2023ApJ...949...22M

Broadband photometric reverberation mapping (PRM) has been investigated for active galactic nuclei (AGNs) in recent years, but mostly on accretion disk continuum RM. Due to the small fraction of broad emission lines in the broad band, PRM for emission lines is very challenging. Here, we present an ICCF-Cut method for broadband PRM to obtain the Hα broad-line lag and apply it to four Seyfert 1 galaxies: MCG+08-11-011, NGC 2617, 3C 120, and NGC 5548. All of them have high-quality broadband lightcurves with daily/subdaily cadences, which enable us to extract Hα lightcurves from the line band by subtracting the contributions from the continuum and host galaxy. Their extracted Hα lightcurves are compared with the lagged continuum-band lightcurves, as well as the lagged Hβ lightcurves obtained by spectroscopic RM (SRM) at the same epochs. The consistency of these lightcurves and the comparison with the SRM Hβ lags provide support for the Hα lags of these AGNs, in a range from 9 to 19 days, obtained by the ICCF-Cut, JAVELIN, and χ² methods. The simulations for evaluating the reliability of the Hα lags and the comparisons between the SRM Hβ and PRM Hα lags indicate that the consistency of the ICCF-Cut, JAVELIN, and χ² results can ensure the reliability of the derived Hα lags. These methods may be used to estimate the broad-line region sizes and black hole masses of a large sample of AGNs in large multi-epoch, high-cadence photometric surveys such as LSST in the future.