2004MNRAS.350L..67W

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 ∼1.4-1.6x10²²/cm2, whilst a warm absorber could have up to ∼10²³/cm2 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.