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 NH=2.0−1.5+2.0 %. We measure an intrinsic luminosity at 2–10 and 10–40 keV equal to ∼ 1.5 × 1045 erg s−1, comparable with luminous local and intermediate-redshift quasar properties. Moreover, the X-ray to optical power-law slope value (αOX = −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 MBH ∼ 3 × 109 M⊙ and a molecular hydrogen mass MH2∼2×1010 M⊙, in good agreement with estimates obtained from previous studies. We finally discuss strengths and limits of our analysis.RM acknowledges support from the ERC Advanced Grant 695671 ‘QUENCH’ and the Science and Technology Facilities Council (STFC)
