The nature of X-ray spectral variability in MCG-6-30-15
  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 $kT_{\rm
BB} \sim 100$ 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.
