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Well-collimated proportional counters serisitive to 1-10-keV X-rays have been flown on an attitudecontrolled Aerobee 150 rocket and have detected discrete sources at the positions of the extragalactic objects 3C 273, NGC 5128 (Cen A), and M87. The X-ray emission from Cen A is confined within 20 of the optical galaxy. The X-ray flux of Cen A falls below the extrapolated radio spectrum.
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Bowyer et al. (1970) studied this question.
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