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Session 100 - Pulsars & X-ray Binaries.
Oral session, Friday, January 09
Monroe,
We present analysis of two types of QPOs in Circinus X-1 observed by the Rossi X-ray Timing Explorer (RXTE) PCA instrument and discuss the impact of the results on the unique standing of Cir X-1 among low-mass x-ray binaries (LMXBs). It has been proposed that the atoll and Z classes of LMXBs differ in both magnetic field and accretion rate. Higher magnetic fields in Z sources could produce the magnetospheric horizontal-branch oscillations (HBOs, 13-55 Hz). Very high accretion rates in Z sources could result in the radiation-pressure driven normal/flaring branch oscillations (N/FBOs, 6-20 Hz). Based on EXOSAT observations, it has been suggested that Cir X-1 might be a unique atoll source that sometimes reaches high accretion rates. Thus Cir X-1 should sometimes show N/FBOs but never HBOs. The 5-20 Hz QPOs observed from Cir X-1 by EXOSAT are cited as the possible N/FBOs. PCA observations of Cir X-1 when the intensity is relatively high (\sim1.0 Crab, 2-30 keV) show the same evolving QPO at 1.3-35 Hz. During periods of flaring to higher intensity (\sim1.2-2 Crab), a broad constant-frequency 4 Hz peak is observed. In light of this 4 Hz QPO at high intensity and the expanded frequency range of the evolving QPO, the identification of the latter with N/FBOs is now in doubt. We discuss alternate interpretations for these two types of QPOs in Cir X-1.
