ESA Science & Technology30-Jun-2005 16:44:23

Beyond the Heliosphere

Gamma-Ray Bursts

Cosmic gamma-ray bursts are the most powerful explosions in the known universe. They appear to be generated by the collapse of very massive stars (~100 solar masses). One of Ulysses' scientific objectives is to be flagship of the 3rd Interplanetary Network (IPN) of gamma-ray burst detectors, which locates bursts precisely by comparing their arrival times at widely separated spacecraft. The distances to about a dozen sources have now been measured, showing that they are at cosmological distances comparable to those of quasars. If the bursts emit their energy uniformly into space, their output is 1053 erg in gamma-rays alone. In addition to being interesting in their own right, gamma-ray bursts are being used by cosmologists as probes of the early universe.

Hubble Space Telescope
image of the galaxy where
the gamma-ray burst of
March 1 2000 originated.
This burst came from a
distance of 11 billion
light-years. Ulysses and the
Rossi X-Ray Timing
Explorer determined its

 In the course of studying gamma-ray bursts, Ulysses has played a major role in the study of so-called soft gamma repeaters, or 'magnetars'. These are neutron stars in our own galaxy, which are thought to possess magnetic fields with strengths of >1014 G. They are especially interesting as cosmic laboratories where the behaviour of matter and its associated radiation can be studied under extreme conditions. Magnetars occasionally emit giant flares that produce the strongest X-ray fluxes ever measured at Earth from an extra-solar object. One such event, observed by Ulysses on August 27 1998, was so intense that it produced a major ionospheric disturbance.

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