1 December 1997
The explosions come from a 'micro-quasar' called GRS1915 in the constellation of Aquila the Eagle at a distance of 40,000 light years on the other side of the Milky Way.
GRS1915 was discovered by an x-ray telescope on the Russian satellite GRANAT in 1992. Rob Fender, from Brighton, currently working at the University of Amsterdam, explained "GRS1915 is believed to consist of a black hole several times more massive than our Sun, in orbit with a normal star. Matter is heated to intense temperatures and radiates x-rays as it spirals in towards the black hole. Most of it is sucked in and lost forever. The process is so violent that unpredictable explosions occur, producing these spectacular jets."
MERLIN is the only instrument in the world capable of observing the radio outbursts accompanying the x-ray flares in such detail.
As Ralph Spencer from Jodrell Bank points out: "Since GRS1915 is so far away from us, we need an instrument with a resolution even better than the Hubble Space Telescope, but at radio wavelengths, to be able to see what is happening close to the black hole. That is precisely what MERLIN is able to do."
MERLIN (the Multi Element Radio Linked Interferometer Network) is run from Jodrell Bank by the University of Manchester on behalf of the Particle Physics and Astronomy Research Council. It consists of six individual radio telescopes spread out across England connected together to give the same resolution as a single huge dish 135 miles across. MERLIN can see detail as small as a penny piece viewed at a distance of 50 miles.
On October 29th this year, astronomers around the world were alerted by the Ryle radio telescope at Cambridge that GRS1915 had flared. The radio telescopes of the MERLIN array then swung into action, and were able to follow the evolution of the outburst. In only 2 weeks it became too faint to be detected. Nonetheless, MERLIN's unique combination of resolution and sensitivity meant astronomers at Jodrell Bank were able to make a detailed time-lapse movie of this fleeting event *).
The images show two streams of bullets of ultra-hot gas being shot out in opposite directions with an apparent speed of expansion of more than twice that of light. This is an effect predicted by Einstein's Special Theory of Relativity. It shows that one jet is moving towards us at an angle, and the other away from us, with real velocities greater than 90% of the speed of light.
*) This colour picture, showing Jodrell Bank radio images of ultra-hot gas which appear to shoot away from a black hole faster than the speed of light, is available from the Jodrell Bank Web Site