SIMBAD references

We report on our XMM-Newton observations of the high-redshift quasars BR 2237-0607 (z=4.558) and BR 0351-1034 (z=4.351). We also report on XMM-Newton observations of 19 other z>4 objects available in the public archive of which 14 were detected. We find that the optical to X-ray spectral index α_ox is correlated with the luminosity density at 2500 Å but does not show a correlation with redshift, which is consistent with earlier results. Radio-loud quasars are brighter and have flatter X-ray slopes compared to radio-quiet quasars. There is some evidence for the jet-dominated sources to be intrinsically absorbed. The mean intrinsic 2-10 keV power-law slope of the 10 high-redshift radio-quiet quasars in our sample for which a spectral analysis can be performed is α_X=1.21±0.52 (ranging between 0.32 and 1.96), which is more like the α_X=1.19±0.10 found from the ASCA observations of low-redshift narrow-line Seyfert 1 galaxies (NLS1s) but different from the α_X=0.78±0.11 found for low-redshift broad-line Seyfert galaxies. The steep X-ray spectral index suggests high Eddington ratios L/L_Edd. These observations give credence to the hypothesis of Mathur that NLS1s are low-luminosity cousins of high-redshift quasars. Comparison with other results from the literature indicates that perhaps the most luminous quasars, from low to high redshift, have similarly steep X-ray spectra suggestive of high Eddington luminosities.