2. The X-Ray Light Curve

The X-ray emission is influenced by the source's physical characteristics. The luminosity gives a direct measure of the instantaneous mass accretion rate $\dot{M}$. The X-ray light curve provides information on fluctuations in $\dot{M}$.

In the case of an eclipsing binary the eclipse duration can be used to obtain the eclipse half-angle $\theta_{e}^{}$ as shown in Figure 5.2. This may then be used to constrain the inclination angle i (see Figure 5.2). If the tidally distorted companion is characterized by a sphere of the same volume, its radius R_c is given by (1984)

$$\approx \sqrt{\cos^2i+\sin^2i\sin^2\theta_e}$$ (36)

If R_c is taken to be $\beta$ times the Roche lobe radius R_L, i is given by

$$\approx {\sqrt{1-\beta^2\left({R_L\over a}\right)^2}\over \cos\theta_e}$$ (37)

where R_L may be computed from Equation 1.9.

Figure 12: A high-mass X-ray binary system as seen from Earth showing the definition of the eclipse half-angle $\theta_{e}^{}$.

Figure 13: A high-mass X-ray binary system showing the definitions of the various geometrical parameters.