2014A&A...561L...8P

Using robotic telescopes of the Universitaetssternwarte Bochum near Cerro Armazones in Chile, we monitored the z=0.0377 Seyfert-1 galaxy WPVS 48 (2MASX J09594263-3112581) 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 NIR variation features of WPVS 48 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 K_s emission, tracing the hot (∼1600 K) dust echo, lags the B and R variations by on average τ= 64 ±4 days and 71 ±5 days, respectively (restframe). WPVS 48 lies on the known τ- M_V relationship. However, the observed lag τ is about three times shorter than expected from the dust sublimation radius r_sub inferred from the optical-UV luminosity, 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 τ and r_sub. Longer-wavelength dust reverberation data might enable one to distinguish between the scenarios.