THE MASS OF THE BLACK HOLE IN THE QUASAR PG 2130+099

We present the results of a recent reverberation-mapping campaign undertaken to improve measurements of the radius of the broad-line region and the central black hole mass of the quasar PG 2130+099. Cross-correlation of the 5100 angstrom continuum and H beta emission-line light curves yields a time lag of 22.9(-4.3)(+4.3) days, corresponding to a central black hole mass M-BH = (3.8 +/- 1.5) x 10(7) M-circle dot. This value supports the notion that previous measurements yielded an incorrect lag. We reanalyze previous data sets to investigate the possible sources of the discrepancy and conclude that previous measurement errors were apparently caused by a combination of undersampling of the light curves and long-term secular changes in the H beta emission-line equivalent width. With our new measurements, PG 2130+099 is no longer an outlier in either the R-BLR-L or the M-BH-sigma* relationship.