Using a revolutionary new X-ray telescope, a team of American and British astronomers has obtained the most detailed X-ray spectrum of a celestial body ever - and at a fraction the cost of a large orbiting satellite. Scientists from the US Naval Research laboratory, the University of Leicester (UK) the Mullard Space Science Laboratory of University College London (UK) and the Lawrence Livermore National Laboratory (US) collaborated to build the J-PEX spectrograph, packed full of new technology. It was flown on a NASA sounding rocket from White Sands Missile Range in New Mexico on 21 February 2001 and was able to observe its target star for 5 minutes before parachuting back down to Earth.
Since the flight Dr Martin Barstow and Nigel Bannister at the University of Leicester have been working on the information returned by J-PEX. The results were presented for the first time at an international scientific meeting ("Continuing the challenge of extreme ultraviolet astronomy") in San Francisco on 24 July. Dr Barstow said "J-PEX is the most sensitive X-ray instrument yet launched into space and its first flight was a great success. We discovered what we were looking for - evidence for ionised helium in the spectrum of the white dwarf star G191-B2B."
X-ray astronomers working in this part of the spectrum have so far had to use "grazing incidence" mirrors to collect and focus the radiation. Compared to conventional telescopes used for observations of visible light for example, they are relatively inefficient and need a large and expensive satellite to accommodate them. J-PEX solves this problem with multi-layer mirror technology, which allows the use of normal incidence like in an optical telescope. It gives high sensitivity with a smaller, less massive payload. The only disadvantage is that multi-layer mirrors operate over a restricted wavelength range but they can be "tuned" to particularly interesting regions of the X-ray spectrum.
Dr Ray Cruddance of the US Naval Research Laboratory, who led the development of J- PEX said, "J-PEX has brought us to a new threshold in X-ray and EUV astronomy, which promises spectroscopic observations of unparalleled resolution and sensitivity."
The results from this experiment are important to understanding white dwarf stars and the history of our locality in the Milky Way Galaxy. The helium in their atmospheres gradually separates out from the hydrogen and sinks downwards, like a mixture of oil and later separating. Measuring how much helium is present throws light on how quickly this process proceeds. And high levels of ionised helium can only be the result of a nearby supernova exploding a few million years ago. Earlier observations of G191-B2B, which is 220 light years away, hinted at the presence of ionised helium. Now, early analysis of the new data from J-PEX shows there is definitely ionised helium in the interstellar gas though the researchers cannot yet say how much, if any, is in the atmosphere of the star itself.
Though no more flights of J-PEX are guaranteed at the present time, the team are hoping for at least two more flights, in about two to three years' time. They also hope to build, for a cost of about 25 million dollars, a version that could go on a satellite observatory.
The Low Energy Transmission Spectrometer on the Chandra X-ray Observatory, currently in orbit, has an effective area of 10 square centimetres and a resolving power of 500. J-PEX has about half the effective area but ten times the resolving power. J-PEX instrument alone cost around 3 million dollars. The total cost of Chandra was about 500 million dollars.
Further information is available from Dr Martin Barstow, Department of Physics and Astronomy, tel 0116 252 3492. Web site: www.star.le.ac.uk/edu/jpex/
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