# What is the “radiation sound speed” inside an accretion column?

I was trying to read "A New Two-fluid Radiation-hydrodynamical Model for X-Ray Pulsar Accretion Columns" and its follow-up paper both by West et al. and came across a quantity called "radiation sound speed", $$a_r$$.

It is defined as $$a_{r}^{2}=\frac{\gamma_{r} P_{r}}{\rho},$$ where $$\gamma_{r}$$ is the ratio of specific heats for the radiation field, whatever that is supposed to mean, $$P_{r}$$ is the radiation pressure and $$\rho$$ the mass density.

My question now is: How can the radiation field have specific heats and a speed of sound?
I always though only compressible fluids, gases, etc. can have that.

• Radiation does not require medium for the energy transportation, but sound does. I think this equation explains how a radiation field interacts with matters, which are the medium for the sound wave. – Kornpob Bhirombhakdi Nov 22 '19 at 20:59