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 water 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.
Notes for Editors
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:
Dr Martin Barstow
University of Leicester
(+44) (0)116 252 3493
mab@star.le.ac.uk
Dr Raymond Cruddace
E. O. Hulbert Center for Space Research
US Naval Research Laboratory, Code 7620
4555 Overlook Ave SW
Washington DC 20375-5352, USA
Phone (+1) 202 767 2344
Fax (+1) 202 767 0497
e-mail cruddace@xip.nrl.navy.mil
RELATED LINKS
* The J-PEX project page
http://www.star.le.ac.uk/edu/jpex/
