X-ray behaviour of Circinus X-1 - I. X-ray dips as a diagnostic of periodic behaviour

We examine the periodic nature of detailed structure (particularly dips) in the Rossi X-Ray Timing Explorer/All Sky Monitor (RXTE/ASM) light-curve of Circinus X-1. The significant phase wandering of the X-ray maxima suggests their identification with the response on a viscous time-scale of the accretion disc to perturbation. We find that the X-ray dips provide a more accurate system clock than the maxima, and thus use these as indicators of the times of periastron passage. We fit a quadratic ephemeris to these dips, and find its predictive power for the X-ray light-curve to be superior to ephemerides based on the radio flares and the full archival X-ray light-curve. Under the hypothesis that the dips are tracers of the mass transfer rate from the donor, we use their occurrence rate as a function of orbital phase to explore the (as yet unconstrained) nature of the donor. The high term in the ephemeris provides another piece of evidence which shows that Cir X-1 is in a state of dynamical evolution, and thus is a very young post-supernova system. We further suggest that the radio `synchrotron nebula' immediately surrounding Cir X-1 is, in fact, the remnant of the event that created the compact object, and discuss briefly the evidence for and against such an interpretation.