The flash of gamma rays was detected on Aug. 27 by at least seven spacecraft in Earth orbit and in deep space.
It capped several months of observations of an object known as SGR 1900+14, a Soft Gamma Repeater located in the constellation Aquila (the eagle) near Sagittarius (the archer).
 Left: SGR 1900+14 - observed by the Italian-Dutch Beppo SAX satellite - during a dormant phase in 1997 (left side) and in September 1998 (right side) after it erupted with a series of energetic flares. 720x486-pixel. Credit: Chryssa Kouveliotou, USRA, and Peter Woods, UAH. |
The Aug. 27 boomer may have been caused by an out-of-control magnetic field of a neutron star realigning itself in a manner similar to what happens inside solar flares.
A neutron star is created when a massive star explodes in an event called a supernova. The remaining core of the supernova, which has slightly more mass than our own sun but can no longer burn fuel, compresses under gravity into a neutron star about 20 km (12 mi) across.
A magnetar, or neutron star with a huge magnetic field, may explain the observation.
According to the magnetar theory, the common flashes that are the hallmark of SGRs are due to starquakes. As a magnetar's huge magnetic field drifts though the neutron star, the magnetic strain on the iron crust causes it to deform, often breaking it. Like an earthquake, this vibrates the star with seismic waves, and drives shaking magnetic waves outward to energize particles outside the star and emit flashes of soft gamma-rays. To cause starquakes, the magnetic field must be enormous - at least 1014 gauss, at least 100 times stronger than a "normal" pulsar!
In the first moments of a magnetar flare, the release of pure magnetic energy drives out an tremendous explosion of superheated particles and gamma rays.
This was observed as the intense gamma-ray pulse in the first second of the Aug. 27 event.The explosion leaves behind a residue of hot particles which are held close to the star by the magnetic field, because charged particles cannot freely flow across a strong magnetic field. This magnetically trapped cloud cools and shrinks by emitting soft gamma rays and X-rays, as observed in the fading tail of the Aug. 27 event. As the star rotates, the trapped cloud of particles is seen from different angles, causing the intensity to rise and dip regularly over each 5.16-second rotation cycle.
Just such a series of flashes was seen this summer by a number of spacecraft, notably the Rossi X-ray Timing Explorer (RXTE), Beppo-SAX, the Advanced Satellite for Cosmology and Astrophysics (ASCA), and four U.S. defense and civilian weather satellites, all in Earth orbit; Wind, about a million miles from Earth; the Ulysses solar polar probe near the orbit of Jupiter, and the Near-Earth Asteroid Rendezvous (NEAR) spacecraft near the orbit of Mars.
SGR 1900+14 was observed with the RXTE between May 31 and June 9 for a total of 41,700 seconds (11.6 hours) and found it had a period of 5.159142 seconds.
Comparing the various RXTE observations with each other and with earlier data from ASCA, Kouveliotou's team calculated that SGR 1900+14 was slowing down ever so slightly. Each rotation is about 0.00000000057 second slower than the one before. That's about one second every 290 years. It doesn't seem like much, but for the fact that since SGR 1900+14 is a neutron star, it weighs at least 1-1/2 times as much as our Sun. Something incredibly powerful is putting the brakes on it.
The most spectacular event came on Aug. 27 when RXTE was not even pointed at SGR 1900+14. The burst was so strong and clear that RXTE's Proportional Counting Array detected it through the shielding intended to keep stray radiation out.
Then the array locked up because of the overload. Because Ulysses and NEAR were in deep space, scientists could triangulate the position by slight differences in arrival times.