Bentz, Misty C.2; Denney, Kelly D.3; Cackett, Edward M.2; Dietrich, Matthias2; Fogel, Jeffrey K. J.2; Ghosh, Himel2; Horne, Keith D.2; Kuehn, Charles2; Minezaki, Takeo2; Onken, Christopher A.2; Peterson, Bradley M.2; Pogge, Richard W.2; Pronik, Vladimir I.2; Richstone, Douglas O.2; Sergeev, Sergey G.2; Vestergaard, Marianne3; Walker, Matthew G.2; Yoshii, Yuzuru2
1 Dark Cosmology Centre, The Niels Bohr Institute, Faculty of Science, Københavns Universitet2 unknown3 Dark Cosmology Centre, The Niels Bohr Institute, Faculty of Science, Københavns Universitet
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 x 10^42 ergs s^-1. We determine a rest-frame time lag between flux variations in the continuum and the Hbeta 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) x 10^7 M_sun 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 ~ 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.