November 3, 2000
RELEASE: 00-173

NASA'S CHANDRA CAPTURES TELLING GAMMA-RAY AFTERGLOW

     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 
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

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