X-ray spectroscopy of the z = 6.4 quasar SDSS J1148+5251

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 greater than or similar to 9 sigma). The X-ray spectrum is best fitted by a power law with photon index Gamma = 1.9 absorbed by a gas column density of N-H = 2.0(-1.5)(+2.0) x 1023 cm(-2). We measure an intrinsic luminosity at 2-10 and 10-40 keV equal to similar to 1.5 x 10(45) erg s(-1), comparable with luminous local and intermediate-redshift quasar properties. Moreover, the X-ray to optical power-law slope value (alpha(OX) = -1.76 +/- 0.14) of J1148 is consistent with the one found in quasars with similar rest-frame 2500 angstrom luminosity (L-2500 similar to 10(32) erg s(-1) angstrom(-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(circle dot)) molecular clouds (MC) distributed on similar to kpc scales in the host galaxy. Our analysis favours a black hole mass MBH similar to 3 x 10(9) M-circle dot and a molecular hydrogen mass M-H2 similar to 2 x 10(10) M-circle dot, in good agreement with estimates obtained from previous studies. We finally discuss strengths and limits of our analysis.