Cambridge, MA — 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.

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.”

Signature of a Supernova

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.”

Brightest Explosions in the Universe

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.

NOTE: Images and other information are available at:
http://www.nd.edu/~pgarnavi/grb011121/

A paper by Garnavich and Stanek’s team is online at
http://xxx.lanl.gov/pdf/astro-ph/0204234.

A second team led by Joshua Bloom of CalTech also made observations
of the afterglow and SN 2001ke. Their results are presented in an
abstract at http://xxx.lanl.gov/abs/astro-ph/0203391 and a press
release available at http://pr.caltech.edu/media/releases/.

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.