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EUROPE'S X-RAY TELESCOPE XMM READY FOR LAUNCH

When on December 10, 1999, an Ariane 5 launches from Kourou in French Guayana, a lot is at stake for the European space branch. This flight not only is the the first commercial Ariane 5 launch: Flight 504 is to launch the biggest yet scientific payload built for the European Space Agency - the X-ray telescope XMM (X-ray Multi Mirror), a satellite weighing almost four tons and being more than ten meters long.

Only six months after a Space Shuttle has launched the American X-ray satellite Chandra into orbit, the scientists will have a second of the X-ray telescope available. Over the next ten years, XMM will be searching the universe for X-ray sources such as Black Holes, pulsars or exploding stars.

Like the colour of visible light is holding important information such as temperature, composition and movement of objects in space, X-rays are holding valuable chemical and physical information, but over a much wider energy range. However, since X-rays are shielded by the Earth's atmosphere, the telescopes have to be stationed on a spacecraft outside of the atmosphere.

First experiments with X-ray detectors on rockets were conducted in the fourties in the USA. Up until the seventies it was not possible to generate images of any kind from the X-rays. The main mission back then was prove the existence of such sources.

The first X-ray imaging satellite was the Einstein observatory which was launched in 1978. The X-ray astronomy reached a next leap with the launch of Rosat, an X-ray satellite built under the industrial lead of the German company Dornier Satellitensysteme. At the time of Rosat's launch, there were roughly 1000 X-ray sources known. Rosat, working up until the beginning of 1999, discovered almost 150000 sources. With XMM and its much finer energy resolution of 0,1 to 10 kev scientists are planning to find more than one million X-ray sources that are unknown today.

Like with Rosat, Dornier Satellitensysteme also had the industrial lead for ESA's X-ray Multi Mirror mission. A total of 35 European companies participated in the 230 million Euro program.

Along with the flight unit, two engineering and test models of the satellite were built. One of the test units was taken to ESA's test center in Noordwijk, the Netherlands, at the end of 1997 to verify the satellite's performance in conditions that can be expected during launch and once it is in space.

The specific configuration of the satellite and its enormous focal length of 7,5 meters was a challenge as far as the form stability under drastically changing temperature conditions was concerned. Most of the satellite's structure is made of carbon-fiber with an aluminium honeycomb core.

A X-ray telescope is built very differently to optical telescopes. X-rays are absorbed by the mirror material if the rays hit the mirror in an angle of more than one degree. Because of this, X-ray telescopes have barrel-shaped mirrors which allow to glance the rays off in a very shallow angle. By subtly angling the mirror along its length it is possible to focus them. In order to increase the gathering power, several of the mirrors are nested one inside another like Russian dolls.

Each of XMM's three telescope systems has 58 nested mirrors with a maximum diameter of 70 cm. The XMM mirrors are made of nickel and are gold plated on their reflecting surface. The mandrels were supplied by Carl Zeiss in Germany and the mirrors were made by Media Lario in Italy, a subsidiary of the German company Kayser-Threde.

The telescopes can gather 60 percent of the incoming X-rays. As a comparison: The American Chandra observatory, which has a single telescope system with four nested mirrors can focus only ten percent of the X-rays entering from the field of view. Even though the aperture area of an XMM telescope is only one third of Chandra's, its gathering power is about three times as great, or ten times with the three telescopes.

The images of the observed X-ray sources are generated by the so called EPIC systems (European Photon Imaging Camera). These are equipped with CCD detectors (charged coupled devices). The CCD cameras in XMM's three telescopes can distinguish X-rays of different wavelength by the energy delivered to a picture element by each X-ray particle, or photon.

For a more thorough analysis of the energy spectrum, two telescopes will divert part of the incoming beam with stacks of gratings that diffract the X-rays, fanning out the various wavelength on to a secondary CCD strip at a secondary focus, allowing the system to gather more detailed information about the X-rays' sources, such as temperature, densitiy and its chemical composition. The spectrometers are working in the engergy range between 0,35 and 2,5 keV. The astronomers are hoping to be able to achieve a spectroscopic examination of up to 30000 sources.

Additional to the X-ray telescopes, XMM is fitted with an optical monitor. Employed for the first time with an X-ray satellite, this combination will allow to simultanously monitor X-ray sources in the visible light range during observations. All six of XMM's science instruments will work simultanously. This is the main difference to Chandra which is alternating spectroscopic and image generating observation.

Nevertheless, the two new X-ray telescopes are not competitors but can supplement each other due to their specific qualities. While XMM has the larger gathering power, Chandra can see more details in the center of its focal area. Since Chandra has suffered from radiation damage, XMM will have to take over some of Chandras research spectrum.

In early September Chandra scientist found out that damage to the telescope's CCD chips occured every time the telescope traveled through the Earth's radiation belts which contain high concentrations of high-energy protons. Because of the damage, the satellite has lost its "color vision". The satellite controllers have since prevented more damage by using a motorized mechanism to move the CCD camera out of the focus point. Scientists are also hoping to repair the chips.

In order to protects XMM's CCD cameras, the satellite controllers will move filter wheels in front of the detectors during radiation belt passage.

Joint calibration observations are planned for XMM and Chandra for the second halve of 2000. This would allow to further improve the scientific performance of the two X-ray telescopes. Once both of them are in full operation, says a scientist from the XMM team, this will be a luxury that was unseen yet by the X-ray astronomers, and that will allow a lot of good research.

From page 50 of FLUG REVUE 1/2000