XMMobservations of the high-redshift quasar RX J1028.6 - 0844 atz= 4.276: soft X-ray spectral flattening

We present results from a new XMM-Newton observation 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 study with ASCA previously (Yuan et al. 2000, ApJ 545, 625), is confirmed to be present, however, with reduced column density when modelled by absorption.The inferred column density for absorption intrinsic to the quasar is 2.1( +0.4 -0.3 ) × 10 22 cm -2 for cold matter, and higher for ionised gas.The spectral flattening shows remarkable similarity with those of two similar objects, GB 1428+4217 (Worsley et al. 2004, MN-RAS 350, L67) and PMN J0525-3343 (Worsley et al. 2004, MNRAS 350, 207).The results improve upon those obtained from a previous short-exposure observation for RX J1028.6-0844 with XMM-Newton (Grupe et al. 2004, AJ 127, 1).A comparative study of the two XMM-Newton observations reveals a change in the power-law photon index from Γ ≃1.3 to 1.5 on timescales 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.