X-ray spectral variability of the Seyfert galaxy NGC 4051

We report on the X-ray spectral variability of the Seyfert 1 galaxy NGC 4051 observed with the Rossi X-ray Timing Explorer (RXTE) during a 1000-d period between 1996 May and 1999 March. The spectra were obtained as part of monitoring observations and from two long observations using the RXTE Proportional Counter Array (PCA). During the monitoring period the 2-10 keV flux of NGC 4051 varied between 10(-12) and 7 x 10(-11) erg cm (-2) s (-1). We re-analysed RXTE PCA observations from a distinct low state in 1998 May using the latest background and detector response models. The RXTE and BeppoSAX observations of NGC 4051 during the low state show a very hard spectrum with a strong unresolved fluorescence line. This emission, probably caused by reflection from a molecular torus, is likely to be constant over long time-scales and is therefore assumed as an underlying component at all flux states. By subtracting the torus component we are able to determine the spectral variability of the primary continuum. In the variable component we observe a strong anticorrelation of X-ray flux and spectral hardness in the PCA energy band. We show that the changes in hardness are caused by slope variability of the primary power-law spectrum rather than by changing reflection or variable photoelectric absorption. The primary spectral index varies between <Gamma= 1.6 for the faintest states and Gamma= 2.3 during the brightest states, at which level the spectral index approaches an asymptotic value. We find that the response of the flux of the 6.4-keV iron fluorescence line to changes in the continuum flux depends on the time-scale of the observation. The profile of the line is very broad and indicates an origin in the innermost regions of the accretion disc.