Supermassive Black Hole and Broad-line Region in NGC 5548: 2023 Reverberation Mapping Results

TL;DR

This study presents 2023 reverberation mapping results for NGC 5548, revealing the structure, dynamics, and stability of its broad-line region and supermassive black hole through spectroscopic monitoring over multiple years.

Contribution

It provides new measurements of emission line lags, black hole mass, and BLR kinematics, demonstrating the stability and evolution of the BLR over nearly a decade.

Findings

Measured emission line lags for multiple lines, revealing ionization stratification.

Estimated supermassive black hole mass as (2.6±1.1)×10^8 M_sun.

Observed temporal evolution of BLR kinematics from virialized to inflow/outflow signatures.

Abstract

We present the results of the 2023 spectroscopic reverberation mapping (RM) campaign for active galactic nuclei (AGN) of NGC 5548, continuing our long-term monitoring program. Using the Lijiang 2.4-meter telescope, we obtained 74 spectra with a median cadence of 1.9 days. Through detailed spectral decomposition, we measured the light curves of the optical continuum at 5100~\AA\ and the broad He~{\sc ii}, He~{\sc i}, H$\gamma$, and H$\beta$ emission lines. The time lags of these lines relative to the continuum are measured as $1.3^{+1.6}_{-0.6}$, $2.3^{+1.5}_{-2.1}$, $10.0^{+2.0}_{-1.8}$, and $15.6^{+2.6}_{-2.9}$ days (rest-frame), respectively. Velocity-resolved lag profiles for H$\gamma$ and H$\beta$ were constructed. Combined with data from previous seasons (2015$-$2021), we find that the radial ionization stratification of the broad-line region (BLR) is stable; the average virial mass of the supermassive black hole in NGC~5548 is $(2.6\pm1.1)\times 10^{8}M_{\odot}$, consistent with the $M_{\rm BH}-\sigma_*$ relation; the broad He~{\sc ii} line exhibits the largest responsivity, followed by broad He~{\sc i} (or H$\gamma$) and H$\beta$ lines; the BLR kinematics show significant temporal evolution, transitioning from virialized motions to signatures of inflow and outflow. Furthermore, an analysis of 35 years of historical data confirms a 3.5-year time lag between variations in the optical luminosity and the BLR radius, potentially implicating the role of radiation pressure or dynamical structure changes in the inner accretion disk. Long-term campaign demonstrates that the BLR in NGC 5548 is a robust yet dynamically evolving entity, providing crucial insights into AGN structure and accretion physics.