We present results of a comprehensive study of the distribution of absorption dips with orbital phase in Cygnus X-1 .
Firstly , the distribution was obtained using archival data from all major X-ray observatories and corrected for the selection effect that phase zero ( superior conjunction of the black hole ) has been preferentially observed .
Dip occurrence was seen to vary strongly with orbital phase \phi , with a peak at \phi \sim 0.95 , i.e .
was not symmetric about phase zero .
Secondly , the RXTE ASM has provided continuous coverage of the Low State of Cygnus X-1 since Sept. 1996 , and we have selected dip data based on increases in hardness ratio .
The distribution , with much increased numbers of dip events , confirms that the peak is at \phi \sim 0.95 , and we report the discovery of a second peak at \phi \sim 0.6 .
We attribute this peak to absorption in an accretion stream from the companion star HDE 226868 .
We have estimated the ionization parameter \xi at different positions showing that radiative acceleration of the wind is suppressed by photoionization in particular regions in the binary system .
To obtain the variation of column density with phase , we make estimates of neutral wind density for the extreme cases that acceleration of the wind is totally suppressed , or not suppressed at all .
An accurate description will lie between these extremes .
In each case , a strong variation of column density with orbital phase resulted , similar to the variation of dip occurrence .
This provides evidence that formation of the blobs in the wind which lead to absorption dips depends on the density of the neutral component in the wind , suggesting possible mechanisms for blob growth .