Abstract:
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
Ks 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 Ks 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
Ks). Given the large error bars, these lags are consistent with each other. However, considering the lag between V and Ks 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
