Photometric reverberation mapping of several active galactic nuclei

The broad emission line region size of active galactic nuclei (AGN) is an important parameter for calculating the mass of the central black hole. Usually, the size is measured by spectral reverberation mapping to obtain the time lag between the broad emission line and continuum radiations, but measurement using intermediate or large optical telescopes is time-consuming. Here, we adopt another method to perform photometric reverberation mapping. We used a 60-cm telescope at the Xinglong station of the National Astronomical Observatories of China (NAOC) to observe six AGNs at redshift z ≃ 0 . 3. We used the V broad band to trace the continuum and the narrow OIII band to trace the redshifted H β emission line. To eliminate the influence of the continuum in the OIII band, we assumed that the continuum flux in the narrow band is equal to a fixed fraction α of the flux in the continuum band for each AGN. Under this assumption, we subtracted the continuum band flux with the ratio α from the narrow-band flux and obtained the light curve of the H β line via the improved interpolated cross-correlation function (ICCF) method (ICCF-Cut). The extracted H β light curves were compared with the lagged continuum band light curves. We also used another method JAVELIN to calculate the H β time lags. The H β time lags obtained from the ICCF-Cut and JAVELIN were consistent with the results of spectroscopic reverberation mapping for two AGNs (Mrk 335 and 3C 120). The H β time lags obtained from ICCF-Cut were 7 . 8 + 8 . 0 − 11 . 4 and 13 . 5 + 13 . 0 − 8 . 8 d. To explain why only two of the six AGNs had good results, we used the damped random walk (DRW) model to simulate the light curves of AGNs and found that the photometric reverberation mapping required su ffi cient observations with high cadence. The observation and simulation results revealed that combining the ICCF-Cut and JAVELIN results may enable us to obtain reliable emission line lags for AGNs with large variability and high cadence, although the errors of lags are large.