Scattering of X-ray emission lines by molecular hydrogen

As was shown by Sunyaev and Churazov (1996) and Vainshtein et al. (1998), d istortions in the profiles of X-ray emission lines during the scattering by hydrogen and helium atoms can in principle be used to determine the helium abundance in astrophysical objects. The differences between the scattering s of X-ray photons by atomic and molecular hydrogen are briefly discussed. The most obvious and significant difference is an increase in the elastic s cattering cross section (by a factor of 2 per electron) for molecular hydro gen. For many astrophysical applications, standard analytical expressions f or the scattering cross section for a hydrogen atom (with the elastic scatt ering cross section increased by a factor of 2) can be used to describe the scattering of Xray photons by a hydrogen molecule.