Long-Term X-Ray Variability of Circinus X-1

We present an analysis of long-term X-ray monitoring observations of Circinus X-1 (Cir X-1) made with four different instruments: Vela 5B, Ariel 5 all-sky monitor (ASM), Ginga ASM, and RXTE ASM, over the course of more than 30 yr. We use Lomb-Scargle periodograms to search for the ~16.5 day orbital period of Cir X-1 in each of these data sets. From this we derive a new orbital ephemeris based solely on X-ray measurements, which we compare with the previous ephemerides obtained from radio observations. We also use the phase dispersion minimization (PDM) technique, as well as fast Fourier transform analysis, to verify the periods obtained from periodograms. We obtain dynamic periodograms (both Lomb-Scargle and PDM) of Cir X-1 during the RXTE era, showing the period evolution of Cir X-1 and also displaying some unexplained discrete jumps in the location of the peak power.