XMM-Newton observation of the ultraluminous quasar SDSS J010013.02+280225.8 at redshift 6.326

A 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^-1. 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 × 10⁴⁵ erg s^-1. The 1σ upper limit to any intrinsic absorption column density is N_H = 6.07 × 10²² cm^-2. No significant iron emission lines were detected. We derive an X-rayto- optical flux ratio α_ox 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.