We describe results from a new ground-based monitoring campaign on NGC 5548 , the best studied reverberation-mapped AGN .
We find that it was in the lowest luminosity state yet recorded during a monitoring program , namely L _ { 5100 } = 4.7 \times 10 ^ { 42 } ergs s ^ { -1 } .
We determine a rest-frame time lag between flux variations in the continuum and the H \beta line of 6.3 ^ { +2.6 } _ { -2.3 } days .
Combining our measurements with those of previous campaigns , we determine a weighted black hole mass of M _ { BH } = 6.54 ^ { +0.26 } _ { -0.25 } \times 10 ^ { 7 } M _ { \odot } based on all broad emission lines with suitable variability data .
We confirm the previously-discovered virial relationship between the time lag of emission lines relative to the continuum and the width of the emission lines in NGC 5548 , which is the expected signature of a gravity-dominated broad-line region .
Using this lowest luminosity state , we extend the range of the relationship between the luminosity and the time lag in NGC 5548 and measure a slope that is consistent with \alpha = 0.5 , the naive expectation for the broad line region for an assumed form of r \propto L ^ { \alpha } .
This value is also consistent with the slope recently determined by Bentz et al . for the population of reverberation-mapped AGNs as a whole .