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Session 44 - Stellar X-Ray Sources.
Display session, Tuesday, January 14
Metropolitan Ballroom,
We present results of a 10-hr RXTE PCA observation of the degenerate low mass binary X1820-303 in its low state, during which high frequency (580-680 Hz) quasi-periodic oscillations (HF QPO) were detected for the first time. X1820-303 is thus the seventh LMXB to show HF QPO in its persistent emission. The QPO centroid frequency in X1820-303 correlates strongly with source intensity, increasing from 585\pm4 Hz to 681\pm2 Hz as the count rate increases from 1650 c/s to 1800 c/s. The QPO width of 14-23 Hz may also be weakly correlated with intensity. The rms amplitude ranges from 3.7-4.6 percent in the 1-9 keV band to 8.6-9.4 percent at 9-20 keV. The known lower-frequency noise peak (LFN) is also detected, with centroid 28.4\pm0.7 Hz, FWHM 16.1\pm2.3 Hz, and rms 4.5 percent (1-8 keV) and 5.6 percent (8-20 keV). The coherence of the LFN component is two orders of magnitude weaker than that of the HF QPO, and there are no correlations between the two oscillatory effects, suggesting they originate in different physical regions. We will also show details of the 685-s orbital modulation (visible in the raw light curves) and spectral fitting results, and comment on previous RXTE observations in 1996 March and June during which X1820-303 was brighter and HF QPO were not detected. Finally, the various HF QPO sources and current models for their behavior will be summarized.
