Long-term X-ray spectral variability in Seyfert 1 galaxies

Direct time-resolved spectral fitting has been performed on continuous Rossi X-Ray Timing Explorer monitoring of seven Seyfert 1 galaxies to study their broadband spectral variability and Fe Kalpha variability characteristics on timescales of days to years. Variability in the Fe Kalpha line is not detected in some objects but is present in others; e.g., in NGC 3516, NGC 4151, and NGC 5548 there are systematic decreases in line flux by factors of similar to2-5 over 3-4 years. The Fe Kalpha line varies less strongly than the broadband continuum, but, like the continuum, exhibits stronger variability toward longer timescales. Relatively less model-dependent broadband fractional variability amplitude (F-var) spectra also show weaker line variability compared with the continuum variability. Comparable systematic long-term decreases in the line and continuum are present in NGC 5548. Overall, however, there is no evidence for correlated variability between the line and continuum, severely challenging models in which the line tracks continuum variations modified only by a light-travel time delay. Local effects such as the formation of an ionized skin at the site of line emission may be relevant. The spectral fitting and F-var spectra both support spectral softening as continuum flux increases.