The flux-flux plot ( FFP ) method can provide model-independent clues regarding the X-ray variability of active galactic nuclei .
To use it properly , the bin size of the light curves should be as short as possible , provided the average counts in the light curve bins are larger than \sim 200 .
We apply the FFP method to the 2013 , simultaneous XMM-Newton and NuSTAR observations of the Seyfert galaxy MCG–6-30-15 , in the 0.3–40 keV range .
The FFPs above \sim 1.6 keV are well-described by a straight line .
This result rules out spectral slope variations and the hypothesis of absorption driven variability .
Our results are fully consistent with a power-law component varying in normalization only , with a spectral slope of \sim 2 , plus a variable , relativistic reflection arising from the inner accretion disc around a rotating black hole .
We also detect spectral components which remain constant over \sim 4.5 days ( at least ) .
At energies above \sim 1.5 keV , the stable component is consistent with reflection from distant , neutral material .
The constant component at low energies is consistent with a blackbody spectrum of kT _ { BB } \sim 100 eV .
The fluxes of these components are \sim 10 - 20 \% of the average continuum flux ( in the respective bands ) .
They should always be included in the models that are used to fit the spectrum of the source .
The FFPs below 1.6 keV are non-linear , which could be due to the variable warm absorber in this source .