The mass of the central black hole in the Seyfert galaxy NGC 4151

In order to improve the reverberation-mapping-based estimate of the mass of the central supermassive black hole in the Seyfert 1 galaxy NGC 4151, we have reanalyzed archival ultraviolet monitoring spectra from two campaigns undertaken with the International Ultraviolet Explorer. We measure emission-line time delays for four lines, C IV lambda 1549, He II lambda 1640, C III] lambda 1909, and Mg II lambda 2798, from both campaigns. We combine these measurements with the dispersion of the variable part of each respective emission line to obtain the mass of the central object. Despite the problematic nature of some of the data, we are able to measure a mass of (4.14 +/- 0.73) x 10(7) M-circle dot, although this, like all reverberation-based masses, is probably systematically uncertain by a factor of 3-4.