REVERBERATION MAPPING OF THE BROAD-LINE REGION IN NGC 5548: EVIDENCE FOR RADIATION PRESSURE?

NGC 5548 is the best-observed reverberation-mapped active galactic nucleus with long-term, intensive monitoring. Here we report results from a new observational campaign between 2015 January and July. We measure the centroid time lag of the broad H beta emission line with respect to the 5100 angstrom continuum and obtain tau(cent) = 7.20(-0.35)(+1.33) days in the rest frame. This yields a black hole mass of M-center dot = 8.71(-2.61)(+3.217) x 10(7)M(circle dot) using a broad H beta line dispersion of 3124 +/- 302 km. s(-1) and a virial factor of f(BLR) = 6.3 +/- 1.5 for the broad-line region (BLR), consistent with the mass measurements from previous H beta campaigns. The high-quality data allow us to construct a velocity-binned delay map for the broad H beta line, which shows a symmetric response pattern around the line center, a plausible kinematic signature of virialized motion of the BLR. Combining all the available measurements of H beta time lags and the associated mean 5100 angstrom luminosities over 18 campaigns between 1989 and 2015, we find that the H beta BLR size varies with the mean optical luminosity, but, interestingly, with a possible delay of 2.35(-1.25)(+3.47) years. This delay coincides with the typical BLR dynamical timescale of NGC 5548, indicating that the BLR undergoes dynamical changes, possibly driven by radiation pressure.