The origin of the correlation between the UV and X-rays in NGC 4151

Abstract

We propose that the UV/X-ray correlation observed in NGC 4151 by EXOSAT and IUE is due to reemission of the X-ray flux absorbed by cold clouds in the line of sight. This transmission model satisfies the energy balance, provides a good fit to the X-ray and UV data, and predicts no Compton reflection of X-rays (as observed). The correlation can alternatively be explained by reprocessing of X-rays and gamma-rays by a cold accretion disk. A specific model in which X-rays and gamma-rays are emitted by a dissipative patchy corona and have the spectrum cut off at about 100 keV (as seen by OSSE) is shown to be energetically possible even if it does not give as satisfactory a fit as the transmission model. However, the model predicts a Compton- reflection spectral component, whose presence remains to be tested by future detectors sensitive above 10 keV.