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 re-emission 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). Alternatively, the correlation can be explained by reprocessing of X-rays and gamma-rays by a cold accretion disc. A specific model in which X-rays and gamma-rays are emitted by a dissipative patchy corona and have the spectrum cut off at ~100 keV (as seen by GRO/OSSE) is shown to be energetically possible even if it does not give as satisfactory a fit as does the transmission model. However, the model predicts a Compton reflection spectral component, the presence of which remains to be tested by future detectors sensitive above 10 keV.