New class of cosmic explosion identified

The discovery strongly suggests a continuum between the two previously known classes of explosions.

In this week's issue of Nature, astronomers from the Space Research Institute of the Russian Academy of Sciences and Caltech outlined in two related papers the discovery of the explosion.

The explosion was detected Dec. 3, 2003 by the European-Russian Integral satellite, then observed in detail at ground-based radio and optical observatories, astronomers said.

The burst, known by its birthdate, GRB031203, appeared in the constellation Puppis and is about 1.6 billion light-years away, according to astronomers.

Although the burst was the closest gamma-ray burst to Earth ever studied - all others have been several billion light-years away - researchers noticed that the explosion was extremely faint - releasing only one-thousandth of the gamma rays of a typical gamma-ray burst.

However, the burst was also much brighter than supernovae explosions, which led to the conclusion that a new type of explosion had been found.

Both supernovae and gamma-ray blasts are cosmic explosions marking the death of massive stars. Astronomers have long pondered the cause of the seemingly dramatic differences between these events.

The question of how stars die is a focus of stellar research, and is particularly directed toward the energetic explosions that destroy a star in one cataclysmic event, astronomers say.

Stars are powered by the fusion, or burning, of hydrogen in their interiors. Upon exhaustion of fuel in the interior, the core of massive stars collapse to compact objects - typically a neutron star and occasionally a black hole.

The energy released as a result of the collapse explodes the outer layers, the visible manifestation of matter in the universe, according to astronomers.

However, astronomers say this nuclear energy may be insufficient to power the supernova explosions. One theory is that additional energy is generated from the matter falling onto the newly produced black hole. Many astronomers believe this is what powers the luminous gamma-ray bursts.

But the connection between such extreme events and the more common supernovae is not yet clear, and if they are, indeed, closely related, then there should be a continuum of cosmic explosions, ranging in energy from that of "ordinary" supernovae to that of gamma-ray bursts.

In 1998, astronomers discovered an extremely faint gamma-ray burst coincident with a nearly luminous supernova. The supernova also showed evidence for an underlying engine, albeit a weak one.

The question for astronomers was whether the event was a freak explosion, or whether it was indicative of a larger population of low-powered cosmic explosions with characteristics in between the cosmological gamma-ray bursts and typical supernovae.

Sergey Sazonov of the Space Research Institute is the first author of one of the two papers published in Nature.

"I knew this was an exciting find," he said, "because even though this was the nearest gamma-ray burst to date, the gamma-ray energy measured by Integral is one thousand times fainter than typical cosmological gamma-ray bursts."

The event was studied in further detail by the Chandra X-Ray Observatory and the Very Large Array, a radio telescope facility located near Socorro, N.M.

Graduate student Alicia Soderberg is principal author of the other Nature paper.

"I was stunned that my observations from the Very Large Array showed that this event confirmed the existence of a new class of bursts," she said. "It was like hitting the jackpot."

Implications of the discovery include the possible existence of a significant new population of low-luminosity gamma-ray bursts lurking within the nearby universe, said Shrinivas Kulkarni, the MacArthur Professor of Astronomy and Planetary Science at Caltech and Soderberg's faculty adviser.