New Results on the X-Ray Emission and Its Correlation with the Ultraviolet in NGC 4151
Piro, L.; Perola, G. C.; Altamore, A.; Fiore, F.; Boksenberg, A.; Penston, M. V.; Snijders, M. A. J.; Bromage, G. E.; Clavel, J.; Elvius, A.; Ulrich, M. H.
Italy, United Kingdom
Abstract
Results from observations of the Seyfert galaxy NGC 4151 obtained with Exosat (0.1-25 keV) and IUE (1200-3000 A) during November 1983, and between December 1984 and January 1985, are presented. The stability of the flux during large-scale variations in the UV and X-ray regions suggests that the soft X-ray excess noted by Holt et al. (1980) represents a new spectral component whose temperature is estimated to be 1.8 million K, and luminosity to be 1.7 x 10 to the 42nd erg/s, if it is thermal emission from a hot gas. The observed variable UV and X-ray continua are well correlated down to a time scale of days, and the standard model of a power law with a uniform cold absorber gives a bad fit at low energies in the 1.2-25 keV range. The spectral index of the power law is found to correlate closely with the flux in the 2-10 keV range, with a shallower slope for lower intensity, and the iron fluorescence line at 6.4 keV is seen to respond very slowly to changes in the continuum.