Hard X-ray observations of Circinus X-1 during an outburst

It is stated that the Ariel V rotation modulation collimator detected an outburst from Cir X-1 during observations on 15 February 1976. A strong signal was also detected by the Imperial College hard X-ray scintillation telescope during this period. Only upper limits were obtainable in the latter case for the hard flux during the outburst. The observations in the two cases are here combined to obtain an overall view of the spectral behaviour of Cir X-1 during the outburst. It is suggested that the large scalar X-ray outbursts may be interpreted as corresponding to the periodic removal of a dense screening gas to expose a relatively stable source with a power law spectrum. During the outburst the soft X-ray flux increases by at least two orders of magnitude, but there was no indication of any comparable increase in the flux of hard X-rays. Data obtained by other workers are also mentioned. It is considered that Cir X-1 may provide an extreme example of a source for which large variations in the amount of surrounding material dramatically change the spectral shape. (U.K.)