conditions in the plasma in proximity to the compact object
aretraditionally 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)).