How can I calculate the density in a plasma based on the XSPEC normalization?
After a fit using an APEC model, XSPEC returns a norm, which is simply the emission measure of the gas scaled by the distance, where n_e, n_H are the electron and hydrogen densities and D_A is the angular diameter distance to the source (for a Galactic source, this is simply the normal distance). If the source is extragalactic, then the distinction is relevant, however. Using concordance cosmology values with the best-fit redshift z, D_A can be easily calculated. Then, however, some assumptions must be made about the volume and shape of the emitting plasma. Assuming the source is spherical, of uniform density, and fully ionized with 10\% He (implying n_e ~ 1.2 n_H), these equations can be rewritten as: where theta is the angular size of the source.
