The non-constant slope of the C IV Baldwin effect in NGC 4151

Context. The relationship between the emission-line equivalent width and the continuum luminosity, called the Baldwin effect, is important for studies of the physics of the broad-line region of AGNs. Some recent studies have revealed the non-constant slope of the intrinsic Baldwin effect for several Seyfert 1 galaxies. Aims. Using the archived ultraviolet spectra obtained by IUE, HST, and HUT in 1978-2002, we investigated the intrinsic C iv Baldwin effect of the well-studied Seyfert 1 galaxy NGC 4151. Both its continuum flux and C iv emission-line flux varied about two orders of magnitude in more than two decades, making it one of the best targets for studying the slope variations of the Baldwin effect. Methods. We fitted the C iv line profile of the 490 archived UV spectra of NGC 4151 with a spectral model consisting of a few Gaussian components and derived the slope in the log-log plot for the total flux of C iv emission line against the UV continuum flux in different observation epochs. Results. We found that the slope is not constant for NGC 4151, as it varies from 0.58 in the highest flux epoch to 0.83 in the lowest flux epoch. The slope evidently decreases as the continuum flux increases, which reinforces the previous findings of the non-constant slope in the H β Baldwin effect of NGC 5548 and the C iv Baldwin effect of Fairall 9. Conclusions. Our result suggests that such a non-constant slope may not be unusual for AGNs. Its physical origin is probably related to the different non-linear responses of the emission line to the variable ionizing continuum caused by the different accretion modes at different luminosity levels. We briefly discuss the effects of various absorption components in the C iv line profile of NGC 4151 and argue that the slope variation is not driven mainly by the absorption effect. Intensive, long-term, and high-resolution spectral observations of strongly variable AGNs are needed in the future to confirm our result.