The soft X-ray spectrum of NGC 4151.

Abstract

EXOSAT observations of the Seyfert galaxy NGC 4151 over the period 1983 July - 1984 April have revealed a steeply decreasing flux in the 2 - 10 keV band. Other observations have shown that this coincided with a fall in the whole infrared-to-ultraviolet continuum and a marked weakening of the broad emission lines. The X-ray spectral shape, however, remained unchanged, although - in accord with previous measurements - a power-law spectrum attenuated with a simple column of cold gas does not provide a satisfactory spectral fit below about 3 keV, where a lower relative opacity is required. Inclusion of additional low energy data from the EXOSAT telescopes allows the modelling of the absorbing column to be improved and reveals a second, separate, soft X-ray spectral component. It is suggested that this soft component may be thermal emission from a hot intercloud medium in the narrow emission line region.