Gamma-ray astronomers get a special star on their Christmas tree

Roll over, Beethoven - this blast is for you

Dec. 22, 1999: Astronomy is ending the year with a bang as scientists across the world take advantage of a unique bit of NASA teamwork that quickly located a gamma-ray burst, one of the most violent events in the universe.

As a result, several major observatories, including the Chandra X-ray Observatory, were able to swing into position within hours or days of the blast and discover its X-ray, optical and radio counterparts for the burst. One astronomer has nicknamed the blast Beethoven because it fell on the anniversary of the composer's birth (Dec. 16, 1770). Its official name is GRB 991216.

One of the leading theories for the cause of gamma ray bursts is the collapsar or failed supernova theory. A super-massive star, after burning all of its nuclear fuel, starts to explode as a supernova, but the overlying atmosphere is too massive to blow off, and the explosion collapses, forming jets of matter that burrow out through the poles and then rip the star apart. The scale of the events at the core is about the same size as our Earth. For more details, see the story (link to Taking a Ringside Seat, below) from the 5th Huntsville Gamma Ray Burst Symposium. Credit: Stan Woosley, University of California, Santa Cruz.

"This is the first major success in a three-year program to use instruments on two NASA satellites to get a rapid location of a burst," explained Dr. Marc Kippen. Kippen is a University of Alabama in Huntsville astrophysicist working at NASA's Marshall Space Flight Center. He's part of the Burst and Transient Source Experiment (BATSE) team at NASA/Marshall.

BATSE, one of four instruments aboard the Compton Gamma Ray Observatory, usually is the first (sometimes only) instrument to detect a gamma ray burst. BATSE's eight detector modules point from each corner of the Compton Observatory so they will capture anything that happens above Earth's horizon.

(The Robotic Optical Transient Search Experiment, or ROTSE, which caught GRB 990123, was in daylight and thus unable to detect this burst.)

The price for this all-sky capability is reduced precision in determining the location of a burst. With a little processing on the ground, BATSE data can be used to locate a burst inside a circle 4 degrees across, about eight times the apparent width of the Moon. That's still far too wide for most telescopes, whose high powers also mean very narrow fields of view.

Now the ball is picked up by the Rossi X-ray Timing Explorer, operated by NASA's Goddard Space Flight Center. Rossi does not have imaging instruments. Even its sensitive Proportional Counting Array has a 1 degree field of view, still too large for most telescopes. But it can be used to produce coarse images.

"What they do is a scan across a region," Kippen explained. It's a little like the raster scan of an electron beam across the face of a TV screen: across, down one, back across, down one, and so on.

"We've done about 25 bursts where we got the location with BATSE and gave it to Rossi and they spent a frantic time trying to reprogram the spacecraft to do this scan," he continued. It can take several hours to reprogram the satellite. With Beethoven, they got lucky.

"They were able to get this one in four hours," Kippen said. "They got the source in the first four scans and just happened to see the source in two of those scans."

This brought the location down to a narrow box, just 0.04-by-0.3 degree in size. Another scan 10 hours later placed a similar box at right angles over the location, now pinning it down to 0.04-by-0.08 degrees.

"At that time, using the Rossi location, fairly large telescopes started making observations," Kippen said. "The earlier attempts didn't catch it" when using coarser data. But those using the refined BATSE and Rossi data did.

The hunt started with a low-intensity trigger, followed 20 seconds later by an intense blast of gamma radiation. The trigger event was a flash of radiation just powerful enough to switch on the BATSE alerting system. In about 20 percent of bursts, a small precursor burst comes from a few seconds to several minutes before the main event. No one is yet sure why. Links to 730x478-pixel, 10KB GIF. Credit: BATSE team, NASA/Marshall Space Flight Center

The burst is at 77.38 right ascension, 11.30 declination. The first observatory to catch the optical afterglow was the MDM - the University of Michigan, Darthmouth College, Massachusetts Institute of Technology, Columbia University observatory at Kitt Peak, Ariz. - which quickly recorded a fading magnitude 18.7 source. Since then, several observatories have trained on the burst and recorded a steadily declining afterglow.

"There is even a radio source coincident with the X-ray and optical sources," Kippen explained. "Practically every waveband they've looked at they've seen something."

Even the Chandra X-ray Observatory was able to catch the fading embers just four days after the event, a remarkable bit of reprogramming for so complex a facility.

"It is an excellnt choice for Chandra's first GRB observation," Kippen said. "It's guaranteed to be well studied."

While detailed understanding will have to await some study, followed by publication, one early result has been published in astronomical circulars. An approximation of the red shift of the blast puts it more than 10 billion light years away, roughly 2 billion years after the Big Bang.

"It's amazing that such a bright burst would have a Z of 1," Kippen said, referring to the red shift measurement. A red shift of z=1 indicates that the burst has had its emission red-shifted by a factor of 2 (1+z) due to the cosmological expansion of the universe."

The coordinated observations - which ensure a large data set - alone would be enough to produce a series of professional papers about GRB 991216. In addition, it ties as the second brightest burst recorded by BATSE. It didn't just ring the bell; it rang all eight.

"When you get real high intensities, some photons trickle through the backside of the detectors," Kippen said. "For really bright events, it will light up everything."

BATSE's main large area detectors, or LADs, are large, single-crystal sheets of sodium iodide. Gamma rays passing through will make the crystals sparkle or scintillate. This light is picked up by special sensors at the other end of the bucket holding the crystal.

Right: Hunting a gamma-ray burst's location is a lot like detective work. The blue circles represent the area BATSE indicated the burst should be; the larger the circle, the greater the probability of including the burst. The diagonal represents the arc of sky carved out by correlating the different times of arrival as seen by BATSE and by interplanetary spacecraft with small detectors. Finally, Rossi searched inside the narrow overlap area (inset) and quickly found the burst source. Links to 496x382-pixel, 10KB GIF.

Usually, just three or four BATSE detectors see a burst event. A detector facing the burst will see it the brightest; those at an angle will see it more dimly. By applying a little trigonometry, scientists can determine the approximate position in the sky.

In the case of GRB 991216, all eight detectors were triggered (there are methods of telling which side is illuminated, so there was no confusion about the direction) because the radiation was strong enough to go through the spacecraft.

The brightest seen by BATSE, on Sept. 24, 1996 (GRB 960924), was a mere two times brighter than Beethoven. The previous No. 2 was the "Superbowl burst" that went off Jan. 31, 1993. BATSE scientists were enjoying the game when their pagers were set off by computers in BATSE control room.

Since then, the alerting system has been refined so notices are sent automatically, thus speeding the process of putting observatories on notice, and increasing the chances of catching the burst in the act with more than BATSE.

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