Variability study of Seyfert 2 galaxies with XMM-Newton

We present the results of timing analysis of XMM-Newton observations of Seyfert 2 galaxies in order to search for differences in the mean properties of Seyfert 1 galaxies and Seyfert 2 galaxies. We selected 13 Seyfert 2 galaxies from the XMM-Newton archive that have hard X-ray components in their spectra and calculated the excess variance (sigma(2)(rms)) in the 2-10 keV band. We found that six Seyfert 2 galaxies (3C 98, IRAS 05189-2524, MCG - 5-23-16, NGC 6300, UGC 4203, and PKS 1814-637) have buried luminous nuclei and that the nuclei have timing properties similar to those of Seyfert 1 nuclei. This indicates that these galaxies are candidates for having buried Seyfert 1 nuclei as expected by the unified Seyfert model. The first five galaxies show significant time variability. The amplitude of the time variability of IRAS 05189-2524 is similar to that of narrow-line Seyfert 1 galaxies. In contrast, the amplitude of variability of the seven other galaxies is quite small, much smaller than that of Seyfert 1 galaxies with similar X-ray luminosity. The lack of short time variability in these objects is explained by the dominance of the reflection component in three galaxies (Mrk 3, Mrk 463, and NGC 7582), and by the presence of very massive black holes and an inferred low accretion rate in the other three galaxies (NGC 1052, NGC 4507, and NGC 7172). For Mrk 348, the significant time variability that is expected based on the estimate of the central black hole mass was not detected.