On the Absorption of X-Rays in the Interstellar Medium

We present an improved model for the absorption of X-rays in the interstellar 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 >~100 eV. Compared with previous work, our formalism includes recent updates to the photoionization cross section and revised abundances of the interstellar medium, as well as a treatment of interstellar grains and the H₂ molecule. We review the theoretical and observational motivations behind these updates and provide a subroutine for the X-ray spectral analysis program XSPEC that incorporates our model.