Variable Soft X-Ray Absorption and Excess of VELA X-1

We present preliminary results from a study of the stellar wind modulation of the X-ray spectrum of Vela X-1 using the broad-band (2-200 keV) data obtained in period 1988-1991 with the TTM and HEXE instruments on-board the MIR space station. We find that the hydrogen column density of the absorbing material varies from a negligible value up to 7 x 10²³ atoms cm^-2. A soft X-ray flux excess, when compared with the extrapolation of the spectrum measured at higher energies with absorption due to cold matter, is detectable at all binary phases. It is variable and exhibits two different patterns of behavior. The soft excess is strong in the phase range when the column density is low and is then approximately correlated with the source flux. When the column density is high no such correlation is seen.