Evidence of Thick Obscuring Matter Revealed in the X-Ray Spectrum of the z = 4.28 Quasar RX J1028.6–0844

We report the discovery of an unambiguous, substantial low-energy cutoff in the broadband X-ray spectrum of the radio quasar RX J1028.6-0844 at a redshift of 4.276 obtained with the ASCA satellite, which we preferably explained as an indication of excess X-ray absorption. The equivalent hydrogen column density of the absorbing matter, depending on the redshift and metallicity, ranges from 2.5 × 1021 cm-2 for local absorption up to 2.1 × 1023 cm-2 (solar metallicity) or 1.6 × 1024 cm-2 (10% solar metallicity) for absorption at the quasar redshift. Such a value is among the largest found for high-redshift radio quasars. The absorption, if interpreted as being produced close to the quasar, may indicate the presence of a remarkably large amount of obscuring matter in the quasar environment in the early universe. Implications of the result for the possible origins of the absorbing matter are discussed, concerning especially galactic intervening matter, cool intracluster gas, and ambient medium around the quasar jet. The quasar itself has an enormous apparent luminosity of at least about 2.6 × 1047 ergs-1 (H0 = 50 km s-1 Mpc-1, q0 = 0.5) and a power-law photon index of 1.67 in the 2-50 keV band in the source rest frame.