2014A&A...563A..46M

We probe the emission mechanism of the accreting super massive black holes in the high redshift Universe and, to do this, we study the X-ray spectrum of ULAS1120+064, the highest redshift quasar detected so far at z=7.085, which has been deeply observed (340ks) by XMM-Newton. Despite the long integration time, the spectral analysis is limited by the poor statistics, with only 150 source counts being detected. We measured the spectrum in the 2-80keV rest-frame (0.3-10keV observed) energy band. Assuming a simple power law model, we find a photon index of 2.0±0.3 and a luminosity of 6.7±0.3x10⁴⁴erg/s in the 2-10keV band, while the intrinsic absorbing column can only be loosely constrained (N_H<10²³cm^–2). Combining our measure with published data, we calculate that the X-ray-to-optical spectral index α_OX is 1.8±0.1, in agreement with the α_OX-UV luminosity correlation that is valid for lower redshift quasars. This is the second time that a z>6 quasar has been investigated through a deep X-ray observation. In agreement with previous studies of z∼6 AGN samples, we do not find any hint of evolution in the broadband energy distribution. Indeed from our dataset, ULAS 1120+0641 is indistinguishable from the population of optically bright quasar at lower redshift.