Temporal and spectral variability of AGN with RXTE

Abstract

We present RXTE PCA observations of temporal and spectral variability of the four X-ray bright Seyfert galaxies NGC 4051, NGC 5506, NGC 5548 and MGC-6-30-15 covering a period of 18 months from February 1996. We find evidence of a knee in the long timescale power spectrum of NGC 4051 at a frequency of 5×10 −5 Hz. There is possible evidence of a knee at a slightly lower frequency in the higher luminosity source MCG-6-30-15 but no knees have been found in the still higher luminosity sources NGC 5506 and NGC 5548. If the knee frequency scales inversely with the central black hole mass then, by comparison with galactic black hole X-ray binaries, we deduce a mass of approximately 10 5 M ⊙ in NGC 4051. Strong continuum reflection components and iron fluorescent lines are seen in all four galaxies and we find evidence of strong X-ray spectral variability in all four galaxies. Here we discuss the spectra of NGC 5506 and MCG-6-30-15 as NGC 4051 is discussed elsewhere in these proceedings [1] and a possibly confusing BL Lac lies within the PCA field of view of NGC 5548. In both NGC 5506 and MCG-6-30-15 we find strong evidence for a steeping of the continuum spectral slope as the intensity increases. In MCG-6-30-15 the iron line flux increases only weakly as the continuum flux rises and, indeed, the iron line flux is consistent with being constant. In NGC 5506, however, there is an approximately linear relationship between the iron line flux and continuum flux with only weak evidence for an additional steady component. One explanation, although by no means confirmed, of the spectral index variability and relative quiescence of the iron line flux is that, in addition to rapidly varying steep spectrum continuum emission, there is also a harder reflected component from the surrounding torus. The torus component would not vary on the timescales of our observations and would dominates at very low flux levels. The difference between the variability of NGC 5506 and MCG-6-30-15 may be a lesser contribution from the torus in NGC 5506.