Conclusion

We have investigated in detail the MCS model for the geometry of the plasma surrounding BHCs. In particular, we have calculated the full spectrum produced by this model using a Monte Carlo simulation, as well as the time dependent light curves produced by Comptonization of a soft radiation flare at time zero, and the hard time-lag curve between different energy bands. This is the first time the light curves and the hard time-lags from a plasma with the MCS geometry have been investigated. It is found that Monte Carlo investigation of photon time-lags between different energy bands can be used to constrain this multi-temperature geometry for the accretion disk corona around galactic BHCs.

We note that the energy spectrum is largely insensitive to the values of the radii used for the inner and outer clouds, as in general a Comptonized spectrum is sensitive only to the temperature and optical depth of the plasma (and the initial photon distribution if the plasma is optically thin). However, we find that the time-lag curve between energy bands is sensitive to the ratio of the two radii. This is the main result of this work. We find that a sharp rise in the time-lag curve at low Fourier frequencies is the temporal signature of the MCS model.

MCS also consider the case where bremsstrahlung photons from the tenuous outer-corona are a significant spectral component. However, if the bremsstrahlung photon flux is stationary, then this component will make no contribution to the Fourier transform of the light curves, and therefore no contribution to the time lag curve. This will be the case as long as any fluctuations in the bremsstrahlung flux are small compared to the amplitude of the central Comptonized pulse. Similarly, any time independent processes such as annihilation should not contribute significantly to the time-lags.

Thus the time delay or lag can be used to determine the overall size of the corona and the ratio of the inner and outer radii, if this two-component corona geometry exists. This information combined with values for the optical depth obtained from spectral modeling can then tell us the overall density of the corona. Such a result would then give us all the physical parameters of the plasma cloud.

We have demonstrated that timing observations at X-ray and gamma-ray energies can in principle determine the physical properties of the two-component corona in the MCS model. It is expected that more zones will be added in future versions of the code. This would enable us to investigate density and temperature gradients in more detail.

© Copyright Astronomical Society of Australia 1997