The nature of X-ray spectral variability in MCG-6-30-15

TL;DR

This study uses flux-flux plots to analyze X-ray spectral variability in MCG-6-30-15, revealing a dominant variable power-law component, relativistic reflection, and stable spectral components, with implications for modeling active galactic nuclei spectra.

Contribution

First application of the flux-flux plot method to simultaneous XMM-Newton and NuSTAR data of MCG-6-30-15, clarifying the nature of spectral variability and stable components.

Findings

Spectral slope variations are ruled out.

Variable power-law component with a fixed slope of ~2.

Detection of stable reflection and blackbody components.

Abstract

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.