Context.The Lockman Hole field represents the sky area of lowest Galactic line-of-sight column density cm-2. It was observed by the XMM-Newton X-ray observatory in 18 pointings for a total of 1.16 Ms (raw EPIC pn observing time) constituting the deepest XMM-Newton exposure so far. After the removal of time intervals with high particle background, the final effective exposure was 637 ks.
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Brunner et al. (2007) studied this question.