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 black body component for the soft excess ) and of a two–component one ( power law plus ionized reflection from the accretion disc ) .
Both models reproduce the source spectral variability and can not 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 black body law ( L _ { BB } \propto T ^ { 4 } ) 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 PG quasars .
Moreover , although the spectral slope is correlated with flux , which is consistent with spectral pivoting , the hardest photon indexes are so flat ( \Gamma \sim 1.3–1.4 ) as to require rather unusual scenarios .
Furthermore , the very low flux states exhibit an inverted \Gamma –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 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 from the central black hole .