The conditions in the plasma in proximity to the compact object are  traditionally described by the ionization parameter (flux/density), which is a measure of the dominance of radiation processes over collisional processes.  Contours of constant ionization parameter (logarithmic, in cgs units) are shown for the HMXRB Vela X-1 (above, right; Figure from Sako et al. Ap.J., 525, 921 (1999)).