The mass of the black hole in 3C 273

In this paper we apply the reverberation method to determine the mass of the black hole in 3C 273 from the Ly alpha and C.. emission lines using archival IUE observations. Following the standard assumptions of the method, we find a maximum-likelihood estimate of the mass of 6.59 x 10(9) M-circle dot, with a 1 sigma confidence interval 5.69-8.27 x 10(9) M-circle dot. This estimate is more than one order of magnitude larger than that obtained in a previous study using Balmer lines. We reanalyze the optical data and show that the method applied to the H alpha, H beta, and H gamma Balmer lines produce mass estimates lower by a factor 2.5, but already much larger than the previous estimate derived from the same lines. The finding of such a high mass in a face-on object is a strong indication that the gas motion is not confined to the accretion disk. The new mass estimate makes 3C 273 accreting with an accretion rate about six times lower than the Eddington rate. We discuss the implications of our result for the broad-line-region size and black-hole mass vs. luminosity relationships for the set of objects for which reverberation black-hole masses have been obtained. We find that, while objects with super-Eddington luminosities might theoretically be possible, their existence is not necessarily implied by this sample.