Reverberation Mapping of the Broad-line Region in NGC 5548: Evidence for Radiation Pressure?

TL;DR

This study presents new reverberation mapping results of NGC 5548, measuring the Hβ line lag and black hole mass, and finds evidence suggesting the broad-line region is influenced by radiation pressure causing dynamical changes.

Contribution

The paper provides high-quality velocity-resolved delay maps and long-term analysis indicating radiation pressure impacts the BLR dynamics in NGC 5548, a novel insight into AGN physics.

Findings

Measured Hβ lag of 7.20 days in 2015 campaign.

Constructed symmetric velocity-binned delay map of the BLR.

Found a possible 2.35-year delay in BLR size variation linked to luminosity changes.

Abstract

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 $\tau_{\rm cent} = 7.20^{+1.33}_{-0.35}$ days in the rest frame. This yields a black hole mass of $M_{\bullet}=8.71^{+3.21}_{-2.61} $x$ 10^{7}M_{\odot}$ using a broad H$\beta$ line dispersion of $3124\pm302$ km s$^{-1}$ and a virial factor of $f_{_{\rm BLR}}=6.3\pm1.5$ 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.