XMM-Newton observations 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 the spectral slope varied between both XMM-Newton observations and 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 x 10^22 cm^-2 whilst a warm absorber could have up to ~10^23 cm^-2 and an ionization parameter ~10^2. 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. 
M.A. Worsley, A.C. Fabian, A. Celotti, K. Iwasawa