Optical behaviour of HDE226868 during a Cyg X-1 X-ray transition

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As part of regular monitoring of variable X-ray sources with the University College London–Mullard Space Science Laboratory X-ray instrumentation on the Copernicus satellite, we have observed Cyg X-1 continuously for 12 d between 22 October and 3 November, 1975. The X-ray flux was noted to be more variable than it had been during previous monitoring runs, when the source had been in its low state1, and, in particular, a marked increase in flux occurred 8 d into the observing run ( October 29.5 UT, 1975). After this date, the mean 3–8-keV flux increased steadily, and by 3 November had risen by 30% compared with its level during the first part of the observation. The X-ray flux data, integrated in time blocks of 30 min, are shown in Fig. 1b. In Fig. 1a we show the spectral hardness ratio (defined as the 5.5–8.0-keV flux divided by the 3.0–5.5-keV flux) integrated over intervals of 0.5 d. This quantity decreased progressively after 30 October, until by 3 November it was down by an amount equivalent to a drop of 0.1 in the power law spectral index of the source.

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