2007ApJ...663.1207C

We report results from a 30 ks observation of Cygnus X-1 with the High Energy Transmission Grating Spectrometer (HETGS) on board the Chandra X-Ray Observatory. 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 of the present observation. For instance, the flux of the Ne X line at 12.14 Å was about 5x10^–3 photons/cm2/s in the early part of the observation but became subsequently undetectable, with a 99% upper limit of 0.06x10^–3 photons/cm2/s on the flux of the line. This implies that the line weakened by nearly 2 orders of magnitude on a timescale of hours. The overall X-ray flux of the source did also vary during the observation but only by 20%-30%. For Cyg X-1, the absorption lines are generally attributed to the absorption of X-rays by ionized stellar wind in the binary system. Therefore, they may provide valuable diagnostics on the physical condition of the wind. We discuss the implications of the results.