X-ray Masses for Clusters
Fabricant, Lecar, and Gorenstein:
Bremsstrahlung spectrum is roughly flat up to X-ray
energies, above which it cuts off exponentially.  Cut-off is
related to temperature.  E = hν ~ kBT.  The measurement is
a projection onto 2D space.  Integrating emissivity and
converting to intensity (surface brightness at the projected
radius a):