2017MNRAS.467.3590G

We present the 78 ks Chandra observations of the z = 6.4 quasar SDSS J1148+5251. The source is clearly detected in the energy range 0.3-7 keV with 42 counts (with a significance >=9σ). The X-ray spectrum is best fitted by a power law with photon index Γ = 1.9 absorbed by a gas column density of N_ H_=2.0+2.0^_–1.5×10²³  cm^–2. We measure an intrinsic luminosity at 2-10 and 10-40 keV equal to ∼ 1.5 x 10⁴⁵ erg s^- 1^, comparable with luminous local and intermediate-redshift quasar properties. Moreover, the X-ray to optical power-law slope value (α_OX = -1.76 ± 0.14) of J1148 is consistent with the one found in quasars with similar rest-frame 2500 Å luminosity (L₂₅₀₀ ∼ 10³² erg s^- 1^ Å^- 1^). Then we use Chandra data to test a physically motivated model that computes the intrinsic X-ray flux emitted by a quasar starting from the properties of the powering black hole and assuming that X-ray emission is attenuated by intervening, metal-rich (Z >= Z_☉) molecular clouds (MC) distributed on ∼kpc scales in the host galaxy. Our analysis favours a black hole mass M_BH ∼ 3 x 10⁹ M_☉ and a molecular hydrogen mass M_ H₂∼2×10¹⁰ M_☉, in good agreement with estimates obtained from previous studies. We finally discuss strengths and limits of our analysis.