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We report results from a 30 ks observation of Cygnus X-1 with the High EnergyTransmission Grating Spectrometer (HETGS) on board the {\em Chandra X-rayObservatory}. Numerous absorption lines were detected in the HETGS spectrum.The lines are associated with highly ionized Ne, Na, Mg, Al, Si, S, and Fe,some of which have been seen in earlier HETGS observations. Surprisingly,however, we discovered dramatic variability of the lines over the duration ofthe present observation. For instance, the flux of the Ne X line at 12.14 \AA\was about $5 \times 10^{-3}$ photons cm$^{-2}$ s$^{-1}$ in the early part ofthe observation but became subsequently undetectable, with a 99% upper limit of$0.06 \times 10^{-3}$ photons cm$^{-2}$ s$^{-1}$ on the flux of the line. Thisimplies that the line weakened by nearly two orders of magnitude on a timescaleof hours. The overall X-ray flux of the source did also vary during theobservation but only by 20--30%. For Cyg X-1, the absorption lines aregenerally attributed to the absorption of X-rays by ionized stellar wind in thebinary system. Therefore, they may provide valuable diagnostics on the physicalcondition of the wind. We discuss the implications of the results.

Dramatic Variability of X-ray Absorption Lines in the Black Hole Candidate Cygnus X-1