XMM-Newton study of the complex and variable spectrum of NGC 4051

We study the X-ray spectral variability of the narrow line Seyfert 1 galaxy NGC 4051 as observed during two XMM-Newton observations. To gain insight on the general behaviour, we first apply model-independent techniques such as rms spectra and flux-flux plots. We then perform time-resolved spectral analysis by splitting the observations into 68 spectra (2 ks each). The data show evidence for a neutral and constant-reflection component and for constant emission from photoionized gas, which are included in all spectral models. The nuclear emission can be modelled both in terms of a `standard model' [pivoting power-law plus a blackbody (BB) component for the soft excess] and of a two-component one (power law plus ionized reflection from the accretion disc). Both the models reproduce the source spectral variability and cannot be distinguished on a statistical ground. The distinction has thus to be made on a physical basis. The standard model results indicate that the soft excess does not follow the standard BB law (L_BB ~ T⁴), despite a variation in luminosity by about one order of magnitude. The resulting temperature is consistent with being constant and has the same value as observed in the PG quasars. Moreover, although the spectral slope is correlated with flux, which is consistent with spectral pivoting, the hardest photon indices are so flat (Γ ~ 1.3-1.4) as to require rather unusual scenarios. Furthermore, the very low flux states exhibit an inverted Γ-flux behaviour which disagrees with a simple pivoting interpretation. These problems can be solved in terms of the two-component model in which the soft excess is not thermal, but due to the ionized reflection component. In this context, the power law has a constant slope (about 2.2) and the slope-flux correlation is explained in terms of the relative contribution of the power-law and reflection components which also explains the shape of the flux-flux plot relationship. The variability of the reflection component from the inner disc closely follows the predictions of the light-bending model, suggesting that most of the primary nuclear emission is produced in the very innermost regions, only a few gravitational radii (r_g) from the central black hole.