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 $\Gamma =1.45_{-0.09}^{+0.10}$. The derived optical-to-X-ray spectral slope ${\alpha }_{\mathrm{ox}}$ 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 Angstrom luminosity and the average ${\alpha }_{\mathrm{ox}}$ vs. 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 $N_{\mathrm{H}}=2.8_{-0.6}^{+0.7}\times 10^{23}{\mathrm{cm}}^{-2}$ and $N_{\mathrm{H}}=4.3_{-1.5}^{+1.8}\times 10^{23}{\mathrm{cm}}^{-2}$, assuming a fixed $\Gamma$ of 1.9 and a free $\Gamma$, respectively. The intrinsic rest-frame 2--10 keV luminosity is derived as $(9.4-15.1)\times 10^{43}{\mathrm{erg}\mathrm{s}}^{-1}$, after correcting for lensing magnification ($\mu =51.3$). 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 of intrinsically X-ray weak quasar.

8 pages, 3 figures, 1 table; Accepted for publication in ApJL

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• High Resolution HST Imaging Survey of Local Star-Forming Galaxies I: Spatially-Resolved Obscured Star Formation with H$\alpha$ and Paschen-$\beta$ Recombination Lines