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

Abstract

NGC 2617 attracted a lot of attention after the detection of changes in its spectral type; the geometry and kinematics of the broad-line region (BLR) are still ambiguous. In this paper, we present the high cadence (∼2 days) reverberation mapping campaign of NGC 2617 from 2019 October to 2020 May undertaken at the Lijiang 2.4 m telescope. For the first time, the velocity-resolved reverberation signature of the object was successfully detected. Both H α and H β show an asymmetrical profile with a peak in the velocity-resolved time lags. For both of the 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 of H α , H β , H γ , and He i , with a ratio of τ H α : τ H β : τ H γ : τ 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 α and He i are obtained. Assuming a virial factor of f = 5.5 for the dispersion width of the line, the masses of the black holes derived from H α and H β are and , respectively. Our observed results indicate the complexity of the BLR of NGC 2617.