Dust reverberation-mapping of the Seyfert 1 galaxy WPVS48

Using robotic telescopes of the Universitatssternwarte Bochum near Cerro Armazones in Chile, we monitored the z = 0.0377 Seyfert-1 galaxy WPVS48 (2MASX 109594263-3112581) in the optical ( B and R) and near-infrared (NIR. J and K-s) with a cadence of two days. The light curves show unprecedented variability details. The MR variation features of WPVS48 are consistent with the corresponding optical variations, but the features appear sharper in the NIR than in the optical, suggesting that the optical photons undergo multiple scatterings. The J and Ks emission, tracing the hot (similar to 1600 K) dust echo, lags the B and R variations by on average tau = 64 +/- 4 days and 71 +/- 5 days, respectively (restframe). WPVS48 lies on the known tau - M-V relationship. However, the observed lag tau is about three times shorter than expected from the dust sublimation radius r(sub), inferred from the optical-UV huninosity, and explanations for this common discrepancy are searched for. The sharp NIR echos argue for a face-on torus geometry and allow us to put forward two potential scenarios: 1) as previously proposed, in the equatorial plane of the accretion disk the inner region of the torus is flattened and may come closer to the accretion disk. 2) The dust torus with inner radius r(sub) is geometrically and optically thick, so that the observer only sees the facing rim of the torus wall, which lies closer to the observer than the torus equatorial plane and therefore leads to an observed foreshortened lag. Both scenarios are able to explain the factor three discrepancy between tau and r(sub). Longer-wavelength dust reverberation data might enable one to distinguish between the scenarios.