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.