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 $\gtrsim9\sigma$). The X-ray spectrum is best-fitted by a power-law with photon index $\Gamma=1.9$ absorbed by a gas column density of $\rm N_{ \rm H}=2.0^{ +2.0}_{ -1.5}\times10^{ 23}\,\rm cm^{ -2}$. We measure an intrinsic luminosity at 2-10 keV and 10-40 keV equal to $\sim 1.5\times 10^{ 45}~\rm erg~s^{ -1}$, comparable with luminous local and intermediate-redshift quasar properties. Moreover, the X-ray to optical power-law slope value ($\alpha_{ \rm OX}=-1.76\pm 0.14$) of J1148 is consistent with the one found in quasars with similar rest-frame 2500 \AA ~luminosity ($L_{ \rm 2500}\sim 10^{ 32}~\rm erg~s^{ -1}$\AA$^{ -1}$). 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 ($Z\geq \rm Z_{ \odot}$) molecular clouds distributed on $\sim$kpc scales in the host galaxy. Our analysis favors a black hole mass $M_{ \rm BH} \sim 3\times 10^9 \rm M_\odot$ and a molecular hydrogen mass $M_{ \rm H_2}\sim 2\times 10^{ 10} \rm M_\odot$, in good agreement with estimates obtained from previous studies. We finally discuss strengths and limits of our analysis.
Categories: astro-ph.GA astro-ph.CO