To demonstrate the remarkable ability of XMM-Newton to discover faint X-ray sources, a series of observations is being made in the area known as the ‘Lockman hole’.
In the Ursa Major northern hemisphere constellation — the third-largest constellation better known as Great Bear or as the Big Dipper — the Lockman hole region is one of the best studied fields in X-ray astronomy. In this direction absorbing material such as dust and galactic hydrogen is at its most sparse, and through this ‘hole’ sensitive searches for extragalactic objects hold great promise and have already brought a much greater understanding of X-ray background radiation.
XMM-Newton’s observations of the ‘Lockman Hole’ will eventually
accumulate nearly 2 days worth of exposure time, but as a foretaste of the expected results, the first 8 hours of data have been processed. The first two images show the results from each of the XMM EPIC Imaging cameras with 20,000 second exposures (5.5 hours).
While the ROSAT observatory (launched in 1990) previously imaged this field with great sensitivity, it was only able to detect low energy X-rays, whilst most of the flux from distant faint quasars and active galaxies is expected to arrive at higher X-ray energies.
The Italian-Dutch BeppoSAX observatory had previously offered one of the best high-energy X-ray views of the Lockman Hole (image 3). Exposure duration was 112,000 seconds. But the spatial resolution and efficiency of BeppoSAX is dramatically overtaken by the superb quality of XMM- Newton’s mirrors and detectors.
The BeppoSAX field of view is 1 degree in diameter, about twice that of XMM-Newton. Only the brightest of the XMM-Newton sources are visible in its image — in particular the ‘triangle’ of sources visible just to the left of centre are clearly detected in the XMM-Newton images.
With only 1/5 the observation time, XMM-Newton therefore detects many more sources and with much better precision. However, BeppoSAX, launched in 1996, has other unique capabilities — such as the ability to observe X-rays with energies from the lowest observable by XMM-Newton to over 10 times higher — that make it a powerful observatory in its own right.
Another glimpse of the power of XMM-Newton comes by examining what happens when the energies of the X-rays are separated: two further pictures from the Lockman hole observations show the response in the low energy ROSAT band (0,1KeV – 2.5KeV) and the energies above the ROSAT range.
While many sources are found at the same position in the sky in these two pictures, there are subtle differences. For example above centre and to the right, some of the sources bright in the ROSAT band (red and yellow) are hardly visible at higher energies. Conversely, one or two objects are brighter (red) in the high energy picture but more difficult to see at the lowest energies.
Detecting variations such as these will be vital for XMM-Newton’s work in distinguishing a suspected population of massive black holes, cloaked in a thick X-ray absorbing ring of gas and dust, lurking in young galaxies far out in the early Universe.
ESA’s X-ray space observatory is currently calibrating and verifying the performance of its science instruments before regular science observations start in June.
[Image 1:]
Lockman hole viewed by EPIC MOS1.
[Image 2:]
Lockman hole viewed by EPIC PN.
[Image 3:]
The view of the Lockman Hole obtained with the MECS instrument on BeppoSAX. The "cut-outs" at the upper-left and lower-right of the image are due to the removal of instrumental events.
[Image 4:]
Low energy view of Lockman hole by EPIC PN.
Image 5:]
Same EPIC PN view as Loew energy image, this time showing higher energy sources.