2021MNRAS.501.3905M

We report results of the dust reverberation mapping (DRM) on the Seyfert 1 galaxy Z229-15 at z = 0.0273. Quasi-simultaneous photometric observations for a total of 48 epochs were acquired during the period 2017 July to 2018 December in B, V, J, H and K_s bands. The calculated spectral index (α) between B and V bands for each epoch was used to correct for the accretion disc (AD) component present in the infrared light curves. The observed α ranges between -0.99 and 1.03. Using cross-correlation function analysis we found significant time delays between the optical V and the AD corrected J, H and K_s light curves. The lags in the rest frame of the source are 12.52^+10.00_–9.55 d (between V and J), 15.63^+5.05_–5.11 d (between V and H) and 20.36^+5.82_–5.68 d (between V and K_s). Given the large error bars, these lags are consistent with each other. However, considering the lag between V and K_s bands to represent the inner edge of the dust torus, the torus in Z229-15 lies at a distance of 0.017 pc from the central ionizing continuum. This is smaller than that expected from the radius luminosity (R-L) relationship known from DRM. Using a constant α = 0.1 to account for the AD component, as is normally done in DRM, the deduced radius (0.025 pc) lies close to the expected R-L relation. However, usage of constant α in DRM is disfavoured as the α of the ionizing continuum changes with the flux of the source.