tokens: E.B. thanks the New York University at Abu Dhabi for the kind hospitality during part of this work and Milvia Capalbi for her precious support during the analysis of some XRT observations. We thank Phil Evans for useful discussions. A.P. is supported by a Juan de la Cierva fellowship, and acknowledges grants AYA2012-39303, SGR2009-811, and iLINK2011-0303. P.R. acknowledges contract ASI-INAF I/004/11/0. We thank N. Schartel and the XMM-Newton team for having promptly performed the ToO observation analyzed in this paper. We are indebted to the Swift PI and operations team for the continuous support during the monitoring campaign of X-ray transients.
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