The nature of the X-ray source in NGC 4151

Abstract

Analysis of broad-band X/$γ$-ray spectra of NGC~4151 from contemporaneous observations by {\it GRO}/OSSE, {\it ROSAT}, {\it Ginga} and {\it ASCA}, shows that the data are well modelled with an intrinsic spectrum due to thermal Comptonization. The X-ray spectral index changes from $α\sim$ 0.4 to 0.7, and temperature stays at about 50 keV. The X-ray spectrum varies in such a way that it is consistent with the roughly constant soft $γ$-rays, with pivots at $\sim$ 100 keV. The UV/X-ray correlation observed by {\it EXOSAT} and {\it IUE} can be explained by two specific models with reprocessing of X-rays by cold matter. The first one is based on reemision of the X-ray flux absorbed by clouds in the line of sight. The model predicts no Compton reflection which is consistent with the broad-band spectra. The second model, assumes reprocessing of X-rays and $γ$-rays by a cold accretion disk with dissipative patchy corona. The homogenous corona model is ruled out here, since the hardness of the X-ray spectrum implies that the plasma is photon starved. The accretion disk model predicts Compton reflection which is only marginally allowed by the observations. Both our models satisfy the energy balance, and provide a good fit to the X/$γ$-rays and UV data.