Semi-periodic pulsations in circinus X–1

CIRCINUS X–1 (4U1516–56) was first detected in 1969 (ref. 1) and has since been studied in X-ray, radio, and optical wavelengths. Particular interest in this source has resulted from its resemblance to Cygnus X–1—a suspected black hole. Both objects have X-ray luminosities in the range 1037–1038 erg s−1 (refs 2,3). Both exhibit rapid intensity variations with timescales as low as 1 ms (refs 4,5), and as long as seconds or days6–8. Both X-ray sources are in binary systems; the orbital period reported for Cir X–1 is 16.585 d (ref. 9). Cir X–1 differs from Cyg X–1 in that the total mass of the binary pair is unknown. While an optical candidate has been found10, no estimation of its mass has been made, so that the evidence for its being a black hole is not as strong for Cir X–1. Variable IR emission11 has been reported from the companion. There have been conflicting reports of X-ray periods of the order of seconds: Margon et al.1 suspected a period of 0.685 s and Toor reported a possible 2.2 s period4. We report here our investigations, using HEAO A-1, of these semi-periodic pulsations in Cir X–1.