X-ray spectroscopy of the $\textit{z}$=6.4 quasar SDSS J1148+5251

Abstract

We present the 78 ks $\textit{Chandra}$ observations of the $\textit{z}$ = 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$^{+2.0}{−1.5}$ %. We measure an intrinsic luminosity at 2–10 and 10–40 keV equal to ∼ 1.5 × 10$^{45}$ 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 ($\textit{L}$2500 ∼ 10$^{32}$ erg s$^{-1}$ Å$^{-1}$). Then we use $\textit{Chandra}$ 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 × 10$^{9}$ M⊙ and a molecular hydrogen mass M$_{H2}$∼2×10$^{10}$ M⊙, in good agreement with estimates obtained from previous studies. We finally discuss strengths and limits of our analysis.