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Session 35 - Galactic X-Ray (XTE) Sources.
Display session, Tuesday, January 14
Metropolitan Ballroom,

[35.06] Complex Variability in Circinus X-1 at Transition with RXTE

R. Shirey (MIT), H. Bradt (MIT), A. Levine (MIT), E. Morgan (MIT)

The neutron-star binary Circinus X-1 has been observed with the Rossi X-ray Timing Explorer (RXTE) Proportional Counter Array (PCA) for 60% of 2.0 days (1996 September 20.3-22.3) beginning 0.5 day before phase zero of its 16.55-day cycle. The RXTE All-Sky Monitor (ASM) has shown Cir X-1 to be relatively bright (\sim1 Crab) for most of each cycle since 1996 February and flaring to as bright as 3 Crab after the phase-zero transition. The PCA observations show a complex pattern of dips and flares of varying duration as the source intensity climbs to 2.0 Crab. The spectrum softens significantly as the source reaches its high state. During dips the spectrum generally hardens. However, during strong dips the low-energy channels do not drop below a fixed floor value while the higher channels continue to decrease; thus the low-energy spectrum softens. We will present results of spectral fits to these observations in order to better characterize the nature of these dips and flares, as well as the transition itself. In addition, we will track the fast temporal variability of the intensity, i.e. quasi-periodic oscillations, over the duration of these observations, and we will attempt to correlate the variability with spectral state. The behavior at phase zero will be compared to the spectral and temporal evolution observed during quiescent phases (Shirey et al. 1996, ApJ, 469, L21).

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