Chandra Space Telescope Captures Telling Gamma-Ray Afterglow

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 powerful."

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:

http://chandra.harvard.edu