XMM observations of the high-redshift quasar RXJ1028.6 - 0844 at z= 4.276: soft X-ray spectral flattening

Abstract

We present results from a new XMM-Newton observation of the high-redshift quasar RXJ1028.6 - 0844 at a redshift of 4.276. The soft X-ray spectral flattening, as reported by a previous study with ASCA, 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 is 2.1(^+0.4_-0.3) ×10²² cm^-2 for cold matter, and higher for ionized gas. The spectral flattening shows remarkable similarity with that of two similar object, namely GB1428 + 4217 and PMNJ0525 - 3343. The results improve upon those obtained from a previous short-exposure observation for RXJ1028.6 - 0844 with XMM-Newton. 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 time-scales of about one year. A tentative excess emission feature in the rest-frame 5-10keV band is suggested, which is similar to that marginally suggested for GB1428 + 4217.