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The Sky Seen from Europe : X Marks the Spot

After 7 years, 3491 observations that uncovered 192,000 individual radiation sources, the world’s largest catalogue of cosmic X-ray sources is now complete. Called 2XMM,1  the catalogue represents years of work by dozens of European astronomers dedicated to organizing data gathered by the European Space Agency’s XMM-Newton satellite.
“Our goal was to create a tool to shed light on the new frontiers of astrophysics,” explains Christian Motch. A Strasbourg Observatory2 astronomer, Motch was part of a consortium that included researchers from France, Great Britain, Germany, Italy, and Spain. “This is the largest such catalogue in the world,” says Motch. “It really has no equivalent.”
Many galactic and extragalactic objects emit X-rays, from distant active galactic nuclei to nearby stars (including our sun), and even comets.

Spot X

© Prof M. Watson, XMM-Newton SSC, University of Leicester, UK/Graphical concept courtesy A. Comastry

Illustration comparing different X-ray surveys in terms of total area covered on the sky and the depth of the detected sources.

The kind of radiation released can be used to identify the object emitting it, even if it is not detectable on the visible spectrum. For each of the 192,000 sources, scientists recorded not only their position in the sky, but also signal strength, variability, and “hardness” or “softness,” the X-ray’s mean wavelength or “color.” X-rays are generated by hot energetic sources. They are the most efficient method of identifying sources hidden behind absorbing materials or gathering information about rare objects.
It is this feature that makes the 2XMM catalogue so valuable. Far from being a simple list of space phenomena, the catalogue is now a massive searchable database.
“It has been indexed with other catalogues and astronomical data, such as that contained in Strasbourg’s Astronomical Data Center,” says Laurent Michel, a researcher from the Strasbourg Observatory who supervised much of the construction of the database.
This flexible, online tool for astronomers may even help solve an old astronomical mystery, namely the source of the diffuse high energy
X-ray emission that encompasses the whole sky. “When you look at X-ray diffuse emission, it’s very hot, but its sources appear to be relatively cold,” says Motch. “There is a population of hot sources that are shielded by local matter–and the XMM-Newton satellite is helping us find more and more of these buried X-ray sources,” he enthuses.

Mark Reynolds

Notes :

1. The 2XMM catalogue is available online at: View web site and
View web site
2. CNRS / Université Louis Pasteur.

Contacts :

Christian Motch
Observatoire astronomique de Strasbourg.


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