X-Ray Timing and Spectral Evolution of Circinus X-1 versus Orbital Phase with the Rossi X-Ray Timing Explorer
We have carried out a study of Cir X-1 through (1) detailed sampling over a single 16.5 day intensity cycle with the Rossi X-Ray Timing Explorer (RXTE) proportional counter array and (2) complementary monitoring over six such cycles with the RXTE all-sky monitor (ASM). We report here the temporal and spectral evolution at nonflaring phases, during which Cir X-1 remained unusually bright (~1.0 crab) and relatively steady. The Fourier power density spectrum of the source was observed to vary with strong correlations among low-frequency flat-topped power (<~1--10 Hz), a quasi-periodic oscillation (QPO) peak centered at 1.3--12 Hz, and a broad QPO peak centered at ~20 Hz up to ~100 Hz. As orbital phase increased within the cycle, the rms amplitude of the flat-topped power generally (but not monotonically) decreased while the QPO features generally evolved toward higher frequency. The 1.5--12 keV spectrum was observed to generally harden during the nonflaring phases of each of the six 16.5 day periods monitored by the ASM.