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We observed Cyg X-1 with {\em RXTE} contiguously over its 5.6-day binaryorbit. The source was found to be in the hard state throughout the observation.Many intensity dips were detected in the X-ray light curves. We found that thedips fell into two distinct categories based on their spectral properties. Onetype exhibits strong energy-dependent attenuation of X-ray emission at lowenergies during a dip, which is characteristic of photoelectric absorption, butthe other type shows nearly energy-independent attenuation. While the firsttype of dips are likely caused by density enhancement in an inhomogeneous windof the companion star, as previous studies have indicated, the second typemight be due to partial obscuration of an extended X-ray emitting region byoptically thick ``clumps'' in the accretion flow. It is also possible that thelatter are caused by a momentary decrease in the X-ray luminosity of thesource, due, for instance, to a decrease in the mass accretion rate, or byThomson scattering in highly ionized ``clumps''. We discuss the implications ofthese scenarios.Comment: 18 pages,5 figures. in press, ApJ January 10, 200

Discovery of Two Types of X‐Ray Dips in Cygnus X‐1