Semi-periodic pulsations in circinus X–1

Abstract

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 days^6–8. 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.