Previous abstract Next abstract

Session 100 - Pulsars & X-ray Binaries.
Oral session, Friday, January 09

[100.03] The Unique Standing of Circinus X-1 Among LMXBs in Light of QPOs Observed by RXTE

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

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.

Program listing for Friday