An XMM-Newton detection of the z=5.80 X-ray-weak quasar SDSSp J104433.04012502.2

We report on an XMM-Newton observation of the most distant known quasar, SD SSp J104433.04-012502.2, at z = 5.80. We have detected this quasar with hig h significance in the rest-frame 3.4-13.6 keV band, making it the most dist ant cosmic object detected in X-rays; 32 +/- 9 counts were collected. SDSSp J104433.04-012502.2 is notably X-ray-weak relative to other luminous, opti cally selected quasars, with alpha (ox) = -1.91 +/- 0.05 and a 3.4-13.6 keV luminosity of approximate to1.8 x 10(44) ergs s(-1). The most likely reaso n for a its X-ray weakness is heavy absorption with N-H greater than or sim ilar to 1024 cm(-2), as is seen in some broad absorption line quasars and r elated objects; we discuss this and other possibilities. High-quality spect roscopy from 0.95-1.10 mum to search for blueshifted C IV absorption may el ucidate the origin of the X-ray weakness.