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Session 12 - Binary Stars.
Display session, Monday, June 08
Atlas Ballroom,

[12.03] Orbital Modulation of the X-ray Intensity of Cyg X-1 in its High and Low States

L. Wen, W. Cui, A. M. Levine, H. V. Bradt, R. A. Remillard, S. A. Rappaport (MIT)

We have analyzed over 2 years of RXTE/ASM 2-12 keV light curve data for Cyg X-1. We have detected the 5.6 day orbital period in power spectra of both the light curves and the hardness ratios when Cyg X-1 was in the low state. The folded light curves and hardness ratios reveal a broad intensity `dip' accompanied by spectral hardening centered on the superior conjunction of the X-ray source. The `dip' has a duration of about 30% of the orbital period and depth ranging from 10% to 35% of the non-dip intensities in three energy bands. In contrast, no feature at the orbital period is evident in the power spectra or folded light curves for the high state. We discuss possible mechanisms for the observed periodic modulation.

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