2021MNRAS.502.1158Z

Mrk142 is known as the only outlier in R-L space (the correlation between BLR (broad-line region) radii and continuum luminosity) among the low-redshift local reverberation-mapped broad-line active galactic nuclei (BLAGNs) with moderate accretion rates, as its BLR radius is smaller than the expected R-L value. Here, by considering probable optical transient quasi-periodic oscillations (QPOs), a new approach to assessing the BLR radius is presented. Reliable transient QPOs in high-energy emissions from the vicinity of the black hole have been reported in several normal AGNs; however, to date there are few short-term, low-energy optical transient QPOs in normal BLAGNs (i.e. not the QPOs reported in blazars or in AGNs harbouring binary black hole systems). Using the photometric optical light curves, which are well described directly by sinusoidal functions, we report probable short-term optical transient QPOs with periodicities around 14 and 43 d in BLAGN Mrk142, indicating optical QPOs similar to but scaled from those in high-energy bands. By considering the 14-d QPOs related to the reprocessing procedure, a new approach to estimating the BLR radius is possible for Mrk142 through the reverberation mapping technique. The new BLR radius of Mrk142 obeys the R-L relationship well, demonstrating that this relationship is fundamental in local normal BLAGNs without ultra-high accretion rates.