Hard X-ray spectral variability of the brightest Seyfert AGN in the Swift/BAT sample

Aims. We analyse data from a 58 month long, continuous Swift/Burst Alert Telescope (BAT) series of observations of the five brightest Seyfert galaxies at hard X-rays, to study their flux and spectral variability in the 20-100 keV energy band. The column density in these objects is less than 10(24) cm(-2), which implies that the Swift/BAT data allow us to study the "true" variability of the central source. Methods. We used 2-day binned light curves in the (20-50) and (50-100) keV bands to estimate their fractional variability amplitude, and the same band 20-day binned light curves to compute hardness ratios and construct "colour-flux" diagrams. We also considered a thermal Comptonization model, together with a reflection component with constant flux, and produced model "colour-flux" diagrams, assuming realistic variations in the model parameter values, which we then compared with the observed diagrams. Results. All objects show significant variations, with an amplitude that is similar to the AGN variability amplitude at energies below 10 keV. We found evidence of an anti-correlation between variability amplitude and black hole mass. The light curves in both bands are well-correlated with each other, with no significant delays on time scales as short as two days. We find that both NGC 4151 and NGC 2110 display no spectral variability, but that there is a significant anti-correlation between hardness ratios and source flux in NGC 4388 (and NGC 4945, IC 4329, to a lesser extent). This "softer when brighter" behaviour is similar to that observed at energies below 10 keV, and cannot be explained if the continuum varies only in flux; the intrinsic shape should also steepen with increasing flux. Conclusions. The detection of significant flux variations indicate that the central source in these objects is intrinsically variable on time scales as short as similar to 1-2 days. The intrinsic slope of the continuum varies with the flux (at least in NGC 4388). The positive "spectral slope-flux" correlation can be explained if the temperature of the hot corona decreases with increasing flux. The lack of spectral variations in two objects (NGC 4151 and NGC 2110) might indicate that they are in a different "state", as their accretion rate is less than 1% of the Eddington limit (significantly lower than the rate of the other three objects in the sample).