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NASA's Swift Satellite Discovers New Kind of Black Hole Explosion

27 December 2006

Image of the host galaxy of GRB 060614.
Click image to enlarge

Image of the host galaxy of GRB 060614 taken with the Hubble Space Telescope. The top panel shows the galaxy on June 27, 2006 with the gamma-ray burst afterglow clearly visible in the circle to the upper right. The bottom panel shows the galaxy on July 15 with the gamma-ray burst afterglow faded away and no light from a supernova detected. Credit: A. Gal-Yam (Caltech)

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.

However this hybrid burst exhibits properties of the two known classes of gamma-ray bursts, yet possesses features that remain unexplained. It is thus unclear what kind of object or objects exploded or merged to create the new black hole.

NASA's Swift satellite first discovered the burst on June 14, 2006. Since then, 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 last 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."

Model of Long Duration Gamma-Ray Burst (Video Description)

Model of Short Duration Gamma-Ray Burst (Video Description)

These animations illustrate the leading models for long and short duration gamma-ray bursts. Left: Long gamma-ray bursts (more than four seconds in duration) are thought to be due to massive collapsing stars. This animation shows the center of a dying star collapsing minutes before the star implodes and emits a gamma-ray burst that is seen across the Universe. (2.2 Mb - no audio). Right: Shorter bursts (less than two seconds in duration) are thought to be caused by mergers of binary systems with black holes or neutron stars. This video shows the merger of two neutron stars. (1.9 Mb - no audio). (Credit for both videos: NASA/Dana Berry, Skyworks Digital)

The brightness of the burst and its lag-luminosity relationship (a measure of the arrival time of photons of various energies) suggest that the burst behaved more like a short burst (from a merger) than a long burst (from a supernova). 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."

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Imagine the Universe! is a service of the High Energy Astrophysics Science Archive Research Center (HEASARC), Dr. Alan Smale (Director), within the Astrophysics Science Division (ASD) at NASA's Goddard Space Flight Center.

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