Flare observations of Circinus X-1 at 2-cm wavelength

Abstract

We report observations at 2.1 cm (14.4 GHz) of a radio flare with at least three peaks from Cir X-1 on 1978 April 24–26. Peaks (∼ 300 mJy max) were separated by ∼ 23 hr. Onset of the first peak occurred a few minutes after predicted X-ray phase zero (i.e. the time of rapid transition from high to low X-ray flux), in contrast to the delay of ∼ 7 to 8 hr observed for an earlier flare. A previous flare onset is used to determine a 2-cm flare period P = 16^d.590 ± 0^d.002, averaged over 25 cycles of the source, consistent with the most accurate X-ray period of 16^d.594 ± 0^d.003.