The WISSH quasars project - III. X-ray properties of hyper-luminous quasars S. Martocchia, E. Piconcelli, L. Zappacosta, F. Duras, G. Vietri, C. Vignali, S. Bianchi, M. Bischetti, A. Bongiorno, M. Brusa et al. (7 more) A&A, 608 (2017) A51 Published online: 06 December 2017 DOI: 10.1051/0004-6361/201731314