The exceptional X-ray variability of the dwarf Seyfert nucleus NGC 4395

An analysis of the X-ray variability of the low-luminosity Seyfert nucleus NGC 4395, based on a long XMM-Newton observation, is presented. The power spectrum shows a clear break from a flat spectrum (alpha approximate to 1) to a steeper spectrum (alpha approximate to 2) at a frequency f(br) = 0.5-3.0 x 10(-3) Hz, comparable to the highest characteristic frequency found previously in a Seyfert galaxy. This extends the measured M-BH - f(br) values to lower M-BH than previous studies of Seyfert galaxies, and is consistent with an inverse scaling of variability frequency with black hole mass. The variations observed are among the most violent seen in an active galactic nuclei to date, with the fractional rms amplitude (F-var)exceeding 100 per cent in the softest band. The amplitude of the variations seems intrinsically higher in NGC 4395 than most other Seyfert galaxies, even after accounting for the differences in characteristic frequencies. The origin of this difference is not clear, but it is unlikely to be a high accretion rate (L/L-Edd less than or similar to 20 per cent for NGC 4395). The variations clearly follow the linear rms-flux relation, further supporting the idea that this is a ubiquitous characteristics of accreting black holes. The variations are highly coherent between different energy bands with any frequency-dependent time delay limited to less than or similar to 1 per cent.