CfA: Culprit caught in gamma-ray burst mystery
Posted: May 17, 2002

Gamma-ray bursts appear to be the most powerful explosions in the Universe. Discovered by the Defense Department in the 1960's while looking for Soviet nuclear tests, their existence was a military secret for years. Now an international team of astronomers combining ground-based and NASA Hubble Space Telescope observations has discovered the culprit: a supernova hidden beneath the powerful glow of a gamma-ray burst. The presence of the exploding star, designated SN 2001ke, is the best evidence to date that some gamma-ray bursts (GRBs) are generated from the death of massive stars.

Color Hubble Space Telescope image
The gamma-ray burst, designated GRB 011121, was detected by the Italian-Dutch satellite Beppo-SAX on November 21, 2001. Ten hours later the team, led by Kris Stanek of the Harvard-Smithsonian Center for Astrophysics and Peter Garnavich of the University of Notre Dame, searched for visible evidence of the high-energy burst using a 1.3-meter Polish telescope in Chile and found the rapidly fading optical 'afterglow' of the event. The team then made spectroscopic observations with the Magellan 6.5-meter Walter Baade Telescope. They found that the source of the burst was a galaxy more than 6 billion light-years away, meaning that the Earth hadn't even formed yet when the gamma-rays exploded outward on their long journey.

"We needed to precisely locate the burst quickly while the optical afterglow was still easily visible," says team co-leader Peter Garnavich. "Then, we monitored the burst both visually and spectroscopically to watch for clues to the origin of the burst. We knew that, if the gamma-rays came from a supernova, we might be able to detect the light from the supernova as the bright afterglow from the gamma-ray burst dimmed over time."

More than a week after the burst when the optical emission from the GRB afterglow had faded, Magellan and the NASA Hubble Space Telescope continued to focus on the host galaxy and were rewarded in their vigil when a 'bump' was seen in the light output. This flash of excess light peaking a week or two after the gamma-rays is the signature of a supernova, the death throes of a star much more massive than our Sun. Data from Magellan also provided the first spectrum of a supernova associated with a GRB. The supernova was bluer than expected and faded faster than comparable stellar explosions.

"The hunt for the source of gamma-ray bursts has been a detective story as challenging as any faced by the famous Lieutenant Columbo. We were thrilled to be the first to catch a supernova 'in the act,' so to speak," says Kris Stanek. "Moreover, this supernova has surprising properties, which means a variety of stellar explosions must be able to make gamma-rays bursts."

Recent satellite experiments such as Beppo-SAX and the NASA Compton Gamma-Ray Observatory have revolutionized our understanding of GRBs by pinpointing their locations and permitting study over the entire electromagnetic spectrum.

Once thought to be the result of neutron star or black hole mergers, it is now clear that at least some of the gamma-ray flashes are produced when a star more than eight times more massive than the Sun reaches the end of its life. A supernova is seen when the core of such a massive star can no longer hold itself up against gravity and collapses into a neutron star or black hole. The collapse apparently sends out narrow jets of matter and energy at nearly the speed of light. When the jets are pointed toward Earth we see a burst of gamma-rays as well as radio and optical light.

CoMagellan infrared image
Other members of the collaboration led by Kris Stanek are:

David Bersier, Saurabh Jha, Robert Kirshner, Tom Matheson (Harvard-Smithsonian CfA)
Stephen Holland (Notre Dame)
Lukasz Wyrzykowski (Warsaw University Observatory)
Leopoldo Infante and Eduardo Bendek (Pontificia Universidad Catolica de Chile)
Kevin Krisciunas (Cerro Tololo Interamerican Observatory)
Mark Phillips (Carnegie Institution of Washington)
Ray Carlberg (University of Toronto)

Headquartered in Cambridge, Massachusetts, the Harvard-Smithsonian Center for Astrophysics (CfA) is a joint collaboration between the Smithsonian Astrophysical Observatory and the Harvard College Observatory. CfA scientists organized into seven research divisions study the origin, evolution, and ultimate fate of the universe.

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