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Session 23 - Degenerate Stars, Pulsars.
Display session, Tuesday, June 10
South Main Hall,
Observations of Circinus X-1 with the ASM and PCA instruments on the Rossi X-ray Timing Explorer (RXTE) from 1996 February to 1997 April have shown Cir X-1 to be brighter than about 1.0 Crab (2-10 keV) for this entire period. For part of each 16.5-day flaring cycle, the intensity remains relatively steady at about 1.0 Crab, while the spectrum gradually hardens. During this period the power density spectrum (PDS) shows evolving flat-topped, band-limited noise with a quasi-periodic oscillation (QPO) peak at or above the (varying) cutoff frequency (Shirey et al. 1996, ApJ, 469, L21). Complex dipping due to absorption occurs near phase zero of the 16.5-day cycle (Shirey et al. 1996 BAAS, 28, 1315). After phase zero, the intensity flares to 2-3 Crab for 3-4 days (or more). During this "active" state, the spectrum is very soft and PDSs show strong very-low-frequency noise (VLFN) and a broad, stationary QPO centered at about 4 Hz. When the intensity reaches its highest levels, the 4 Hz QPO appears to evolve into band-limited noise with a powerlaw decline at high frequencies. We use both timing and spectral behavior to characterize the active state and show that active-state behavior is often observed at phases beyond the bright flaring period, i.e., a moderately strong secondary flare, a slow decline of the primary flare over most of the cycle, or a small enhancement in intensity during the latter portion of the cycle. These periods also show strong VLFN, stationary 4 Hz QPO, and spectral softening. The behavior of Cir X-1 during the active state may be due to the accretion rate approaching the Eddington limit.
