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.
@article{arxiv.astro-ph/0702644,
title = {NGC 5548 in a Low-Luminosity State: Implications for the Broad-Line Region},
author = {Misty C. Bentz and Kelly D. Denney and Edward M. Cackett and Matthias Dietrich and Jeffrey K. J. Fogel and Himel Ghosh and Keith D. Horne and Charles Kuehn and Takeo Minezaki and Christopher A. Onken and Bradley M. Peterson and Richard W. Pogge and Vladimir I. Pronik and Douglas O. Richstone and Sergey G. Sergeev and Marianne Vestergaard and Matthew G. Walker and Yuzuru Yoshii},
journal= {arXiv preprint arXiv:astro-ph/0702644},
year = {2011}
}
Comments
24 pages, 3 tables, 7 figures, accepted for publication in ApJ