The origin is investigated for a prominent and rapidly variable emission feature near the 4640-A line which has been observed in the optical counterparts of Sco X-1, Cyg X-2, Her X-1, Cen X-3, 3U 0900-40, and 3U 1700-37. It is suggested that the emission is generated in the vicinity of a compact X-ray source by the interaction of X-rays with nearby gaseous matter and that three likely sources of the emission are the ions C III, N III, and O II. Results are presented for a detailed mathematical model of an X-ray source surrounded by a uniform distribution of matter and are used to evaluate possible emission mechanisms. It is concluded that selective emission of N III (4634 to 4641 A) in the Bowen fluorescence process may be the responsible mechanism.
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McClintock et al. (1975) studied this question.