Flux-Flux Analysis

The flux-flux plot (FFP) is a model-independent technique used to characterize spectral variability in Seyfert galaxies. It consists in modeling the shape of the flux in various energy bands plotted versus the flux in a reference band, in order to search for any constant emission component. I showed that this technique can be a robust and straightforward way to break degeneracies between various spectral models, since it allows us to identify the various variable and stable components in the X-ray spectra of a given source.

I presented guidelines for its use, by identifying the optimal choice of time and energy bins and the effect of the Poisson noise on its results. I applied this technique to XMM-Newton and NuSTAR observations of three sources: IRAS 133224-3809, MGC-06-30-15, and SWIFT J2127.4+5654. These analyses led to the identification of variable and constant components in these systems.

Variable component in MCG-6-30-15

Variable component in
Swift J2127.4

The constant components in the latter two sources consist of a blackbody-like spectrum dominating below 2 keV and a neutral reflection dominating in the 2-40 keV range. The presence of partial-covering absorbers in the line of sight has been proposed as an alternative to the relativistic reflection, in order to explain part of the X-ray variability, the apparent, broad red wing of the Fe line, and the spectral curvature below 10 keV. However, using this method, I could explain the dependence of the light curve variance on energy, and rule out the presence of variable partial covering absorption which could mimic a broad red-wing of the Fe line.

The constant component in Swift J21217.

Observed variance versus Expected variance obtained from the FFP analysis in Swift J2127.

The energy spectrum of Swift J2127 in the low and high flux states, fitted with the model derived from the FFP analysis.