Evidence for a 16.6 day period from Circinus X-1

Abstract

Analysis of All-Sky Monitor observations of Cir X-l (3U 1516-56) over the period 1975 October-1976 April has revealed a well-defined modulation of the 3-6 keV flux at a period of 16 d 585 ± 0 d 01. The light-curve is characterized by an abrupt drop in emission occurring on a time scale of <0 d 07, with epoch JD 2,442,877.181 ± 0.07. No clear correspondingly sharp increase in emission is observed during the cycle, so that a noneclipse origin for this effect cannot be ruled out.