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.
