2021ApJ...909...18F

The changes of broad emission lines should be a crucial issue in understanding the physical properties of changing-look active galactic nuclei (CL-AGNs). 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 ∼3 days. The rest-frame time lags of Hα and Hβ are τ_Hα=7.56_–2.10^+4.42 and τ_Hβ=7.50_–0.77^+2.05 days, respectively. From an rms Hβ line dispersion of σ_line = 1713.3 ± 46.7 km s^–1 and a virial factor of f_σ = 5.5, the central black hole mass of NGC 3516 is estimated to be M_BH=2.4_–0.3^+0.7×10⁷M_☉, which is in agreement with previous estimates. The velocity-resolved delays show that the time lags increase toward negative velocity for both Hα and Hβ. The velocity-resolved RM of Hα is done for the first time. These RM results are consistent with other observations before the spectral-type change, indicating a basically constant broad-line region structure during the CL process. The CL model of changes of accretion rate seems to be favored by long-term Hβ variability and RM observations of NGC 3516.