And Then There Were Four
September 19, 1997
Gamma ray bursters are transient, one-time events that appear on the cosmic stage and are never seen again. But some other transients are in a special category that keep coming back for curtain calls. This week, scientists announced that they had found yet another soft gamma ray repeater - SGR - to add to the cast of three that have been discovered since 1979.
Editor's note (July 7, 1998): The science team that discovered and located this object has decided not to confirm it as an SGR. Although it appeared to be an SGR, the scientists later decided that the data were too tentative to warrant confirmation at this time.
However, in May the science team announced that extensive observtions of another SGR allowed them to confirm its identity as the first known magnetar, or highly magnetized neutron star.
"This is the most intense SGR seen by BATSE," said Dr. Chryssa Kouveliotou of the Universities Space Research Association. BATSE - the Burst and Transient Source Experiment on board the Compton Gamma Ray Observatory - was designed to detect gamma ray bursts that have mystified scientists since 1973, when they were first announced to the wider community (they were first discovered in 1967).
Unlike gamma ray bursts, which emit large amounts of high energy gamma radiation, SGRs have a larger proportion of lower-energy X-ray radiation. Also, in contrast to gamma ray bursts, which can rumble on for many minutes, SGRs pop off - like a cap gun - in as short a time as it takes to snap your fingers.
The first SGR was discovered by a Russian spacecraft in 1979. Two others were not recognized as a new phenomenon until 1986, when several scientists gathered in Toulouse, France, to decide how to classify and name this newly found class of objects. Spurred by Dr Kevin Hurley, of the University of California at Berkeley, the scientists reviewed these unusual burst sources, and decided they had discovered a new phenomenon. SGR research remained quiet until 1993, when BATSE captured two of the sources emitting again. In late 1996, Kouveliotou was able to use BATSE to alert other scientists to point the Rossi X-ray Timing Explorer at the expected position.
And so the population sat at three until this summer.
"On June 29, BATSE triggered on an extremely intense burst that had all the characteristics of SGR emission," said Kouveliotou. "Immediately we alerted Kevin for additional information from the Ulysses spacecraft."
"Ulysses [a probe orbiting the sun] saw the burst and it also detected an additional short intense burst," Hurley said.
This extra burst was also seen by the Konus detector aboard the Wind geoscience satellite. An International Planetary Network (IPN) arc between BATSE and Ulysses and between Ulysses and Konus placed these two events on unknown grounds: no other SGR source was detected from their location.
"Still we decided to pay it safe," said Kouveliotou. "We stood by and waited for the next burst to clinch the source location. On Friday (September 12), we got it! We heard from the All Sky Monitor on the Rossi X-ray Timing Explorer that they had seen a very bright event that literally shut their instrument off. Dan Smith, an ASM team member at the Massachusetts Institute of Technology, immediately informed all other spacecraft teams. When data from BATSE came in, we realized we had another intense SGR burst; the event was also seen by Ulysses, and Hurley immediately put all three events on the map."
"The locations agreed," he said. "We concluded that we are observing a new SGR."
And on Thursday, even more good news arrived: Fred Vrba of the U.S. Naval Observatory at Flagstaff, Arizona, announced that the observatory had found an object that is glowing very bright in infrared. It is not yet confirmed as the optical counterpart, but nothing else has been nominated.
"It's very hot now," Kouveliotou said. "Everybody's looking at it, scanning the error box." The error box is the small section of sky where instruments aboard other spacecraft indicate the source can be found. The June 29 observation by Ulysses, BATSE and Konus produced a long, thin error box that included too much sky, while BATSE showed that the burst was similar to past SGRs.
"We decided to wait and get a better measurement when the two spacecraft had moved apart," Kouveliotou said.
By September 12, when the second burst went off, Ulysses had moved far enough away that their combined detection produced a much smaller error box.
"Kevin computed the position and it was clearly a new location," Kouveliotou said.
For all their differences from other bursts, SGRs have at least one similarity: we don't know what causes them. Among the possibilities are young neutron stars - only 6,000 years old - energizing a large cloud of gas cast off in a supernova explosion. They are temporary phenomena, dying around their 10,000th birthday. Another possibility is X-ray binary stars that accrete matter at irregular intervals and belch gamma rays when the matter lands.
(At right is an artist's conception of an x-ray binary system, showing matter being pulled away from the left-most star onto the accretion disk of its partner.)
As with other bursts, the answer will evolve from continued attendance as the SGRs stage their shows.