PASADENA, Calif.-Astronomers have identified a new class of cosmic
explosions that are more powerful than supernovae but considerably
weaker than most gamma-ray bursts. 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 the California
Institute of Technology announce in two related papers the discovery
of the explosion, which was first detected on December 3, 2003, by
the European-Russian Integral satellite and then observed in detail
at ground-based radio and optical observatories. The burst, known by
its birthdate, GRB031203, appeared in the constellation Puppis and is
about 1.6 billion light-years away.

Although the burst was the closest gamma-ray burst to Earth ever
studied (all the others have been several billion light-years away),
researchers noticed that the explosion was extremely faint–releasing
only about 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 the rare but brilliant gamma-ray bursts are
cosmic explosions marking the deaths of massive stars. Astronomers
have long wondered what causes the seemingly dramatic differences
between these events. The question of how stars die is currently a
major focus of stellar research, and is particularly directed toward
the energetic explosions that destroy a star in one cataclysmic event.

Stars are powered by the fusion (“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
which is a supernova. In this process, new elements are added to the
inventory of matter in the universe.

However, 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 that 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,
GRB 980425, coincident with a nearby luminous supernova. The
supernova, SN 1998bw, also showed evidence for an underlying engine,
albeit a very weak one. The question that arose was whether the
event, GRB 980425/SN 1998bw, 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.

“I knew this was an exciting find 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,” says Sergey Sazonov of the Space Research
Institute, the first author of one of the two Nature papers.

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

“I was stunned that my observations from the Very Large Array showed
that this event confirmed the existence of a new class of bursts,”
says graduate student Alicia Soderberg, who is the principal author
of the other Nature paper. “It was like hitting the jackpot.”

There are several exciting implications of this discovery, including
the possible existence of a significant new population of
low-luminosity gamma-ray bursts lurking within the nearby universe,
said Shrinivas Kulkarni, who is the MacArthur Professor of Astronomy
and Planetary Science at Caltech and Soderberg’s faculty adviser.

“This is an intriguing discovery,” says Kulkarni. “I expect a
treasure trove of such events to be identified by NASA’s Swift
mission scheduled to be launched this fall from Cape Canaveral. I am
convinced that further discoveries and studies of this new class of
hybrid events will forward our understanding of the death of massive
stars.”