2004MNRAS.347.1345U

The narrow-line Seyfert 1 galaxy, NGC 4051, was observed in one of its prolonged low flux states by XMM-Newton in 2002 November. Here we present the results of an analysis of the European Photon Imaging Camera (EPIC) pn data obtained during the observation. Within the low state, the source shows complex spectral variability which cannot easily be explained by any simple model. However, by making a `flux-flux' plot which combines the low state data with data obtained during a normal flux state, we demonstrate that the extremely hard spectrum observed above 2 keV results from a continuation of the spectral variability seen in the normal state, which is caused by spectral pivoting of the power-law continuum. The pivoting power law appears to be attached to a Comptonized thermal component of variable flux (blackbody temperature kT ∼ 0.1 keV, consistent with the small black hole mass in NGC 4051) which dominates the soft X-ray band in the low state, and is probably the source of seed photons for Comptonization. Additional constant thermal and reflection components, together with absorption by ionized gas, seem to be required to complete the picture and explain the complex X-ray spectral variability seen in the low state of NGC 4051.