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W. Yuan, M. Matsuoka, S. Ueno (NASDA, Japan)
We report on the evidence for a strong low-energy spectral cutoff in the radio-loud quasar RXJ1028.6-0844 at a redshift of 4.28 obtained with an ASCA observation. The most likely explanation is photoelectric absorption of the soft X-rays. Our result confirmed that the excess X-ray absorption is common for radio-loud quasars at redshifts extending up to z>4. If the absorber lies close to the quasar, the derived H column density (~2\times 1023\,cm-2) is the largest among those found in high-redshift radio quasars. This may indicate the presence of a remarkably large amount of obscuring matter in the quasar environment in the early universe. Possible origins of the absorber are discussed.