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 beta lags and linewidths from which we estimate a black hole mass of log (M-center dot/M-circle dot) = 7.0, uncertain by similar to 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 t proportional to lambda(4/3), as expected for a temperature structure of T(R) proportional to R-3/4 for a thin accretion disc, or t proportional to lambda(2) 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) similar to const. Modelling the SED predicts an Eddington ratio of lambda(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.