An absorption feature appears at 600 eV and extends to higher energies by 50 - 100 eV in a spectrum that covers the range from 0.38 to 1.6 keV obtained with the objective grating spectrometer on the Einstein Observatory. The spectrum can be fitted by a single absorption edge or trough plus two power laws with slopes that are consistent with simultaneous measurements of the 2 - 10 keV flux. The absence of a low-energy cutoff is consistent with a neutral column density of NH I ≡ 4×1020 cm-2. Absorption by ionized oxygen is by far the most likely explanation of the sharp feature, and possible detailed interpretations are discussed.
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CANIZARES et al. (1984) studied this question.