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tokens: The authors wish to thank Ettore Perozzi, Michael Freyberg, Frank Haberl, Matteo Guainazzi and the XMM–Newton calibration team for checking that the observed jitter has no instrumental origin. We thank Jan-Uwe Ness, Ignacio de la Calle and the rest of the XMM–Newton scheduling team for the enormous support that made the new XMM–Newton observations possible. We also thank Barbara De Marco, Thomas Tauris, Jonathan Ferreira, Pierre-Olivier Petrucci, Henri Gilles and Rosario Iaria for very useful discussion as well as the referee for the very useful comments that significantly improved the paper. This research has made use primarily of data obtained with XMM–Newton , an ESA science mission with instruments and contributions directly funded by ESA Member States and NASA , and on data obtained from the Chandra and ASCA Data Archives. The GC XMM–Newton monitoring project is supported by the Bundesministerium für Wirtschaft und Technologie/Deutsches Zentrum für Luft- und Raumfahrt (BMWI/DLR, FKZ 50 OR 1408 and FKZ 50 OR 1604) and the Max Planck Society . KD would like to thank MPE for the hospitality during his stay, and the German Academic Exchange Service (DAAD) for the fellowship which supported his participation in the project. TMD acknowledges support by the Spanish Ministry of Economy and Competitiveness (MINECO) under the grant AYA2013-42627 . LS acknowledges partial support from ASI INAF I/004/11/1 .