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The relationship between the emission-line equivalent width and the continuumluminosity, so called the Baldwin effect, plays an important role in studyingthe physics of the broad line region of AGNs. Using the archived ultravioletspectra obtained by IUE, HST, and HUT in 1978-2002, we investigated theintrinsic CIV Baldwin effect of a well-studied Seyfert 1 galaxy NGC 4151. Bothits continuum flux and CIV emission-line flux varied about two orders ofmagnitude in more than two decades, making it one of the best targets forstudying the slope variation of the Baldwin effect. We fitted the CIV lineprofile of the 490 archived UV spectra of NGC 4151 with the same modelconsisting of a few Gaussian components, and derived the slope in the log-logplot for the total flux of CIV emission line against the UV continuum flux indifferent observation epochs.We found that the slope is not constant for NGC4151, as it varies from 0.58 in the highest flux epoch to 0.83 in the lowestflux epoch. The slope evidently decreases as the continuum flux increases,which reinforces the previous findings of the non-constant slope in the H_betaBaldwin effect of NGC 5548 and the CIV Baldwin effect of Fairall 9. Our resultsuggests that such a non-constant slope may not be unusual for AGNs. Itsphysical origin is probably related to the different non-linear responses ofthe emission line to the variable ionizing continuum caused by the differentaccretion modes at different luminosity levels. We briefly discuss the effectsof various absorption components in the CIV line profile of NGC 4151 and arguethat the slope variation is not driven mainly by the absorption effect.Intensive, long-term and high resolution spectral observations of stronglyvariable AGNs are needed in the future to confirm our result.Comment: 8 pages, 5 figures, references added and typo corrected, accepted for  publication in A&

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