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We present the results from a $\sim500$ ks Chandra observation of the$z=6.31$ QSO SDSS J1030+0524. This is the deepest X-ray observation to date ofa $z\sim6$ QSO. The QSO is detected with a total of 125 net counts in the full($0.5-7$ keV) band and its spectrum can be modeled by a single power-law modelwith photon index of $\Gamma = 1.81 \pm 0.18$ and full band flux of$f=3.95\times 10^{-15}$ erg s$^{-1}$ cm$^{-2}$. When compared with the dataobtained by XMM-Newton in 2003, our Chandra observation in 2017 shows a harder($\Delta \Gamma \approx -0.6$) spectrum and a 2.5 times fainter flux. Such avariation, in a timespan of $\sim2$ yrs rest-frame, is unexpected for such aluminous QSO powered by a $> 10^9 \: M_{\odot}$ black hole. The observed sourcehardening and weakening could be related to an intrinsic variation in theaccretion rate. However, the limited photon statistics does not allow us todiscriminate between an intrinsic luminosity and spectral change, and anabsorption event produced by an intervening gas cloud along the line of sight.We also report the discovery of diffuse X-ray emission that extends for 30"x20"southward the QSO with a signal-to-noise ratio of $\sim$6, hardness ratio of$HR=0.03_{-0.25}^{+0.20}$, and soft band flux of $f_{0.5-2 \: keV}=1.1_{-0.3}^{+0.3} \times 10^{-15}$ erg s$^{-1}$ cm$^{-2}$, that is notassociated to a group or cluster of galaxies. We discuss two possibleexplanations for the extended emission, which may be either associated with theradio lobe of a nearby, foreground radio galaxy (at $z \approx 1-2$), orascribed to the feedback from the QSO itself acting on its surroundingenvironment, as proposed by simulations of early black hole formation.

The 500 ks Chandra observation of the z = 6.31 QSO SDSS J1030+0524