Semi-periodic pulsations in circinus X-1

• Published in: Nature (London) 1979-03, Vol.278 (5703), p.436-438
• Main Authors: SADEH, D, MEIDAV, M, WOOD, K, YENTIS, D, SMATHERS, H, MEEKINS, J, EVANS, W, BYRAM, E. T, CHUBB, T. A, FRIEDMAN, H
• Format: Article
• Language: English
• Subjects: Humanities and Social Sciences; letter; multidisciplinary; Science; Science (multidisciplinary)
• ISSN: 0028-0836; 1476-4687
• DOI: 10.1038/278436a0

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 10 37 –10 38 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 found 10 , 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 emission 11 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 period 4 . We report here our investigations, using HEAO A-1, of these semi-periodic pulsations in Cir X–1.