The X-ray variability of the Seyfert 1 galaxy MCG-6-30-15 from longASCAandRXTEobservations

Abstract

We present an analysis of the long Rossi X-ray Timing Explorer (RXTE) observation of the Seyfert 1 galaxy MCG–6-30-15, taken in 1997 July. We have previously used the data to place constraints for the first time on the iron abundance–reflection fraction relationship, and now expand the analysis to investigate in detail the spectral X-ray variability of the object. Our results show that the behaviour is complicated. We find clear evidence from colour ratios and direct spectral fitting that changes to the intrinsic photon index are taking place. In general, spectral hardening is evident during periods of diminished intensity, and in particular, a general trend for harder spectra is seen in the period following the hardest RXTE flare. Flux-correlated studies further show that the 3–10 keV photon index Γ3–10 steepens, while that in the 10–20 keV band, Γ10–20, flattens with flux. The largest changes come from the spectral index below 10 keV; however, changes in the intrinsic power-law slope (shown by changes in Γ3–10), and reflection (shown by changes in Γ10–20) both contribute in varying degrees to the overall spectral variability. We find that the iron-line flux FKα is consistent with being constant over large time intervals on the order of days (although tentative evidence exists which show that FKα changes on shorter time intervals of order ≲10 ks during time periods surrounding flare events), and has an equivalent width which anticorrelates with the continuum flux and reflection fraction. A possible interpretation for the iron-line flux constancy and the relative Compton reflection increase with flux from the flux-correlated data is an increasing ionization of the emitting disc surface, while spectral analysis of short time intervals surrounding flare events hints tentatively at observed spectral responses to the flare. We present a simple model for partial ionization where the bulk of the variability comes from within 6rg. Temporal analysis further provides evidence for possible time (≲1000 s) and phase (φ∼0.6 rad) lags. Finally, we report an apparent break in the power density spectrum (∼4–5×10−6 Hz) and a possible 33-hour period. Estimates for the mass of the black hole in MCG–6-30-15 are discussed in the context of spectral and temporal findings.