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We aim to study the variability properties of bright hard X-ray selectedActive Galactic Nuclei (AGN) with redshift between 0.3 and 1.6 detected in theChandra Deep Field South (XMM-CDFS) by a long XMM observation. Taking advantageof the good count statistics in the XMM CDFS we search for flux and spectralvariability using the hardness ratio techniques. We also investigated spectralvariability of different spectral components. The spectra were merged in sixepochs (defined as adjacent observations) and in high and low flux states tounderstand whether the flux transitions are accompanied by spectral changes.The flux variability is significant in all the sources investigated. Thehardness ratios in general are not as variable as the fluxes. Only one sourcedisplays a variable HR, anti-correlated with the flux (source 337). Thespectral analysis in the available epochs confirms the steeper when brightertrend consistent with Comptonisation models only in this source. Finding thistrend in one out of seven unabsorbed sources is consistent, within thestatistical limits, with the 15 % of unabsorbed AGN in previous deep surveys.No significant variability in the column densities, nor in the Comptonreflection component, has been detected across the epochs considered. The highand low states display in general different normalisations but consistentspectral properties. X-ray flux fluctuations are ubiquitous in AGN. In general,the significant flux variations are not associated with a spectral variability:photon index and column densities are not significantly variable in nine out ofthe ten AGN over long timescales (from 3 to 6.5 years). The photon indexvariability is found only in one source (which is steeper when brighter) out ofseven unabsorbed AGN. These results are consistent with previous deep samples.

The XMM deep survey in the CDF-S. X. X-ray variability of bright sources