New results on the X-ray emission and its correlation with the ultraviolet in NGC 4151

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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.

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