Velocity-resolved Reverberation Mapping of Changing-look AGN NGC 2617.

NGC 2617 has attracted a lot of attention after the detection of the changes in spectral type, and its geometry and kinematics of broad-line region (BLR) are still ambiguous. In this paper, we present the high cadence ($\sim$ 2 days) reverberation mapping campaign of NGC 2617 from 2019 October to 2020 May undertaken at Lijiang 2.4 m telescope. For the first time, the velocity-resolved reverberation signature of the object was successfully detected. Both H$\alpha$ and H$\beta$ show an asymmetrical profile with a peak in the velocity-resolved time lags. For each of both lines, the lag of the line core is longer than those of the relevant wings, and the peak of the velocity-resolved lags is slightly blueshifted. These characteristics are not consistent with the theoretical prediction of the inflow, outflow or Keplerian disk model. Our observations give the time lags ofH$\alpha$, H$\beta$, H$\gamma$, and He I, with a ratio of $\tau_{\rm{H}\alpha}$:$\tau_{\rm{H}\beta}$:$\tau_{\rm{H}\gamma}$:$\tau_{\rm{He~I}}$ = 1.27:1.00:0.89:0.20, which indicates a stratified structure in the BLR of the object. It is the first time that the lags of H$\alpha$ and He I are obtained. Assuming a virial factor of $f$ = 5.5 for dispersion width of line, the masses of black hole derived from H$\alpha$ and H$\beta$ are $\rm{23.8^{+5.4}_{-2.7}}$ and $\rm{21.1^{+3.8}_{-4.4}} \times 10^{6}M_{\odot}$, respectively. Our observed results indicate the complexity of the BLR of NGC 2617.