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 c onfirm the presence of a complex soft X-ray spectrum first seen with the RO SAT PSPC. Flattening below a rest-frame energy of 5 keV can be accounted fo r by absorption from an equivalent column density of (cold) gas with N-H si milar 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. Th ese findings are consistent with and extend to higher redshifts the correla tion between increasing soft X-ray flattening and increasing z, previously pointed out for large samples of radio-loud quasars. We propose that such f eatures, including X-ray absorption and soft excess emission as well as abs orption 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 nucl ear regions of the host galaxy. High-energy X-ray emission consistent with the extrapolation of the medium-energy spectrum is detected up to similar t o 300 keV (rest frame).