After a lonely nine months, Ulysses has a new partner in gamma-ray burst detection. On 21 November, the ESA/NASA spacecraft in orbit high above the Sun's poles, and Mars Odyssey, NASA's spacecraft recently arrived at the Red Planet, detected their first gamma-ray burst together.

The simultaneous detection by the two widely-separated spacecraft, plus detection by a third spacecraft (the Italian-Dutch BeppoSAX) close to Earth, allowed astronomers to locate the source of the burst and look for its afterglow using ground-based telescopes.

The blue dot in the centre of this image (shown by the arrow) is the optical light signature of GRB011121, the gamma-ray burst detected by Ulysses, Mars Odyssey and BeppoSAX on 21 November 2001. The gamma-ray burst briefly appeared brighter than the rest of the Universe in gamma rays, before rapidly fading away. This optical image, taken with the National Science Foundation's Blanco 4-meter telescope at Cerro Tololo Inter-American Observatory in Chile during the morning of 23 November 2001, shows the optical transient a day and a half after the initial burst. The transient had faded in brightness from its peak by more than a factor of 100 in that period of time. The reddish dot adjoining the gamma-ray burst, at its lower left, is probably the core of the galaxy in which the gamma-ray burst occurred. Credit: M. Brown, R. Schommer, K. Olsen, B. Jannuzi, A. Dey (NOAO), A. Fruchter, J. Rhoads (STSci) AURA/NSF

Gamma-ray bursts were first detected in the 1960s, but it was not until 1997 that the first association was made between a burst and its afterglow. The problem is that bursts are so short-lived (from milliseconds to 1000 seconds) that locating their sources requires careful coordination between observatories.

Ulysses is part of a network of spacecraft and ground-based telescopes coordinated in this way. Although a few thousand gamma-ray bursts have been detected since Ulysses was launched in 1990, optical counterparts have been found for only about 30 so far. One of the most notable occurred on 31 January 2000 when Ulysses helped to establish that it was the most distant gamma-ray burst ever recorded (see Yet another record: Ulysses detects most distant gamma-ray burst).

Astronomers now think that gamma-ray bursts are very frequent, occurring about once a minute somewhere in the sky. They originate in the most powerful explosions in the Universe, and at least some are thought to occur when massive rapidly rotating stars collapse into black holes. When observed at great distances they become tools to study the Universe during its infancy.

The burst on 21 November was located in the constellation of the Chamaeleon, in the southern sky. The Very Large Telescope in Chile found its optical counterpart and measured its redshift from which its distance was found. Redshifts and hence distances have been found for only about half of the gamma-ray bursts with an optical counterpart so far.

At a distance of 4,000,000,000 light-years (corresponding to a redshift of 0.3), the burst was the closest ever recorded but, paradoxically, it was not particularly strong.

"It turns out that weak bursts can be close, and strong bursts can be very distant; this means that we are probably observing emission which is beamed. If we happen to be right in the beam, the burst will look strong, regardless of its distance," says Kevin Hurley from the University of California at Berkeley and principal investigator for the gamma-ray burst experiment (GRB) on Ulysses.

Ulysses is the longest serving spacecraft with a gamma-ray burst detector. Since its launch, other spacecraft with similar detectors have come and gone. There are always two or three orbiting Earth, but keeping available at least one other spacecraft far from Earth has not always been easy.

NASA's Pioneer Venus Orbiter, Mars Observer and NEAR spacecraft have taken on the role at various times. But when NEAR completed its mission on the asteroid Eros last February, Ulysses was left without a distant companion until Mars Odyssey was well underway.

The detection of bursts by three distantly separated spacecraft allows astronomers to use triangulation methods to locate the source of the bursts. The greater the separation of the spacecraft, the higher is the accuracy of the method. Spacecraft operating close to Earth which have taken on the role of detecting gamma-ray bursts, include NASA's WIND and HETE-II satellites, the Italian-Dutch BeppoSAX, the Indian satellite SROSS-C2, the Japanese X-ray observatory Yohkoh and the Chinese SZ-2 mission. In October 2002, ESA's gamma-ray observatory, INTEGRAL, will join them.

"Although Ulysses is primarily a mission to study the Sun and its heliosphere, it is also making major contributions to a wide range of astrophysical studies," says Richard Marsden, ESA's project scientist for Ulysses. "The exciting results from the gamma-ray burst experiment are a good example."