On the absorption of X-rays in the interstellar medium

We present an improved model for the absorption of X-rays in the interstell ar medium (ISM) intended for use with data from future X-ray missions with larger effective areas and increased energy resolution such as Chandra and the X-Ray Multiple Mirror mission, in the energy range greater than or simi lar to 100 eV. Compared with previous work, our formalism includes recent u pdates to the photoionization cross section and revised abundances of the i nterstellar medium, as well as a treatment of interstellar grains and the H -2 molecule. We review the theoretical and observational motivations behind these updates and provide a subroutine for the X-ray spectral analysis pro gram XSPEC that incorporates our model.