BATSE Pins Down Space Oddity
December 13, 1996
Continental plates push against each other, form a mountain range, then crash the few millimeters in a flash of gamma rays to the face of the neutron star.
Or perhaps the neutron star just zips through a cloud of dust and rock, swallows everything, and belches gamma rays as the meal hits bottom.
Since 1980, astronomers have puzzled over an odd class of stellar objects called soft gamma-ray repeaters (SGR). SGRs are unique because they appear to violate the so-called Eddington limit which describes how radiation pressure will slow the fall of matter into a dense star. SGR bursts turn on and off more quickly than so-called "normal" bursts.
In addition, they repeat now and then - usually a few years apart - which most gamma ray bursters don't do. Three SGRs are known, two near the center of our galaxy, and a third in a small satellite galaxy called the Large Magellenic Cloud.
Dr. Chryssa Kouveliotou, an astronomer at NASA's Marshall Space Flight Center, is getting closer to uncloaking these oddities thanks, in part, to recent observations by Marshall's Burst and Transient Source Experiment (BATSE) aboard the Compton Gamma Ray Observatory (CGRO). On Oct. 31, BATSE detected an SGR outburst, and Kouveliotou was able to get another satellite aimed to capture part of the source's activity.
"We know SGRs emit in an energy range that BATSE can detect," said Kouveliotou, a senior member of the BATSE team employed by the Universities Space Research Association. "But what about energies below BATSE's threshold? It's a series of microbursts. We found out that the source emits a series of bursts, only one of which is detectable by BATSE."
Kouveliotou has been studying SGRs since the first two were discovered by Russian scientists reviewing data from early gamma-ray burst detectors aboard the Venera spacecraft bound for Venus. In their data they found bursts that had less energy - were "softer"- and shorter in duration than most bursts. A third SGR was identified in 1986 in data from a Russian Prognoz satellite.
The Hunters Go BATSE
Thus alerted, scientists hunted and found the three SGRs in other astronomical data. But early detectors could not pin the location any finer than a large box that encompassed too many stars to allow a telescope to scan and locate the SGR while it was active. It was for just such challenges that GRO was equipped with the eight BATSE detectors to observe the entire sky all the time.
BATSE was launched in April 1991 and soon was returning data about gamma ray bursts. Finally, in the fall of 1993, BATSE detected three SGR emissions in one day. Over 40 days six bursts were detected, all from SGR 1806-20 (the numbers refer to its position in the sky near the constellation Sagittarius). Kouveliotou at Marshall soon gave refined position estimates to scientists operating other astronomy satellites.
On Oct. 9, 1993, a Japanese team commanded their Advanced Satellite for Cosmology and Astrophysics (ASCA) to scan that part of the sky and caught the dying embers of a burst. The location coincided nicely with a rare, young supernova remnant, G10.0-0.3 (its catalog listing) about 50,000 light years away, near the center of our galaxy. G10.0-0.3 is known as a plerion, a type of supernova remnant powered by a rotating neutron star much like the more famous Crab Nebula.
That Crushing Feeling
A popular notion about supernovas is that they all create black holes. Few are supernovas that massive. Instead, they leave neutron stars about the size of the Earth with the mass of the sun. Their surface gravity is so intense that a mountain range would be no more than a mere ripple just a few millimeters high.
That leads to the theory that SGR outbursts are the energy released by quakes as a neutron star's intense magnetic field and rapid rotation drive the continental plates. The other principal SGR theory is that bursts come from the star scavenging meteoric debris from interstellar space.
Proving either theory - or coming up with a new one - requires data. Knowing that SGR 1806-20 - or another SGR - would act up eventually, Kouveliotou requested observing time on the Rossi X-ray Timing Explorer, a relatively new satellite operated by NASA's Goddard Space Flight Center. She waited for an "event" that would tell GRO's computers to record data at high speed so scientists can study the details like a high-speed movie replayed in slow motion.
Although the two satellites' names denote different parts of the radiation spectrum, they overlap a little. "Hard" X-rays for RXTE are "soft" gamma rays for GRO. Most X-ray bursts have an average energy of about 2,000 electron volts, or 2 keV (by comparison, visible light is about 1 eV). The one known bursting pulsar emits X-rays at mostly at 10 keV, and SGRs at 17 to 35 keV, the band where hard X-rays become soft gamma rays, at least in the way we name them. Most of the gamma ray bursts that BATSE detects are much harder, at around 200 to 300 keV.
When BATSE saw a burst from SGR 1806-20, Kouveliotou asked the RXTE team to repoint the satellite and catch two days' worth of data before the Earth's motion put 1806-20 on a line close to the sun.
"RXTE observation has revealed a whole new behavior for this object which we had not known but suspected," Kouveliotou said. Part of that behavior includes groups of series of small bursts, sometimes up to 30 clustered together, only a couple of which was strong enough to trigger BATSE.
RXTE has to wait until mid-January for another look at 1806-20. Meanwhile, in addition to RXTE's data, Kouveliotou will look at background data from BATSE for signs that 1806-20 was warming up. And she is thinking about applying for guest observing time on the Advanced X-ray Astrophysics Facility (AXAF), a Marshall telescope that will offer detailed looks at oddities like SGRs or bursting pulsars.