Deep XMM-Newton Observations of an X-ray Weak Broad Absorption Line Quasar at z = 6.5

Abstract

We report X-ray observations of the most distant known gravitationally lensed quasar, J0439+1634 at z = 6.52, which is also a broad absorption line (BAL) quasar, using the XMM-Newton Observatory. With a 130 ks exposure, the quasar is significantly detected as a point source at the optical position with a total of ${358}_{-19}^{+19}$ net counts using the EPIC instrument. By fitting a power law plus Galactic absorption model to the observed spectra, we obtain a spectral slope of ${\rm{\Gamma }}={1.45}_{-0.09}^{+0.10}$ . The derived optical-to-X-ray spectral slope α _ox is $-{2.07}_{-0.01}^{+0.01}$ , suggesting that the X-ray emission of J0439+1634 is weaker by a factor of 18 than the expectation based on its 2500 Å luminosity and the average α _ox versus luminosity relationship. This is the first time that an X-ray weak BAL quasar at z > 6 has been observed spectroscopically. Its X-ray weakness is consistent with the properties of BAL quasars at lower redshift. By fitting a model including an intrinsic absorption component, we obtain intrinsic column densities of ${N}_{{\rm{H}}}={2.8}_{-0.6}^{+0.7}\times {10}^{23}\,{\mathrm{cm}}^{-2}$ and ${N}_{{\rm{H}}}={4.3}_{-1.5}^{+1.8}\times {10}^{23}\,{\mathrm{cm}}^{-2}$ , assuming a fixed Γ of 1.9 and a free Γ, respectively. The intrinsic rest-frame 2-10 keV luminosity is derived as (9.4-15.1) × 10⁴³ erg s^-1, after correcting for lensing magnification (μ = 51.3). The absorbed power-law model fitting indicates that J0439+1634 is the highest redshift obscured quasar with a direct measurement of the absorbing column density. The intrinsic high column density absorption can reduce the X-ray luminosity by a factor of 3-7, which also indicates that this quasar could be a candidate intrinsically X-ray weak quasar.