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Reverberation Mapping of Changing-look Active Galactic Nucleus NGC 3516

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$3 days. The rest frame time lags of H$α$ and H$β$ are $τ_{\rm{H}α}=7.56^{+4.42}_{-2.10}$ days and $τ_{\rm{H}β}=7.50^{+2.05}_{-0.77}$ days, respectively. From a RMS H$β$ line dispersion of $σ_{\rm{line}} = 1713.3 \pm 46.7$ $\rm{km}$ $\rm{s^{-1}}$ and a virial factor of $f_σ$ = 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$α$ and H$β$. The velocity-resolved RM of H$α$ 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$β$ variability and RM observations of NGC 3516.