The column density towards LMC X-1

We measure the neutral absorption towards the black hole X-ray binary system LMC X-1 from six archival soft X-ray spectra obtained with the gratings and/or CCD detectors on Chandra, XMM-Newton, and Swift. Four spectral models for the soft continuum are investigated. While the powerlaw model may overestimate $N_{\rm H}$ considerably, the others give consistent results. Taking the lower metalicity of the Large Magellanic Cloud into account, we find equivalent hydrogen column densities of $N_{\rm H}$ = (1.0-1.3) $\times$ 1022 cm-2, with a systematic dependence on the orbital phase. This variation in the neutral absorption can nearly explain the orbital modulation of the soft X-ray flux recently detected with the All Sky Monitor (ASM) on the Rossi X-ray Timing Explorer (RXTE).