XMM–Newton spectroscopy of high‐redshift quasars

We present XMM–Newton X‐ray spectra and optical photometry of four high‐redshift ( z = 2.96–3.77) quasars, [HB89] 0438−436, [HB89] 2000−330, [SP89] 1107+487 and RX J122135.6+280613; of these four objects, the first two are radio‐loud and the last two are radio‐quiet. Model fits require only a power law with Galactic absorption in each case; additional intrinsic absorption is also needed for [HB89] 0438−436 and RX J122135.6+280613. The spectra are hard ( Γ∼ 1.7 for [HB89] 0438−436, [HB89] 2000−330 and ∼1.4 for RX J122135.6+280613), with the exception of [SP89] 1107+487, which is softer (Γ∼ 2.0) ; the combined Galactic and intrinsic absorption of lower‐energy X‐rays in the last of the sources is much less significant than in the other three. The two intrinsically unabsorbed sources have greater optical fluxes relative to the X‐ray contributions at the observed energies. While there is no need to include reflection or iron line components in the models, our derived upper limits (99 per cent confidence) on these parameters are not stringent; the absence of these features, if confirmed, may be explained in terms of the high power‐law contribution and/or a potentially lower albedo as a result of the low disc temperature. However, we note that the power‐law spectrum can be produced via mechanisms other than the Comptonization of accretion disc emission by a corona; given that all four of these quasars are radio sources at some level, we should also consider the possibility that the X‐ray emission originates, at least partially, in a jet.