SIMBAD references

We observed 17 optically selected, radio-quiet, high-redshift quasars with the Chandra ACIS and detected 16 of them. The quasars have redshifts between 3.70 and 6.28 and include the highest-redshift quasars known. When compared with low-redshift quasars observed with ROSAT, these high-redshift quasars are significantly more X-ray-quiet. We also find that the X-ray spectral index of the high-redshift objects is flatter than the average at lower redshift. These trends confirm the predictions of models in which the accretion flow is described by a cold, optically thick accretion disk surrounded by a hot, optically thin corona, provided the viscosity parameter α≥0.02. The high-redshift quasars have supermassive black holes, with masses of ∼10¹⁰ M_☉, and are accreting material at ∼0.1 times the Eddington limit. We detect 10 X-ray photons from the z=6.28 quasar SDSS 1030+0524, which might have a Gunn-Peterson trough and be near the redshift of reionization of the intergalactic medium. The X-ray data place an upper limit on the optical depth of the intergalactic medium, τ(IGM)<10⁶, compared to the lower limit from the spectrum of Lyα and Lyβ, which implies τ(IGM)>20.