Winds can ‘blow up’ AGN accretion disc sizes

Recent multiband variability studies have revealed that active galactic nucleus (AGN) accretion disc sizes are generally larger than the predictions of the classical thin disc by a factor of 2 ∼ 3. This hints at some missing key ingredient in the classical thin disc theory: here, we propose an accretion disc wind. For a given bolometric luminosity, in the outer part of an accretion disc, the effective temperature in the wind case is higher than that in the no-wind one; meanwhile, the radial temperature profile of the wind case is shallower than the no-wind one. In presence of winds, for a given band, blackbody emission from large radii can contribute more to the observed luminosity than the no-wind case. Therefore, the disc sizes of the wind case can be larger than those of the no-wind case. We demonstrate that a model with the accretion rate scaling as |4Ṁ₀ (R/RS)β| (i.e. the accretion rate declines with decreasing radius due to winds) can match both the interband time lags and the spectral energy distribution of NGC 5548. Our model can also explain the interband time lags of other sources. Therefore, our model can help decipher current and future continuum reverberation mapping observations.