Ai, YanliYanliAiFabian, A. C.A. C.FabianFan, XiaohuiXiaohuiFanWalker, S. A.S. A.WalkerGHISELLINI, GabrieleGabrieleGHISELLINISbarrato, TulliaTulliaSbarratoDou, LimingLimingDouWang, FeigeFeigeWangWu, Xue-BingXue-BingWuFeng, LonglongLonglongFeng2020-08-272020-08-2720170035-8711http://hdl.handle.net/20.500.12386/26883A brief Chandra observation of the ultraluminous quasar SDSS J010013.02+280225.8 at redshift 6.326 showed it to be a relatively bright, soft X-ray source with a count rate of about 1 count ks<SUP>-1</SUP>. In this article, we present results for the quasar from a 65-ks XMM-Newton observation, which constrains its spectral shape well. The quasar is clearly detected with a total of ∼460 net counts in the 0.2-10 keV band. The spectrum is characterized by a simple power-law model with a photon index of Γ = 2.30^{+0.10}_{-0.10} and the intrinsic 2-10 keV luminosity is 3.14 × 10<SUP>45</SUP> erg s<SUP>-1</SUP>. The 1σ upper limit to any intrinsic absorption column density is N<SUB>H</SUB> = 6.07 × 10<SUP>22</SUP> cm<SUP>-2</SUP>. No significant iron emission lines were detected. We derive an X-ray-to-optical flux ratio α<SUB>ox</SUB> of -1.74 ± 0.01, consistent with the values found in other quasars of comparable ultraviolet luminosity. We did not detect significant flux variations either in the XMM-Newton exposure or between XMM-Newton and Chandra observations, which are separated by ∼8 months. The X-ray observation enables the bolometric luminosity to be calculated after modelling the spectral energy distribution: the accretion rate is found to be sub-Eddington.STAMPAenArticle10.1093/mnras/stx12312-s2.0-85023771867000406839900023https://academic.oup.com/mnras/article-abstract/470/2/1587/3906739?redirectedFrom=fulltext2017MNRAS.470.1587AFIS/05 - ASTRONOMIA E ASTROFISICAERC sectors::Physical Sciences and Engineering::PE9 Universe sciences: astro-physics/chemistry/biology; solar systems; stellar, galactic and extragalactic astronomy, planetary systems, cosmology, space science, instrumentation