XMM-Newtonobservations of GB B1428+4217: confirmation of intrinsic soft X-ray absorption

We report the results of XMM–Newton observations of the X-ray bright, radio-loud blazar GB B1428+4217 at a redshift of z = 4.72. We confirm the presence of soft X-ray spectral flattening at energies ≲0.7 keV as reported in previous ROSAT and BeppoSAX observations. At hard X-ray energies, the spectrum is consistent with a power law, although we find that the spectral slope varied between both XMM–Newton observations and it is also significantly different from that reported previously. Whilst we cannot rule out intrinsic cold absorption to explain the spectral depression, we favour a dust-free warm absorber. Cold absorption requires a column density ∼1.4–1.6 × 1022 cm −2, whilst a warm absorber could have up to ∼1023 cm −2 and an ionization parameter ∼102. The spectrum of GB B1428+4217 shows remarkable parallels with that of the z = 4.4 blazar PMN J0525–3343, in which the available evidence is also most consistent with a warm absorber model.