We report the results of a series of Ginga observations of the archetypal Seyfert galaxy NGC 4151 carried out over a period of approximately 20 months from mid-1987 to early 1989. The improved sensitivity of the Ginga Large Area Counter compared to previous proportional counter experiments has allowed much tighter constraints to be placed on many of the spectral properties of NGC 4151 in the medium-energy X-ray regime. We find that the softening of the X-ray spectrum as the source brightens is best modelled in terms of variations in the spectral index of the underlying power-law continuum, a phenomenon which may relate directly to the emission processes giving rise to the broad-band X-ray to gamma-ray continuum of the source. Spectral variations due to changes in the line-of-sight absorption are also evident on time-scales of months to years. The weighted average value for the iron K-edge energy is 7.21 +/- 0.10 keV, indicating that the absorbing gas is not strongly photo-ionized. The bulk of the X-ray absorption can probably be associated with the broad-line emitting clouds, although the intrinsic width of the 6.4-keV iron K-alpha line (approximately 38 000 km s-1 FWHM) also implies the presence of cold matter within 1 light-day of the central source. The flux in the 'soft excess' observed below approximately 4 keV appears to be well correlated with the continuum level. Spatial non-uniformity and/or abundance anomalies in the X-ray absorbing medium currently provide the most plausible explanation of this persistent feature of the X-ray spectrum of NGC 4151.
