XEUS: A new concept in exploring X-ray Universe
Posted: April 10, 2001

XEUS mission logo. Photo: ESA
Which came first, the chicken or the egg? Or, in the case of the Universe, was it massive black holes or galaxies?

To answer this question by studying black holes in the early Universe requires an extremely sensitive X-ray telescope -- one that is even more sensitive than the European Space Agency's XMM-Newton Observatory, which has been operational since 1999.

Even though XMM-Newton has an unprecedented capacity for collecting X-rays, it is not sensitive enough to pick out the individual sources of the most distant X-ray emissions. To do this, ESA, together with industry, universities and research institutes in Europe, Japan and the United States has started to study a revolutionary new mission called XEUS -- the X-ray Evolving Universe Spectroscopy mission.

The scheme is a scientific by-product of ESA's participation in the International Space Station (ISS), which opens the way for researchers in Europe to use the ISS for scientific research. As a result, senior European space scientists have advised the agency to think about a major X-ray astronomy facility associated with the ISS.

XEUS, a mission in search of the first objects ever created, is likely to become an important component in ESA's expanding role in the ISS.

An overview of this exciting X-ray observatory was presented at the UK National Astronomy Meeting in Cambridge last Thursday by Dr. Arvind Parmar (European Space Agency).

Black holes and the X-ray Universe
One of the current hot topics in astronomy is how, and when, the first galaxies formed. Many astronomers now believe that most galaxies have massive black holes at their centres. It is also possible that the presence of these black holes may be necessary to spark the formation of the first stars and thus the complexity that led to intelligent life.

Black holes cannot be observed directly, but by studying the X-rays produced by material falling into a black hole, astronomers can measure their mass and distance, and even how fast they are rotating. They do this by spreading the X-rays into a spectrum (rather like the colours of the rainbow). This allows X-rays characteristic of iron atoms with a sharply defined energy to be identified.

When seen from afar, this energy is altered by the conditions near the black hole as the material swirls rapidly around in an accretion disk. At any moment, some of the iron atoms are rushing away from us, so their apparent energy is decreased, while others are moving towards us, and their apparent energy is increased due to the well-known Doppler effect.

Another effect, known as "gravitational red-shift" causes the X-rays to lose energy as they climb out of the region of very strong gravity near a black hole.

This is not the end of the story of the iron signature. For very distant black holes, the expansion of the Universe shifts the whole pattern into the low-energy part of the spectrum.

So by studying the shape and energy of the iron emission from some of the first black holes, astronomers hope to learn more about the black holes themselves, as well as the composition and properties of the material falling into them.

XEUS and International Space Station
XEUS is a radically different approach compared to previous X-ray observatories. It will consist of two spacecraft -- one carrying a 4.5 m diameter X-ray mirror, and the other a detector -- separated by the 50 m focal length of the optics. The detector spacecraft will use solar-electric propulsion to continuously follow the focal spot of the optics.

A single Ariane 5 launch will place XEUS-1 into a 600 km orbit. In this precursor mission, XEUS-1 will prepare for the sensitive observations of the fully-grown XEUS. After 4 years the mirror spacecraft will dock to the ISS to experience the growth required in order to study the distant Universe.

Illustration of the XEUS deployment. Photo: DASA
Using the European Robotic Arm on the space station, additional mirror segments will be added to the mirror spacecraft, building the world's largest and most powerful X-ray telescope. With an eventual diameter of 10 m, the effective area of the optics will be increased by a factor of 5, reaching 30 sq. m -- comparable in size to the largest ground-based optical telescopes.

The servicing of XEUS at the ISS will be based on the currently foreseen capabilities of the space station and will depend heavily on robotics and the presence of astronauts. Having completed its mission, the detector spacecraft is de-orbited, to be replaced later by the XEUS-2 detector spacecraft, which is equipped with the latest instrument technology.

Due to the phased approach of the mission concept, the development and investment costs are spread over a longer time than for a normal space project. An ambitious mission with powerful challenges for both the scientific community and industry, XEUS promises to become a truly global mission with international collaboration at a significant level, involving many of the partners that have teamed up to build the ISS.

Peering deeper into space than any previous X-ray mission, XEUS-2 will look back in time to when the Universe was only a few percent of its current age, observing and measuring the properties of the first massive objects to develop after the Big Bang.