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 January and July, 2015. We measure the centroid time lag of the broad H$\beta$ emission line with respect to the 5100 \AA continuum and obtain ${\tau }_{\mathrm{c}\mathrm{e}\mathrm{n}\mathrm{t}}=7.20_{-0.35}^{+1.33}$ days in the rest frame. This yields a black hole mass of $M_{\bullet }=8.71_{-2.61}^{+3.21}$x${10}^7{M}_{\odot }$ using a broad H$\beta$ line dispersion of $3124\pm 302$ km s${}^{-1}$ and a virial factor of $f_{{}_{\mathrm{B}\mathrm{L}\mathrm{R}}}=6.3\pm 1.5$ 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 {\AA} 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 $2.35_{-1.25}^{+3.47}$ yrs. This delay coincides with the typical BLR dynamical timescale of NGC 5548, indicating that the BLR undergoes dynamical changes, possibly driven by radiation pressure.