The long-term X-ray variability properties of AGNs in the Lockman Hole region I. E. Papadakis, E. Chatzopoulos, D. Athanasiadis, A. Markowitz and I. Georgantopoulos A&A, 487 2 (2008) 475-483 Published online: 24 June 2008 DOI: 10.1051/0004-6361:200809572