Chandra measurement of the X-ray spectrum of a quasar at z=5.99

Abstract

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 alpha_{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 alpha_{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.