NGC 5548 in a low-luminosity state: Implications for the broad-line region

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; 1042 ergs s(-1). We determine a rest-frame time lag between flux variations in the continuum and the H beta line of 6.3(-2.3)(+2.6) 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) x 10(7) M-circle dot 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 proportional to 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.