Simultaneous EXOSAT and IUE Observations of Fairall 9: Short and Long-Term Variability

Abstract

Simultaneous observations of the Seyfert I galaxy Fairall 9 (F9) were performed with Exosat and IUE during October 1983 and 1984. A decrease in the flux of the source by more than a factor of 2 was observed at all wavelengths between the two epochs. Furthermore, in the X-ray (0.05-10-keV) band, variability of the order of 50 percent on a time scale of days was found during October 1983. The shape of the spectrum varied together with the flux level on both short and long time scales, the ratio between soft and hard X-ray counts being higher when the source was brighter. The complete X-ray light curve for the period 1971-1984 shows that during recent years F9 has faded not only in the UV - which was already known - but also in the X-ray band. The latter decreased more slowly than the former before 1983; then the decline rates in the two energy bands took on comparable large values. Different interpretations of the spectral variability are possible, and three are discussed: the X-ray spectrum can be taken as due to a constant-slope power law, with energy index alpha = 0.96 + or - 0.06 and varying column density; alternatively, the changes can be interpreted as spectral-index changes in a power law with constant absorption, nH = 2 x 10 to the 20th/sq cm; finally, a multicomponent spectrum can be assumed, with a variable soft excess responsible for the soft-X-ray variability. In the last case, the X-ray and UV spectra can be linked, under the assumption that the soft-X-ray excess in the exponential tail of the UV, and a cutoff energy of 86 + or - 15 eV can be determined.