X-RAY REPROCESSING BY COLD CLOUDS IN ACTIVE GALACTIC NUCLEI

Ginga observations have shown that significant spectral features are present in the X-ray spectra of most Seyfert galaxies. These features are usually interpreted in terms of a model in which the primary X-ray continuum is 'reflected' and reprocessed in an accretion disc. Here we consider an alternative geometry, whereby the reprocessing occurs in a distribution of clouds or 'blobs' surrounding the central illuminating source. A Monte Carlo technique is employed to calculate the predicted X-ray spectrum when a single spherical cloud of material is illuminated by a power-law continuum. Using a simplifying approximation, we then derive spectra for a spherical distribution of such clouds. As in the case of disc-reflection models, a significant iron emission line and Compton reflection hump are produced, as long as the optical depth of the individual clouds is large. The spectra show no significant soft X-ray cut-off for covering fractions less than unity. The mean equivalent width of the iron line observed in the Ginga Seyfert sample is obtained in our model for cloud covering fractions of 65-80 per cent. We compare these results to those obtained from accretion-disc models, and suggest means by which the two classes of model may be distinguished.