TIME-RESOLVED SPECTRAL-ANALYSIS OF 3 SEYFERT-GALAXIES OBSERVED WITH GINGA

Observations by Ginga of bright Seyfert galaxies have revealed in their spectra the presence of a hard X-ray component which is probably produced, along with the iron K fluorescence line, by reprocessing of the primary one in optically thick matter. This discovery suggests that spectral variability can arise if the primary and the reprocessed components do not vary simultaneously. We attempted to explain the spectral variations observed by Ginga in IC 4329A, MCG-6-30-15 and NGC 4051 in the framework of the two component model. The behaviour of the first two objects in the range 1.7-18 keV and that of the third one above 4 keV can be well reproduced by the two component model, with a constant slope of the primary continuum, and the differences found can arise from the different sizes of the reprocessing region in individual objects. In IC 4329A the variations in the reprocessed component lags behind those in the primary one more than three days. In MCG-6-30-15 the lag is longer than a few hours. In this source, however, the spectral variations could also be due to changes in the slope of the primary component which are correlated with the intensity, and in this case the time lag would reduce to less than 90 min. In NGC 4051 the modest spectral variations above 4 keV are consistent with nearly simultaneous variations in both components, a result strengthened by the evidence, significant at the 99% confidence level, of variations of the iron fluorescence line, which are correlated with those of the continuum down to time scales as short as a few thousand seconds. On the other hand, in this galaxy the two component model is able to reproduce adequately neither the low energy spectral shape nor its large variations. The two modifications of this model that we tested, namely the assumption of a partial covering absorber or of an unobscured soft component, were both successful. In both cases the effective N(H) appears to be inversely correlated with the source intensity, thus lending support to the case of a "warm absorber" in this galaxy.