Abstract

The correlation observed in NGC4151 between the O, UV and X-ray fluxes is explained in terms of reprocessing of hard X-rays by a thick disk that reradiates the incoming energy into O and UV photons. The flatness of the UV spectrum and the upper limits on X-ray reprocessed components (high energy bump, variable part of iron line) demand tight limits on the mass of the central object, the luminosity (absolute and relative to the Eddington one) and the extension of the spectrum in the γ-ray region.

Introduction

Reprocessing of X-rays by a thick medium has been called for to explain two X-ray features observed in several Seyfert galaxies: the iron line and the high energy bump. The fact that reprocessing of hard X-rays may play an important role also in the optical and UV, an idea firstly advanced in, has been recently proposed to account for the short time-scale correlation of optical and UV light curves, too short to be explained by processes directly connected to accretion by a disk.

Similar considerations apply to the case of NGC4151, where the optical, UV and X-ray fluxes are correlated down to a time scale of 1 l.d. - although the correlation between UV and X-ray breaks at higher UV luminosities (we will comment on this behaviour in the following). In this object, however, the absence of an high energy bump as well as of a broad and variable iron line apparently argues against the presence of a thick reprocessor near the central source.