XMM-Newton Observations of Two High-Redshift Quasars: RX J1028-0844 and BR 0351-1034

We report the results of XMM-Newton observations of two high-redshift quasars, one radio-loud, RX J1028.6-0844 (z=4.276) and one radio-quiet, BR 0351-1034 (z=4.351). We find that the evidence for strong excess absorption toward RX J1028-0844 is marginal at best, contrary to previous claims. The superior sensitivity and broader, softer energy range of XMM-Newton (0.2-10 keV) allows better determination of spectral parameters than much deeper ASCA observations (0.8-7 keV). Our XMM-Newton observations call into question several other ASCA results of strong absorption toward high-redshift radio-loud quasars. RX J1028.6-0844 occupies the same parameter space in broadband spectral properties as the low-redshift BL Lac objects, showing no obvious evolution with redshift. The radio-quiet quasar BR 0351-1024 became fainter between ROSAT and XMM observations by a factor of at least 5, but with the present data we cannot determine whether there is an associated spectral change. These observations do not support previous claims of weaker X-ray emission from high-redshift, radio-quiet quasars. The soft X-ray spectral slope required to reconstruct the ROSAT PSPC hardness ratio of BR 0351-1034 is about α_X=3.5, the steepest X-ray slope ever observed in a high-redshift quasar and similar to that of low-redshift narrow-line Seyfert 1 galaxies.

Based on observations with XMM-Newton, an ESA science mission with instruments and contributions directly funded by ESA member states and NASA.