A search for rapid optical periodicities from Cyg X-l

Abstract

ACCORDING to the most widely accepted model of the X-ray source Cyg X-1, the X rays are produced in a gas disk surrounding and accreting into a black hole with a mass of about 10M⊙. The accretion disk is maintained by matter transferred from the binary companion of Cyg X-1, the blue supergiant HDE226868. The X-ray luminosity of Cyg X-l is known to vary randomly on a wide range of time scales, from many days to a few milliseconds, with the shortest of these time scales corresponding to the dynamical time scale of the innermost stable orbit around the black hole1,2. The optical luminosity of the Cyg X-l system is also known to vary on time scales of days3, but only one case of extremely rapid optical variability has been reported4,5. The properties of the rapid variations were most unusual. The variations were transient and were detected only four times, but they occurred sufficiently frequently that three of these detections occurred during the total of only 2 h of observations accumulated by Auriemma et al. in July 19754,5. The variations were nearly periodic, with periods of 83.53 ms, 83.71 ms, 83.59 ms, and with |ΔP/P| ∼ 2× 10−4. The periodicities were long lived, lasting at least 6 min and probably over 10 min, so that the pulse trains consisted of over 4,000 pulses. The amplitudes of the pulses were large, up to 0.042 mag. No comparable behaviour has been detected in the X-ray light curve of Cyg X-l, and it is far from clear that this behaviour is consistent with our present conceptions of accretion disks. Thus, the detections of these pulses, if confirmed, would be of some importance since they would require a revision of the models for Cyg X-1. Therefore, we have taken an extensive set of observations of Cyg X-l with the purpose of detecting additional examples of the rapid periodicities.