to have no supernova,” said Dr Pall Jakobsson of the University of Hertfordshire, “This may be a more common type of explosion than we expected, possibly a new mechanism for star death.”

Scientists remain divided on whether this was a long-short burst from a merger or a long burst from a star explosion with no supernova for whatever reason. Most conclude, however, that some new process must be at play: either the model of mergers creating second-long bursts needs a major overhaul, or that the progenitor star from an explosion is intrinsically different from the kind that makes supernovae.

Swift, launched in November 2004, is a NASA mission in partnership with the Italian Space Agency and the Particle Physics and Astronomy Research Council, United Kingdom; and is managed by NASA Goddard. Penn State controls science and flight operations from the Mission Operations Center in University Park, Pennsylvania. Los Alamos National Laboratory provides gamma-ray imaging analysis. The Ultra-Violet –Optical Telescope (UVOT) was constructed at UCL’s MSSL. The University of Leicester played a major role in developing Swift’s X-Ray Telescope and hosts the UK Swift Science Data Centre.

PPARC funds UK participation in Swift, ESO and the Gemini telescopes.

Julia Maddock | alfa
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