Spectral variability in celestial X-ray sources.

The Ariel-6 spacecraft was launched during 1979 and remained operational for ∼ 3 years. The satellite payload is discussed, with an emphasis on the medium energy detectors, their calibration and performance. We present observations of spectral variability, in both galactic and extra-galactic sources, made with this instrument in the 1-50 keV energy range. Three galactic sources were observed, these consisted of two X-ray pulsators, 4U0115+63 and GX 1+4; the former being fortuitously seen during outburst. Both sources were found to show a significant phase dependence in their X-ray spectra. In GX 1+4 an Iron emission feature was detected, the equivalent width of which was also phase dependent. Pulse timing was performed on the data from both sources and in the case of 4U0115+63 yields a revised set of orbital parameters. The third galactic source presented is 3A1822-371, we find that our data are not only consistent with the presence of an accretion disc corona in the system, but strongly suggests the presence of such a corona in many disc fed systems. Three Seyfert galaxies, NGC 4151, MCG 8-11-11 and IIIZw2 were observed, this being the first reported X-ray spectrum of IIIZw2. NGC 4151 was found to have a spectrum consistent with that seen ∼ 3 years earlier, whilst MCG 8-11-11 was softer than seen hitherto and now shows evidence for an Iron emission line. The BL Lac, Mkn 421 was found to have a two component spectrum; comparison with earlier reported spectra indicates that both components vary independently of each other and therefore arise in physically differing regions of the source. Finally a forward-look is made which considers the improvements in medium energy spectroscopy that can be anticipated over the next decade or so.