Testing super-eddington accretion on to a supermassive black hole: reverberation mapping of PG 1119+120

We measure the black hole mass and investigate the accretion flow around the local (z = 0.0502) quasar PG 1119+120. Spectroscopic monitoring with Calar Alto provides H β lags and linewidths from which we estimate a black hole mass of log (M_•/M_⊙) = 7.0, uncertain by ~0.4 dex. High cadence photometric monitoring over 2 yr with the Las Cumbres Observatory provides light curves in seven optical bands suitable for intensive continuum reverberation mapping. We identify variability on two time-scales. Slower variations on a 100-d time-scale exhibit excess flux and increased lag in the u' band and are thus attributable to diffuse bound-free continuum emission from the broad-line region. Faster variations that we attribute to accretion disc reprocessing lack a u'-band excess and have flux and delay spectra consistent with either τ ∝ λ^4/3, as expected for a temperature structure of T(R) ∝ R^-3/4 for a thin accretion disc, or τ ∝ λ² expected for a slim disc. Decomposing the flux into variable (disc) and constant (host galaxy) components, we find the disc SED to be flatter than expected with $f_{\nu } \sim \rm {const}$. Modelling the SED predicts an Eddington ratio of λ_Edd > 1, where the flat spectrum can be reproduced by a slim disc with little dust extinction or a thin disc that requires more dust extinction. While this accretion is super-Eddington, the geometry is still unclear; however, a slim disc is expected due to the high radiation pressure at these accretion rates, and is entirely consistent with our observations.