the volume goes as $dm_x \propto r$ as well. Therefore, the X-ray intrinsic emission goes as $$L_{xi} =n_e n_ p V \Lambda \propto \left( \frac{dm_x}{V} \right)^2 V \propto r^{-1},
\label{eq:lxem}$$ where $\Lambda$ is the emissivity, and $n_e$ and $n_p$ are the electron and proton number densities, respectively. The $L_{xi} \propto r^{-1}$ variation is assumed by P09. There are small variations to this dependance. A06 considered the relative motion of the two stars, through its influence on the ram pressures of the two winds. They found that before periastron the X-ray emission is larger than that given by equation (\[eq:lxem\]), while after periastron it is lower.

The relation $L_{xi} \propto r^{-1}$ is generic to X-ray emission from colliding fast winds. For $\eta$ Car the eccentricity is very high, and the increase in the internal X-ray emission from apastron to periastron is by a factor of $\ga 20$. Any model, and any observer from what ever direction, would detect a sharp increase in the X-ray emission when the system is near periastron. The direction to the observer would change the observed flux because of the dependance of the column density of the absorbing gas on the direction. However, there are several unknown 