STARGAZERS WITH X-RAY EYES

Scientists’ understanding of black holes and the origins of the universe, is set to be improved with the help of a new European-funded satellite. Melanie Tringham reports

Black holes may now be a phenomenon familiar to more than just theoretical physicists, but still very little is known about the dense gravitational masses.

As intense concentrations of matter, black holes are so dense they can crush matter and bend light. Einstein created the theory of black holes as defining the beginning and end of galaxies. In the next few years, black holes are expected to give up a few more of their secrets: a European project, X-ray Multimirror (XMM), aims to establish the nature of black holes.

As matter disappears into the hole, it heats up and emits its energy as X-rays. These provide the most obvious evidence of black holes, and XMM will detect the X-rays. When launched into Earth’s orbit in 1999, XMM will take detailed snapshots of the X-ray universe.

X-rays cannot be picked up by conventional optical equipment, because they pass straight through conventional flat lenses. The solution in XMM is one of the European Space Agency’s proudest engineering achievements. Researchers have constructed a nest of 58 cylindrical mirrors made from 1mm thick glass and plated in gold. Each is angled slightly to pick up X-rays at different orientations.

The effect is that 60% of the X-rays entering the telescope will find a mirror positioned to pick them up.

Despite being only 1mm thick, the mirrors will have a diameter of 700mm, and there is a risk they will sag under their own weight when assembled and launched into space on an Ariane 5 rocket.

`Supporting all these mirrors in the right configuration, being able to launch it and control the temperature from different sides of the sun, adds up to a big technological programme,’ says Paul Murdin, director of space science at the British National Space Centre.

German company Dornier has won the prime contract for the project, worth Ecu600m (£415), but the largest subcontract has gone to the Anglo-French company Matra Marconi Space, which is manufacturing the attitude and orbital control sub-system at its Bristol site, a contract worth Ecu34m (£25m). British universities will build crucial instruments. University College London’s Mullard Space Science Laboratory and the University of Leicester are building the instrument that will pick out X-ray sources detected from the mirrors.

The project is being heralded as a financial achievement for ESA. Robert Laine, XMM’s project manager, claims to have saved ESA 10% of the programme budget. Many large programmes tend to run over by 20%, and only then does ESA management take notice. Laine was employed at Alcatel Alsthom until 1992, and since then he has worked in Estec, ESA’s technology centre.

ESA will also need to fight at least two other X-ray satellites in the battle for scientific credibility among the world’s X-ray astronomers. Nasa is expected to launch the Advanced X-ray Astrophysics (AXAF) in September 1998, and Russia will be launching Spectrum-X in 1999. Leicester University researchers are contributing to Russian project.

AXAF, the more serious rival, is claimed by ESA to offer scientists more limited scope in data. Each picture will be more detailed but the area covered will be smaller.

`The best way forward for science is to have cooperation, but with competition in between,’ says Murdin. `If you just use one instrument you get an extremely one-sided view of what’s going on.’

Britain’s track record in astronomy has been noteworthy. The most recent discoveries concerning black holes, vindicating Einstein’s predictions, were made by a team at Cambridge with scientists in Japan.

X-ray research will continue over the next 10 years. ESA and others, including the BNSC, are trying to develop the next generation of X-ray satellites. The driving factor is increasing the size of the telescope mirror without increasing its weight.

XMM and its rivals are designed to be launched on leading-edge, heavy duty rockets such as Ariane 5, and larger launchers are not likely to be available for the foreseeable future. Therefore, the telescope mirrors must get lighter, and that will need a lighter substrate than glass.

But if this finally happens, the real questions about black holes and our beginnings might have a chance of being answered.