X-ray emitting region of Cygnus X-1 constrained by short absorption dips

Abstract Transient dips of Cygnus X-1 are thought to result from the absorption of soft X-rays from the accretion disk due to transits of clumps in the companion star’s winds. Under this model, the time evolution of dips carries information on the sizes of the emitting region and clumps. We investigated the time evolution of absorption dips of Cygnus X-1 detected with NICER. We searched for dips and found 359 strong dips in the low/hard state, while no case was found in the high/soft state. Of the 359 strong dips, 25 are shorter than 10 s, and we investigated the time evolution of 15 of them that were fully captured in good time intervals of observations without overlapping with other long dips. The evolution of the spectrum during dips suggests that the clump is a bunch of small blobs or has a mottled structure, and that the size of the X-ray emitting region is larger than the blobs or the structure. The durations of ingress and egress of the dips limit the size of the X-ray emitting region to be less than $(5.5\pm 0.7)\times 10^{8}[v_{\rm {rel}}/(1.6\times 10^{8}~\rm {cm~s}^{-1})]$ cm, where $v_{\rm {rel}}$ is the relative velocity between the X-ray emitting region and clumps. This corresponds to $\approx$44 times the Schwarzschild radius for a 21.2 $M_{\odot }$ black hole. This size is considered to be close to the actual size of the X-ray emitting region.