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 timescale of 6500 s in the rest frame. The X-ray variation requires an unusually high radiative efficiency of at least 4.2, further supporting the blazar nature of the source. 
Th. Boller (1), A.C. Fabian (2), W.N. Brandt (3) and M.J. Freyberg (1)   ((1) MPE Garching, Germany; (2) Institute of Astron...