ROSAT PSPC detection of soft X-ray absorption in GB 1428+4217:: the most distant matter yet probed with X-ray spectroscopy

We report on a ROSAT PSPC observation of the highly luminous z = 4.72 radio-loud quasar GB 1428+4217 obtained between 1998 December 11 and 17, the final days of the ROSAT satellite. The low-energy sensitivity of the PSPC detector was employed to constrain the intrinsic X-ray absorption of the currently most distant X-ray detected object. Here we present the detection of significant soft X-ray absorption towards GB 1428+4217, making the absorbing material the most distant matter yet probed with X-ray spectroscopy. X-ray variability by 25 +/- 8 per cent is detected on a time-scale of 6500 s in the rest frame. The X-ray variation requires an unusually high radiative efficiency eta of at least 4.2, further supporting the blazar nature of the source.