XMM-Newton observations of the extremely X-ray luminous quasar CFHQS J142952+544717=SRGE J142952.1+544716 at redshift z=6.18
Abstract
We present results from a 20 ks XMM-Newton DDT observation of the radio-load quasar CFHQS J142952+544717 at z=6.18, whose extreme X-ray luminosity was recently revealed by the SRG/eROSITA telescope in the course of its first all-sky survey. The quasar has been confidently detected with a total of 1400 net counts in the 0.2-10 keV energy band (1.4 to 72 keV in the object's rest frame). Its measured spectrum is unusually soft and can be described by an absorbed power-law model with a photon index of = 2.50.2. There are no signs of a high-energy cutoff or reflected component, with an 90 % upper limit on the fluorescence iron Kα equivalent width of ≈ 290 eV and the corresponding upper limit on the iron K-edge absorption depth of 0.6. We have detected, at the > 95\% confidence level, an excess absorption above the Galactic value, corresponding to a column density NH= 32 × 1022 cm-2 of material located at z=6.18. The intrinsic luminosity of CFHQS J142952+544717 in the 1.4 to 72 keV energy band is found to be 5.5-0.6+0.8 × 1046 erg s-1. We did not detect statistically significant flux changes between two SRG scans and the XMM-Newton observation, spanning over 7.5 months, implying that the quasar remained at this extremely high luminosity level for at least a month in its rest frame. We put forward the hypothesis that the extreme X-ray properties of CFHQS J142952+544717 are associated with inverse Compton scattering of cosmic microwave background photons (at z=6.18) in its relativistic jets.