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

Abstract

WHILE the Ariel V satellite was observing in the Circinus–Norma region an outburst was detected from Cir X-1 on 1976 February 15 by the rotation modulation collimator (RMC) experiment¹. One day before the outburst began the Imperial College hard X-ray scintillation telescope (ST) experiment (8° FWHM, 26 keV–1.2 MeV) completed a 4-d observation of this source and a strong signal was detected. For the following eight days, 1976 February 14–22, the source remained within the field of view, though at low sensitivity. Consequently only upper limits were obtained for the hard flux during the outburst and only a limited measure of the spectrum immediately afterwards. We have combined our results with the RMC results from reference 1 to obtain an overall view of the spectral behaviour of Cir X-1 during this outburst. We suggest that the large scale low energy X-ray outbursts of the ‘super-variable’ Cir X-1 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.