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

Andrzej A. Zdziarski

Copernicus Astronomical Center, Warsaw, Poland

Pawel Magdziarz

Astronomical Observatory, Jagiellonian University, Cracow, Poland

We find the UV/X-ray correlation observed in NGC 4151 by EXOSAT and IUE can be explained by reemision of the X-ray flux absorbed by cold clouds in the line of sight. The 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) provides a good fit to the data. However, the model predicts a Compton-reflection spectral component, which presence remains to be tested by future detectors sensitive above 10 keV.

Key words: galaxies: individual: NGC 4151 - galaxies: Syfert - X-rays: galaxies - ultraviolet: galaxies - accretion, accretion disks - ultraviolet: spectra