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   PR # 21127
    We propose to study the most distant QSO so far observed in the X-ray
    region. Q1745+624 located at z=3.87 is recently identified in the all
    -sky survey by the ROSAT satellite. The observation of Q1745+624 will
    supply information to understand the reported evolution for x-ray QSO
    by determining the luminosity at the high redshift. Furthermore, the 
    study of the energy spectrum of the distant QSO is essential to study
    the origin of the cosmic X-ray background. The spectrum expected with
    40,000 seconds observation by GIS is sufficient for the determination
    of the power law index.                                              
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