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

Abstract

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 $\gtrsim 9\sigma$). The X-ray spectrum is best-fitted by a power-law with photon index $\Gamma =1.9$ absorbed by a gas column density of ${\mathrm{N}}_{\mathrm{H}}=2.0_{-1.5}^{+2.0}\times 10^{23}\mathrm{c}{\mathrm{m}}^{-2}$. We measure an intrinsic luminosity at 2-10 keV and 10-40 keV equal to $\sim 1.5\times 10^{45}\mathrm{erg}{\mathrm{s}}^{-1}$, comparable with luminous local and intermediate-redshift quasar properties. Moreover, the X-ray to optical power-law slope value (${\alpha }_{\mathrm{OX}}=-1.76\pm 0.14$) of J1148 is consistent with the one found in quasars with similar rest-frame 2500 Å~luminosity ($L_{2500}\sim 10^{32}\mathrm{erg}{\mathrm{s}}^{-1}$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 ($Z\ge {\mathrm{Z}}_{\odot }$) molecular clouds distributed on $\sim$kpc scales in the host galaxy. Our analysis favors a black hole mass $M_{\mathrm{BH}}\sim 3\times 10^9{\mathrm{M}}_{\odot }$ and a molecular hydrogen mass $M_{{\mathrm{H}}_2}\sim 2\times 10^{10}{\mathrm{M}}_{\odot }$, in good agreement with estimates obtained from previous studies. We finally discuss strengths and limits of our analysis.

9 pages, 3 figures, 1 table, MNRAS in press

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