tokens: The authors thank the anonymous referee for helpful comments that significantly improved the paper. The authors also thank Phil Uttley for useful and constructive comments. This work is based on observations obtained with XMM-Newton , an ESA science mission with instruments and contributions directly funded by ESA Member States and NASA. G.P. acknowledges support via the Bundesministerium für Wirtschaft und Technologie/Deutsches Zentrum für Luft und Raumfahrt (BMWI/DLR, FKZ 50 OR 1408) and the Max Planck Society. T.M.-D. acknowledges support by the Spanish Ministerio de Economía y competitividad (MINECO) under grant AYA2013-42627. Facilities: XMM (EPIC pn) , RXTE (PCA).
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