NGC 2516 viewed by EPIC MOS. Photo: ESA

Calibration of XMM-Newton's science instruments is continuing at a steady pace in view of the start of the operational phase of the mission next month. Since the end of commissioning, ESA's new X-ray space observatory has been viewing an average of one or more calibration target every day. One such target has been NGC 2516, a young open cluster in the southern hemisphere.

At a distance of 1.300 light years from Earth, the cluster is one of several situated in the Carina constellation which contains the second-brightest star in the sky, Canopus (Alpha Carinae).

Open clusters are groups of stars formed together in the spiral arms of a galaxy and held together by mutual gravitational attraction. Usually irregular in shape, they can contain from a few dozen to several hundred relatively young stars in a volume up to 50 light years across. Although more loosely packed than globular clusters, their central star density can be 10 thousand times that in the Sun's neighbourhood. The youngest stars are often surrounded by traces of the cosmic gas and dust from which they were formed. An average open cluster spreads most of its member stars along its path after several hundred million years.

Open cluster NGC 2516, and even its constituent stars, is clearly visible to the naked eye. It was discovered by Abbe Nicholas Louis de la Caille in 1751-52 on the occasion of a visit to South Africa. Its multi-coloured appearance has led it to be called 'the southern jewelbox', although other open clusters (such as NGC 4755) have also been described the same way. The French astronomer would no doubt be amazed at the clarity of the view obtained with XMM-Newton!

The XMM-Newton observation of NGC 2516 took place on April 6 using all three EPIC cameras. The image from the MOS instruments is the result of combining the 22 000 second (5.5 hours) exposures of both MOS1 and MOS2 cameras. The exposure when using the EPIC-PN camera lasted half as long, some 10 000 seconds, hence the lesser number of sources that are visible. The pictures show how well these X-ray imaging instruments exploit the enormous light-gathering potential of the telescope mirrors to detect some 60 stars.

XMM-Newton. Photo: ESA

The open cluster was chosen as a calibration target for XMM-Newton because the locations of each of these stars is already determined with a very high precision by other ground-based and space-borne observatories, including NASA's Chandra. The stars in XMM-Newton's field of view are distributed over the camera detector areas and can be located to an accuracy of one pixel.

The data is used to pursue the detailed calibration of the EPIC instruments, and to line up the EPIC cameras with the Optical Monitor. With this knowledge, X-ray sources detected in the future will be located with a precision of one arcsecond - equivalent to being able to read the text on this page at a distance of one kilometre!

John Pye, Project Manager of XMM-Newton's Science Survey Centre has been studying open clusters for many years. "XMM-Newton gives us a much deeper view of NGC 2516 compared to previous observations, for instance by ROSAT. We can better interpret the energies that have been detected, allowing us to determine the extreme temperatures of the star coronae. Such observations will help answer the question of whether all open clusters of the same age resemble each other or may have different aspects."