X-ray spectral variations in NGC 4151.

Abstract

In their analysis of the EXOSAT ME data on the Seyfert NGC 4151, Yaqoob, Warwick & Pounds found that the observed tendency of the spectrum to soften as the X-ray flux increases is due to variations in the absorbing column, which are anti-correlated with the source intensity, thus favouring a 'warm absorber' model. In this paper a re-analysis of the same data is carried out and it is found that the evidence of variations in the spectral slope which are correlated with the intensity is certainly no less, and possibly more significant than the one claimed for the absorbing column. It is then shown that the argument in favour of the 'warm absorber' model is not so stringent, while the data, when the slope-intensity correlation is taken into account, are in good agreement with the cold gas 'partial covering' model - and in particular with the related expectations of random changes in the configuration of the absorbing gas.