Absorption in the interstellar medium
X-rays produced by an astronomical source are passing through the interstellar medium of our Galaxy. While the interstellar medium is almost a vacuum, it does contain some material – gas, molecules, and dust – which can absorb the X-rays. If we want to study the sources, i.e., if we want to measure their spectral shape in order to learn about their physics, we need to correct for these effects of absorption. In collaboration with others we have developed the de facto standard model for the absorption of X-rays in the interstellar medium. The first version of this model, called tbabs, was published by Wilms, Allen, and McCray (2000, ApJ 542, 914). Since then we have continued to update and improve the model. The newest version of the model can be found here, it is also distributed with the newest releases of the standard X-ray astronomy modeling packages XSPEC and isis.