2020MNRAS.491.2576W

Characterizing the long-term variability of active galactic nuclei (AGNs) is a key legacy of the Rossi X-ray Timing Explorer (RXTE). We carry out a spectral analysis on a sample of 20 Seyfert 1 galaxies, which had been observed by the RXTE for at least 100 times. All 18 335 spectra are fitted in a uniform way using a power-law component plus an additional Gaussian line when necessary. For any source in our sample, we confirm that the spectrum softens or the photon index, Γ, increases with increasing the 2-10 keV luminosity, LX. However, different source holds distinct Γ-LX/LEdd relation, rather than a common one. We also fit the correlation with a function of Γ = C + β x log (flux), where C is a constant and parameter β is the slope of the correlation. In this way, the increase speed of the Γ-LX/LEdd relation can be depicted with the parameter β. Since our sample contains a large sample of long-term monitored sources, it allows us to explore the second-order effect of spectral evolution. We find that there is an anticorrelation between β and the X-ray Eddington ratio, LX/LEdd. That is, the increasing tendency of Γ with increasing X-ray luminosity becomes slower when the source has a larger X-ray Eddington ratio. Our results may indicate changes in the geometry and/or the viscosity parameter of the accretion disc at different Eddington ratios.