Dust Reverberation Mapping of Z229-15
  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 ({\alpha}) between B
and V bands for each epoch was used to correct for the accretion disk (AD)
component present in the infrared light curves. The observed {\alpha} 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)
days (between V and J), 15.63 (+5.05/-5.11) days (between V and H) and 20.36
(+5.82/-5.68) days (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 {\alpha} = 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 {\alpha} in DRM is disfavoured as the
{\alpha} of the ionizing continuum changes with the flux of the source.
