tokens: F.T. thanks A. Bolatto and A. Marscher for useful comments. F.T. and C.S.R. acknowledges support for this work by the National Aeronautics and Space Administration (NASA) through Chandra Award Number GO4-15103A issued by the Chandra X-Ray Observatory Center, which is operated by the Smithsonian Astrophysical Observatory for and on behalf of NASA under contract NAS8-03060. F.T. acknowledges partial support by the Programma per Giovani Ricercatori-anno 2014 “Rita Levi Montalcini.” E.B. is supported by the European Unions Horizon 2020 research and innovation programme under the Marie Sklodowska Curie grant agreement No. 655324, and by the ICORE program of the Planning and Budgeting Committee (grant No. 1937/12).
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