Spectra

Figure 2 shows the single-temperature (isothermal) coronal Comptonized spectrum produced by a spherical homogeneous cloud of plasma of optical depth and electron temperature of kT_e=76.7 keV (dotted line). Figure 2 also shows the spectrum produced by assuming the MCS model (solid line). The inner cloud parameters are the same as in the single cloud case, and the outer cloud is of effective optical depth and electron temperature kT_e=396 keV. These temperatures and optical depths are the best fit values (as determined by MCS) to BATSE-COMPTEL observations of Cyg X-1 taken when it was in the low state. Also shown is the injected source spectrum which is taken to be a blackbody distribution of temperature 130 eV. The ratio of radius of the inner cloud to the outer cloud radius is a free parameter, but the overall energy spectrum is insensitive to this. Here we fix it to be 0.1.

In both cases, photons gain energy by inverse Compton scattering as they diffuse through the cloud, forming a power-law. We see that the single-temperature corona spectrum has a cutoff at below 1.0 MeV, while the multi-temperature model produces substantial numbers of photons with energies in the MeV range. This high energy excess is due to the photons that are scattered up to gamma-ray energies in the high temperature outer cloud.

Note that the X-ray power-law part of the spectrum has the same slope between 2-100 keV for both the single and double corona models. This is required as the canonical single corona model accounts well for the X-ray part of the spectrum. But while the canonical X-ray power-law is still produced, there is now at high energies an excess of photons above the cutoff energy for a single temperature model. The optical depth of the outer cloud is low enough that the shape of the Comptonized spectrum at low energies is not distorted, however it does act to scatter some photons up to higher energies than they could have reached propagating through a cooler isothermal inner cloud. Therefore, this geometry can potentially explain the production of gamma-rays without contradicting inverse Compton models for spectral fits at X-ray energies.

Figure 2: Typical spectrum produced by the MCS model, as well as the injection spectrum and the single-temperature (isothermal) corona spectrum (dotted line, for comparison). The injected spectrum is a blackbody distribution. Photons gain energy by inverse Compton scattering as they diffuse through the cloud, forming a power-law. The single-temperature corona spectrum has a cutoff below 1.0 MeV, while the multi-temperature model extends upwards into the MeV range. This high energy excess is due to the photons that are scattered up to gamma-ray energies in the high temperature outer cloud. Note that the X-ray power-law part of the spectrum has the same slope for both the single and double corona models. This is required as the canonical single corona model accounts well for the X-ray part of the spectrum.

© Copyright Astronomical Society of Australia 1997