The thermal reprocessing hypothesis in active galactic nuclei (AGN), where extreme ultraviolet/X-ray photons are reprocessed by the accretion disc into optical/UV photons, predicts wavelength-dependent time-delays between the optical continuum at different wavelengths. Recent photometric monitoring by Sergeev et al. has shown that the time-delay is observed in 14 AGN, and generally seen to increase with increasing wavelength, as predicted in the reprocessing scenario. We fit the observed time-delays and optical spectral energy distribution using a disc reprocessing model. The model delivers estimates for the nuclear reddening, the product of black hole mass and accretion rate, and the distance to each object. However, the distances at face value give H0 = 44 +/- 5 km s-1 Mpc-1 - a factor of 1.6 smaller than generally accepted. We discuss the implications of this on the reprocessing model.
Cackett, E. M., Horne, K., Winkler, H. 2007. Testing thermal reprocessing in active galactic nuclei accretion discs. MNRAS 380(2), 669-682.