We present ROSAT observations of NGC 4051 from 1991 November 15 (28-ks
exposure) and 1991 December 12 (4.7-ks exposure). During these observ
ations the source spectrum is well described empirically by a simple p
ower law with energy index similar to 1.5 and an absorption edge with
optical depth similar to 0.8 at similar to 0.85 keV or, more physicall
y, by a warm absorber with an underlying power law of energy index 1.1
9, similar to that seen at higher energies, an ionization parameter xi
similar to 300 and a column of warm gas N-warm similar to 8 x 10(22)
cm(-2). During the November observation the source flux varies by a fa
ctor of similar to 10, and there is some evidence that xi may increase
with increasing flux. However, the correspondence between luminosity
and xi is not simple, and there is a suggestion that xi may lag behind
luminosity by a few thousand seconds. Lf this lag represents a coolin
g time-scale for the warm absorber, we can crudely estimate the gas de
nsity as similar to 10(7) cm(-3), and hence a thickness of similar to
5 x 10(15) cm for the warm gas cloud. The December observation is not
well fitted by a simple warm absorber, and the fit is much improved by
the addition of an emission line at 0.63 +/- 0.07 keV, i.e. at the en
ergy of OVIII Ly alpha (0.65 keV). We note that N-warm as deduced from
the November observation is noticeably less than the value deduced by
Pounds et al. ( similar to 3.7 x 10(23) cm(-2)) from observations one
year prior to ours, and there is an indication that the value deduced
from the December observation is also different from that of the Nove
mber observation, demonstrating the presence of large-scale inhomogene
ities and motions in the warm gas in the nuclei of active galaxies.