Dolores Beasley

Headquarters, Washington, DC
(Phone: 202/358-1753)

Steve Roy

Marshall Space Flight Center, Huntsville, AL

(Phone: 256/544-6535)

Megan Watzke

Chandra X-ray Center, Cambridge, MA

(Phone: 617/496-7998)

RELEASE: 00-173

The Chandra Observatory’s sharp-eyed X-ray vision has
detected something never before seen. The discovery may help
find the origin of what many researchers believe are the most
powerful explosions in the Universe.

The clues are found in the afterglow of a gamma-ray burst
(GRB), and could strengthen the case for a “hypernova” model,
where massive collapsed stars generate these mysterious blasts
of high-energy radiation.

An international team of scientists used Chandra to observe
iron emission lines from ejected material surrounding one such
burst known as GRB991216. This is the first time emission lines
associated with GRBs have been unambiguously detected and their
properties precisely measured at X-ray wavelengths.

“The discovery of iron lines in the X-ray spectrum is an
important clue to our understanding of GRBs,” said Luigi Piro,
lead author of the paper that appears in today’s issue of the
journal Science. “Studying the immediate area around the GRB
tells us a great deal about the origin of the GRB itself.”

Astronomers have long debated how GRBs originate. One theory
contends that GRBs result when two “compact objects,” that is,
neutron stars or black holes, collide and coalesce. Another
theory speculates that a “hypernova,” a gigantic star
collapsing on itself under its own weight, could cause these
extremely energetic outbursts.

A shift in the wavelength, or energy, of the detected iron line
emission, relative to what would be seen in a laboratory, tells
researchers the distance to the GRB. The Chandra team
determined it has taken 6 billion years for the X-rays from
GRB991216 to reach Earth.

From the distance and the intensities of the detected X-ray
emission lines, the investigators deduced the properties of the
ejected material and its relationship to the GRB. The team was
able to determine the mass of the medium within a light day or
two of the GRB as approximately equivalent to at least one-
tenth that of the Sun. By analyzing the widths of the detected
spectral lines, the researchers found that the material
surrounding GRB991216 is moving away nearly 10 percent the
speed of light.

“Our data helps rule out the scenario where two neutron stars
or black holes collide,” Piro said. “We think GRBs result from
something similar to a supernova explosion, but much more

Scientists speculate that the initial shedding of material,
perhaps the outer envelope of a hypernova, is followed by an
event at the core of that hypernova – most likely a collapse to
a black hole. Energy released by the fireball of the resulting
GRB would then heat up the ejected material, producing optical
and X-ray afterglows, lasting days or weeks.

GRB991216, first detected by the Burst and Transient Source
Experiment (BATSE) aboard the NASA’s Compton Gamma-ray
Observatory, Dec. 16, 1999, was one of the brightest GRBs ever
found by that instrument. A more accurate GRB position was
obtained by the Rossi X-ray Transient Explorer. Chandra was
able to reorient quickly in order to observe the event, while
the flux level was still high. This allowed Piro and his team
to observe GRB991216, using Chandra’s High Energy Grating
Spectrometer (HETG) in conjunction with the Advanced CCD
Imaging Spectrometer (ACIS) for more than 3 hours.

The research team included Pennsylvania State University’s
Gordon Garmire, principal investigator for the ACIS instrument,
Michael Garcia of the Harvard-Smithsonian Center for
Astrophysics, Cambridge, MA and other colleagues from the
United States, Italy, Japan, and the Netherlands.

The ACIS X-ray camera was developed for NASA by Penn State and
the Massachusetts Institute of Technology (MIT). The High
Energy Transmission Grating Spectrometer was built by MIT.
NASA’s Marshall Space Flight Center in Huntsville, AL, manages
the Chandra program. The Smithsonian’s Chandra X-ray Center,
Cambridge, MA controls science and flight operations. Images
associated with this release can be found at: