Chandra Measurement of the X-Ray Spectrum of a Quasar at z = 5.99

We report the first measurement of the X-ray spectrum of the z = 5.99 quasar SDSSp J130608.26+035626.3 from a 120 ks observation by the Chandra ACIS-S instrument. Between 0.5 and 7 keV, corresponding to 3.5-49 keV in the quasar rest frame, we find an energy index of 0.86 ± 0.2, consistent with the typical indices found for radio-quiet quasars at lower redshifts and inconsistent with the index required to match the diffuse X-ray background. We have a weak indication of a redshifted Fe K line. In comparing the counting rate between an earlier short observation and the longer observation reported here, we find evidence for source variability at the 99.9% confidence level. We note that other nearby X-ray sources would bias the measured α ox = 1.70 by -0.09 if the X-ray flux were determined from within a 60'' extraction circle. Our results for the energy index and the α ox are consistent with no strong evolution in the active galactic nucleus emission mechanism with redshift out to z ≈ 6 and therefore with the picture that massive black holes have already formed less than 1 Gyr after the big bang.