Reverberation Mapping of Changing-look Active Galactic Nucleus NGC 3516

TL;DR

This study conducted a 6-month reverberation mapping campaign of the changing-look AGN NGC 3516, measuring emission line lags and black hole mass, and analyzing BLR structure stability during spectral type changes.

Contribution

First velocity-resolved reverberation mapping of Hα in NGC 3516, providing insights into BLR structure stability during changing-look phenomena.

Findings

Measured Hα and Hβ lags of ~7.5 days.

Estimated black hole mass of ~2.4×10^7 solar masses.

BLR structure remains stable during spectral changes.

Abstract

The changes of broad emission lines should be a crucial issue to understanding the physical properties of changing-look active galactic nucleus (CL-AGN). Here, we present the results of an intensive and homogeneous 6-month long reverberation mapping (RM) monitoring campaign during a low-activity state of the CL-AGN Seyfert galaxy NGC 3516. Photometric and spectroscopic monitoring was carried out during 2018--2019 with the Lijiang 2.4 m telescope. The sampling is 2 days in most nights, and the average sampling is $\sim$3 days. The rest frame time lags of H$\alpha$ and H$\beta$ are $\tau_{\rm{H}\alpha}=7.56^{+4.42}_{-2.10}$ days and $\tau_{\rm{H}\beta}=7.50^{+2.05}_{-0.77}$ days, respectively. From a RMS H$\beta$ line dispersion of $\sigma_{\rm{line}} = 1713.3 \pm 46.7$ $\rm{km}$ $\rm{s^{-1}}$ and a virial factor of $f_{\sigma}$ = 5.5, the central black hole mass of NGC 3516 is estimated to be $M_{\rm{BH}}= 2.4^{+0.7}_{-0.3} \times 10^{7} M_{\odot}$, which is in agreement with previous estimates. The velocity-resolved delays show that the time lags increase towards negative velocity for both H$\alpha$ and H$\beta$. The velocity-resolved RM of H$\alpha$ is done for the first time. These RM results are consistent with other observations before the spectral type change, indicating a basically constant BLR structure during the changing-look process. The CL model of changes of accretion rate seems to be favored by long-term H$\beta$ variability and RM observations of NGC 3516.