A phenomenological study of the accretion disc in the super-Eddington AGN I Zw 1

ABSTRACT The structure of the accretion disc in active galactic nuclei (AGN) is still an unsolved question, especially how it may change with Eddington ratio. Here we examine the accretion disc in the super-Eddington AGN I Zw 1 using reverberation mapping of the optical continuum. We use three years of optical monitoring with Las Cumbres Observatory at subday cadence in $uBgVriz_s$. The lag-wavelength spectrum, calculated using the cross-correlation method and pyroa, shows a u-band excess.pyroa lags are equally well fitted with a thin and slim disc profile. The ultra-violet/optical AGN spectral energy distribution is consistent with a thin disc. The disc size at $4495\:\rm {\mathring{\rm A}}$ for a thin disc model is $4.23\pm 0.24\:\mathrm{ld}$ and for a slim disc model is $1.71\pm 0.09\:\mathrm{ld}$, larger by a factor of $2\!-\!4$ than the fiducial disc size of $1.07\pm 0.15\:\mathrm{ld}$ as determined using the Eddington ratio. We find evidence of different size scales probed with different variability time-scales. Lags evaluated at longer variability time-scales increase as do frequency-resolved lags at low frequencies, which we interpret as an additional secondary reprocessor at large radii consistent with the broad-line region (BLR) in I Zw 1. The high-frequency lags, predicted well with just a disc, are fit with a thin disc profile and a size of $0.61\pm 0.37\:\mathrm{ld}$. This indicates that the actual disc size may be on the order of the fiducial size. We also collate the most extensive set of directly measured internal sizes of an AGN, from optical to mid-infrared with reverberation mapping and optical interferometry. Assuming that the disc is indeed the fiducial size, these show little evidence that the accretion disc extends into the BLR significantly, tentatively disfavouring the failed radiatively accelerated dust driven outflow BLR formation model.