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Fig. 5.

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Long-term evolution of the main spectral parameters. From top to bottom: column density in units of 1022 cm−2, photon index Γ (generally between 1.6−2.0), intrinsic (de-absorbed) 2−10 keV luminosity, EW of the Fe Kα line (only for the ABSPL model and for values from the literature; the inset shows the logarithmic flux of the Fe Kα line in the ABSPL model, for the data considered in this work). The values referring to the three models employed in our analysis are labeled with different empty markers and colors (blue circles, orange squares and green upward triangles for the ABSPL, BORUS, and WARPDSK models, respectively), and are fully consistent with each other. Red diamonds, the blue star and the purple downward triangle refer to ASCA (Reynolds et al. 2000; Terashima et al. 2002), BeppoSAX (Fiore et al. 2001), and Suzaku (Reynolds et al. 2009) observations, respectively. Top panel: dashed black line is a simple sinusoid with a period of 10 years, and it is not a fit to the data. Most notably, the intrinsic de-absorbed 2–10 keV luminosity shows an approximately steady decrease of more than a factor of 3 over ∼20 years.

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