Dwayne Brown
Headquarters, Washington
202-358-1726
Susan Hendrix
Goddard Space Flight Center, Greenbelt, Md.
301-286-7745
RELEASE: 06-373
NASA SATELLITE DISCOVERS NEW KIND OF BLACK HOLE EXPLOSION
GREENBELT, Md. - Scientists using NASA data are studying a newly
recognized type of cosmic explosion called a hybrid gamma-ray burst.
As with other gamma-ray bursts, this hybrid blast is likely signaling
the birth of a new black hole.
It is unclear, however, what kind of object or objects exploded or
merged to create the new black hole. The hybrid burst exhibits
properties of the two known classes of gamma-ray bursts yet possesses
features that remain unexplained.
NASA's Swift first discovered the burst on June 14. Since the Swift
finding, more than a dozen telescopes, including the Hubble Space
Telescope and large ground-based observatories, have studied the
burst.
"We have lots of data on this event, have 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 four reports appearing in this week's edition
of the journal Nature. "All the data seem to point to a new but
perhaps not so uncommon kind of cosmic explosion."
Gamma-ray bursts represent the most powerful known explosions in the
universe. Yet they are random and fleeting, never appearing twice.
Scientists have only recently begun to understand their nature.
Such bursts typically fall into one of two categories, long or short.
The long bursts last more than two seconds and appear to be from the
core collapse of massive stars forming a black hole. Most of these
bursts come from the edge of the visible universe. The short bursts,
which are under two seconds and often last just a few milliseconds,
appear to be the merger of two neutron stars or a neutron star with a
black hole, which subsequently creates a new or bigger black hole.
The hybrid burst, called GRB 060614, after the date it was detected,
originated from within a galaxy 1.6 billion light years away in the
southern 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. Also, the burst's host galaxy has a low
star-formation rate with few massive stars that could produce
supernovae and long gamma-ray bursts. "This was close enough to
detect a supernova if it existed," said Avishay Gal-Yam of Caltech,
Pasadena, Calif., lead author on another Nature report. "Even Hubble
didn't see anything."
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. "This is brand new territory; we have no theories to
guide us," said Gehrels.
The burst is perhaps not unprecedented. Archived data from the Compton
Gamma-Ray Observatory in the 1990s possibly reveal other hybrid
"long-short" bursts, but no follow-up observations are available to
confirm this. Johan Fynbo of the Niels Bohr Institute in Copenhagen,
also lead author on a Nature report, suggests that a burst from May
of this year was also long, but had no associated supernova.
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.
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 the progenitor star from an explosion is intrinsically
different from the kind that make supernovae.
"We siphoned out all the information we could from GRB 060614," said
Massimo Della Valle of the Osservatorio Astrofisico di Arcetri in
Firenze, Italy, another lead author on a Nature report. "All we can
do now is wait for the next nearby hybrid burst."
Swift launched in November 2004. It is a NASA mission in partnership
with the Italian Space Agency and the Particle Physics and Astronomy
Research Council, England, and managed by Goddard. Penn State in
State College controls science and flight operations. Los Alamos
National Laboratory, N.M., provides gamma-ray imaging analysis.
For images and more information on the Internet, visit:
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