H$\alpha$ Time Delays of AGNs from the Zwicky Transcient Facility Broadband Photometry

TL;DR

This study extends broadband photometric reverberation mapping to the ZTF database, successfully estimating Hα time delays for AGNs, which can help determine black hole masses in large sky surveys.

Contribution

It introduces an efficient method combining ICCF-Cut, JAVELIN, and χ² techniques to reliably measure Hα time lags from broadband photometry, validated against spectroscopic results.

Findings

Hα lags are slightly larger than Hβ lags, consistent with models.

The combined methods reliably estimate Hα lags from broadband data.

Results are applicable for black hole mass estimation in future surveys.

Abstract

In our previous work on broadband photometric reverberation mapping (PRM), we proposed the ICCF-Cut process to obtain the time lags of H$\alpha$ emission line from two broadband lightcurves via subtracting the continuum emission from the line band. Extending the work, we enlarge our sample to the Zwicky Transient Facility (ZTF) database. We adopt two criteria to select 123 type 1 AGNs with sufficient variability and smooth lightcurves from 3537 AGNs at $z<0.09$ with more than 100 epoch observations in the $g$ and $r$ bands from the ZTF database. We calculate the H$\alpha$ time lags for 23 of them which have previous spectroscopic reverberation mapping (SRM) results using ICCF-Cut, JAVELIN and $\chi ^2$ methods. Our obtained H$\alpha$ time lags are slightly larger than the H$\beta$ time lags, which is consistent with the previous SRM results and the theoretical model of the AGN broad line region. The comparisons between SRM and PRM lag distributions and between the subtracted emission line lightcurves indicate that after selecting AGNs with the two criteria, combining the ICCF-Cut, JAVELIN and $\chi^2$ methods provides an efficient way to get the reliable H$\alpha$ lags from the broadband PRM. Such techniques can be used to estimate the black hole masses of a large sample of AGNs in the large multi-epoch photometric sky surveys such as the Legacy Survey of Space and Time (LSST) and the survey from the Wide Field Survey Telescope (WFST) in the near future.