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

We observed Cyg X-1 with RXTE continuously over its 5.6 day binary orbit. 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 the dips fell into two distinct categories based on their spectral properties. One type exhibits strong energy-dependent attenuation of X-ray emission at low energies during a dip, which is characteristic of photoelectric absorption, but the other type shows nearly energy-independent attenuation. While the first type of dips is likely caused by density enhancement in an inhomogeneous wind of the companion star, as previous studies have indicated, the second type might be due to partial obscuration of an extended X-ray-emitting region by optically thick "clumps" in the accretion flow. It is also possible that the latter are caused by a momentary decrease in the X-ray luminosity of the source, due, for instance, to a decrease in the mass accretion rate, or by Thomson scattering in highly ionized clumps. We discuss the implications of these scenarios.