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tokens: We thank the anonymous referee for the helpful comments that helped us to improve the manuscript. 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 the USA (NASA). M.D. thanks Paola Grandi, Daniele Dallacasa for useful discussions. M.D. and M.C. aknowledge financial support from the Italian Space Agency under grant ASI-INAF I/037/12/0 and extended PRIN INAF 2012. GL acknowledges financial support from the CIG grant “eEASY” No. 321913, from ASI-INAF 2014-045-R.0 and ASI/INAF I/037/12/0–011/13 grants. G.P. thanks the DLR: the Bundesministerium für Wirtschaft und Technologie/Deutsche Zentrum für Luft-und Raumfahrt (BMWI/DLR, FKZ 50 OR 1408).