Web hosting Custom Email SiteBuilder

GRB 991216

A Special Report by Dr. Arne Henden, Ph.D, USNO (Flagstaff)
Received: 20 December 1999
Uploaded: 20 December 1999
Last Updated:

GRB 991216 occurred at 1607 UT on December 16, 1999. The BATSE gamma-ray detector on Compton Gamma Ray Observatory (CGRO) measured the burst as the third largest ever observed, with about 70 photons/cm^2/sec at peak and with a duration of about 20 seconds. Note that this flux means that a typical human in space would have received a dose of 5 million gamma rays from this burst!

The BATSE detector gives an approximate 8 degree error radius for the initial position. At this time, the wide field telescopes such as LOTIS or ROTSE would slew over and begin collecting data. Unfortunately, the GRB was below their horizon. If such a system were located in Asia, it would have been able to image the field. The prompt optical radiation is an unsolved regime. The only burst with prompt optical radiation out of several dozen that have been observed was GRB 990123, imaged by ROTSE at a peak of about R=9 and decreasing to R < 14 within a half-hour.

At 1702 UT, the LOCBURST position was distributed. This system utilizes the entire burst to derive an improved localization. LOCBURST indicated a position of 05h 19m 15s +10d 47' 24" (J2000), with about a 2.8degree error radius. Now, wide field telescopes such as LONEOS or other converted Schmidts could image the entire error box, either at once or with a 2x2 mosaic. Again, the GRB was beneath the horizon for the U.S., but other bursts have been covered in this manner. If this burst had prompt radiation similar to GRB 990123, it would now be about R="16," easily detectable by such telescopes. Bejing Observatory was the only telescope to start imaging this field, with about 58x58arcmin field of view. They covered most of the error box. (There is a new system just about to be used, called LOCFAST, that promises to give these 2degree error radii in just a few seconds after the end of the burst.)

Now the RXTE satellite slewed over to the position, and started raster scanning the error box using the on-board X-ray detectors (the RXTE-PCA). The first set of scans were in the RA direction, and localized the fading X-ray counterpart at UT 2249 as 05h 09m 31s 11d 11' 24" (J2000), with an estimated error of 2.4arcmin in RA, and a few tenths of a degree in Dec. This is within the field of view of almost all larger telescopes, and observatories in India and Europe started observing. Note that the position is about 2 degrees from the original position; the BAO observations missed the field. Major observatories, such as La Palma, are clouded out. Several observatories in the U.S. and Chile started observing the field around 991217.1-991217.3 in the optical and near-IR. (Note that this burst was not seen by BeppoSAX. If it had been observed, equivalent localizations would have been reported in a few hours after the burst.) A later set of scans in the Declination direction improved the Dec localization to +11:18:00 +/- 3.0 arcmin.

The first report of a detection of an optical transient occurred at 991217 1248 UT from Uglesich, et. al. (GCN 472), based on MDM 1.3-m images. On Dec 17.142 they found a source that was R="18.8" and faded to R="19.4" on Dec 17.372. This source was confirmed by others.

On Dec 17 at 1814 UT, the InterPlanetary Network (IPN) localization was reported. IPN either gives an annulus (where timing from two spacecraft is used) or a diamond (if three or more spacecraft observed the burst) error localization. In this case, only two spacecraft saw the burst. The annulus passes through the RXTE-PCA error box and further confines the position. (Of course, an OT has already been reported by now. This is not always the case.) Several reports of the OT at R and JHK are now given. HET gets low resolution spectra on Dec 18. SAO spectra yield a redshift of 0.707 based on three emission lines. Keck spectra confirm this. VLT spectra indicate that three redshift absorption systems seem to be present, with one at z="1.02." Radio observations start appearing, with an accurate position given by the VLA of 05h09m31.297s +11d17'07.25" (J2000), errors of about 0.1arcsec.

The source fades to R="20.3" at Dec 18.40, and R="20.9" Dec 19.10.

It continues to be monitored at R and H by several observatories.

DSS Image	One of the leading theories for the cause of gamma ray bursts is the collapsar or failed supernova theory. A super-massive star, after bruning 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 evnets at the core are 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 Sympsium. Credit: Stan Woosley, University of California, Santa Cruz.	Light Curve

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.

"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.

---------------------------------------------------------------------------------------------------------------------------------------------

BATSE Trigger Site

Max Planck Society