CSI Astronomers Link Gamma-ray Bursts and Supernovae

Imagine a cosmic explosion so powerful, it blazes across the entire
Universe. Such explosions exist in the form of gamma-ray bursts. Now, in the
best tradition of the crime scene investigations (CSI), astronomers have
assembled the clues pointing to the source of these gigantic blasts.

“There should no longer be doubt in anybody’s mind that gamma-ray bursts and
supernovae are connected,” said Dr. Thomas Matheson of the
Harvard-Smithsonian Center for Astrophysics (CfA), a member of the team that
made this discovery.

The investigation began on March 29 when NASA’s High-Energy Transient
Explorer satellite (HETE) discovered one of the brightest and closest
gamma-ray bursts on record.

Located in the constellation Leo, the 30-second burst outshone the entire
Universe in gamma rays, and its optical afterglow was still over a trillion
times brighter than the Sun two hours later.

Through observations of that afterglow on subsequent nights, astronomers
spotted the telltale signs of a supernova. The team cannot yet determine the
timing of the burst relative to the supernova (whether one preceded the
other or whether both began at the same time), but the same event — a star
explosion — was certainly the trigger for both.

Gamma-ray bursts are incredibly bright flashes of high-energy radiation that
likely signal the birth of black holes. Bursts occur at random locations
scattered across the sky, and few last more than a minute, making them a
challenge to study.

A supernova is the explosion of a star at least eight times as massive as
the Sun. When such stars deplete their nuclear fuel, they no longer have the
energy to support their mass. Their cores implode, forming either a neutron
star or (if there is enough mass) a black hole.

Matheson’s colleagues on this find include Dr. Peter Garnavich of Notre Dame
and Dr. Krzysztof Stanek of the CfA. The data were obtained by nearly two
dozen scientists primarily using the 6.5-meter MMT telescope at Mount
Hopkins, Arizona. When the burst was discovered, the astronomers scheduled
to use the MMT agreed to observe it as a target of opportunity in
collaboration with Garnavich, Matheson, and Stanek.

“For the first time, we were measuring an event no other human beings had
seen before, ” said Stanek. “The MMT was our magic time machine that we used
to capture this catastrophic cosmic event.”

Matheson and the others have detected direct evidence that the burst
afterglow’s light is exhibiting the same patterns as light from a supernova..
Namely, the scientists see changes in light absorbed by silicon and iron
atoms (forged in the supernova) as the afterglow slowly fades away. The
team is continuing to observe and analyze this unique burst.

Previous observations, particularly from NASA’s Chandra X-ray Observatory
controlled at the Harvard-Smithsonian Center for Astrophysics in Cambridge,
MA, have provided convincing indirect evidence of the gamma-ray
burst/supernova connection. Chandra detected iron and other heavy elements,
which are formed in supernovae, in the vicinity of gamma-ray bursts.

“All gamma-ray bursts may have associated supernovae that are too faint to
observe,” Matheson said, “but this burst, named GRB 030329, was one of the
closest known. We caught it in the act.” The burst was approximately two
billion light-years from Earth, as opposed to other bursts located upwards
of 10 billion light-years away. Because the burst was both close and
bright, the supernova was detectable.

Garnavich added that material from the blast was ejected at such high speeds
that the source of the gamma-ray burst might actually be a “hypernova” — an
explosion ten times more powerful than a typical supernova. “We’ve seen such
hypernova explosions in nearby galaxies, although without an accompanying
powerful gamma-ray burst, so we’ll be watching this distant explosion
closely to see if it fits the hypernova profile,” said Garnavich.

The MMT is a joint venture of the Smithsonian Institution and the University
of Arizona. The 6.5-meter-diameter optical telescope is located on the
summit of Mt. Hopkins, the second-highest peak in the Santa Rita Range of
the Coronado National Forest, approximately 30 miles south of Tucson,
Arizona.=20

HETE was built by MIT as a mission of opportunity under the NASA Explorer
Program, with collaboration among U.S. universities; Los Alamos National
Laboratory; and scientists and organizations in Brazil, France, India, Italy
and Japan.

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 six research divisions study the origin,
evolution, and ultimate fate of the Universe.