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   61140
   We propose to observe the high redshift quasar RXJ1028.6-0844(z=4.3).
   This quasar has the hardest X-ray spectrum among the high redshift
   quasars observed with ROSAT. ROSAT could not constrain whether this
   hardness is due to intrinsic flatness or a strong absorption in lower
   energies, but ASCA observation can do so significantly. It is important
   to understand the origin of the hard spectrum because it must place
   constraints on the nature of the X-ray emission and the physical
   environment in the early universe. It may also give clue to the AGN
   evolution and the cosmic X-ray background.
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