SIMBAD references

We report the results of our Chandra observations of six QSOs at z∼3 from the Palomer Transit Grism Survey. Our primary goal is to investigate the possible systematic change of α_ox between z>4 and z∼3, between which a rapid rise of luminous QSO number density with cosmic time is observed. The summed spectrum showed a power-law spectrum with a photon index of Γ~1.9, which is similar to other unabsorbed active galactic nuclei. Combining our z∼3 QSOs with X-ray observations of QSOs at z>4 from literature or archives, we find a correlation of α_ox with optical luminosity. This is consistent with the fact that the luminosity function slope of the luminous end of the X-ray-selected QSOs is steeper than that of optically selected QSOs. We discuss an upper limit to the redshift dependence of α_ox using a Monte Carlo simulation. Within the current statistical errors, including the derived limits on the redshift dependence of α_ox, we found that the behaviors of the X-ray and optically selected QSO number densities are consistent with each other.