The Narrow Line Seyfert 1 galaxy NGC 4051 was observed in one of its
prolonged low-lux states by XMM-Newton in November 2002. Here we present the
results of an analysis of 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 Comptonised 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 Comptonisation.
Additional constant thermal and reflection components, together with absorption
by ionised 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.Comment: Accepted for publication in MNRAS. 13 pages, 10 figs. A higher
resolution eps version of Fig. 8 is included in the source file