XMMobservations of the high-redshift quasar RX J1028.6– 0844 at.

We present results from a newXMM–Newtonobservation of the high-redshift quasar RX J1028.6– 0844 at a redshift of 4.276. The soft X-ray spectral flattening, as reported by a previous study withASCA, is confirmed to be present, with, however, a reduced column density when modelled by absorption. The inferred column density for absorption intrinsic to the quasar isfor cold matter, and higher for ionized gas. The spectral flattening shows remarkable similarity with that of two similar object, namely GB 1428+ 4217 and PMN J0525− 3343. The results improve upon those obtained from a previous short-exposure observation for RX J1028.6– 0844 withXMM–Newton. A comparative study of the twoXMM–Newtonobservations reveals a change in the power-law photon index fromto 1.5 on time-scales of about one year. A tentative excess emission feature in the rest-frame 5–10 keV band is suggested, which is similar to that marginally suggested for GB 1428+ 4217. [ABSTRACT FROM AUTHOR]