The blazar GB 1428+4217:: a warm absorber at z=4.72?

BeppoSAX observations of the high-redshift (z = 4.72) blazer GB 1428+4217 confirm the presence of a complex soft X-ray spectrum first seen with the ROSAT PSPC. Flattening below a rest-frame energy of 5 keV can be accounted for by absorption from an equivalent column density of (cold) gas with N-H similar to 8 x 10(22) cm(-2). Below 2 keV a (variable) excess of a factor of similar to 20 above the extrapolated absorbed spectrum is also detected. These findings are consistent with and extend to higher redshifts the correlation between increasing soft X-ray flattening and increasing z, previously pointed out for large samples of radio-loud quasars. We propose that such features, including X-ray absorption and soft excess emission as well as absorption in the optical spectra, can be satisfactorily accounted for by the presence of a highly ionized nuclear absorber with column N-H similar to 10(23) cm(-2), with properties possibly related to the conditions in the nuclear regions of the host galaxy. High-energy X-ray emission consistent with the extrapolation of the medium-energy spectrum is detected up to similar to 300 keV (rest frame).