X-ray observation of ULAS J1120+0641, the most distant quasar atz= 7.08

Abstract

We aim at probing the emission mechanism of the accreting super massive black holes in the high redshift Universe. We study the X-ray spectrum of ULAS1120+0641, the highest redshift quasar detected so far at z=7.085, which has been deeply observed (340 ks) 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 2-80 keV rest-frame (0.3-10 keV 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+/-0.3 10^44 erg/s in the 2-10 keV band, while the intrinsic absorbing column can be only loosely constrained (NH< 1E23 cm^-2). Combining our data with published data we calculate that the X-ray-to-optical spectral index alpha_OX is1.8+/-0.1, in agreement with the alpha_OX-UV luminosity correlation valid for lower redshift quasars. We expanded to high energies the coverage of the spectral energy distribution of ULAS1120+0641. 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.