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Using robotic telescopes of the Universitatssternwarte Bochum near CerroArmazones in Chile, we monitored the z=0.0377 Seyfert 1 galaxy WPVS48 (2MASXJ09594263-3112581) in the optical (B and R) and near-infrared (NIR, J and Ks)with a cadence of two days. The light curves show unprecedented variabilitydetails. The NIR variation features of WPVS48 are consistent with thecorresponding optical variations, but the features appear sharper in the NIRthan in the optical, suggesting that the optical photons undergo multiplescatterings. The J and Ks emission, tracing the hot (1600 K) dust echo, lagsthe B and R variations by on average 64 +/- 4 days and 71 +/- 5 days,respectively (restframe). WPVS48 lies on the known tau-M_V relationship.However, the observed lag is about three times shorter than expected from thedust sublimation radius r_sub inferred from the optical-UV luminosity, andexplanations for this common discrepancy are searched for. The sharp NIR echosargue for a face-on torus geometry and allow us to put forward two potentialscenarios: 1) as previously proposed, in the equatorial plane of the accretiondisk the inner region of the torus is flattened and may come closer to theaccretion disk. 2) The dust torus with inner radius r_sub is geometrically andoptically thick, so that the observer only sees the facing rim of the toruswall, which lies closer to the observer than the torus equatorial plane andtherefore leads to an observed foreshortened lag. Both scenarios are able toexplain the factor three discrepancy between tau and r_sub. Longer-wavelengthdust reverberation data might enable one to distinguish between the scenarios.

Dust reverberation-mapping of the Seyfert 1 galaxy WPVS48