16705

There is growing evidence that our understanding of accretion onto supermassive black holes during the quasar phase is incomplete comma and that accretion disks (ADs) are larger than theoretically predicted. This has far reaching implications for accretion physics in the general astrophysical context. It further influences our understanding of the growth of supermassive black holes via accretion and mergers over cosmic times comma their immediate active environs comma and the potential of using quasars as standard cosmological candles. Reverberation mapping (RM) has been proven successful in studying quasar interiors comma which cannot be resolved by conventional imaging techniques. Nevertheless comma RM inferred AD sizes are controversial comma and may be affected by non-AD emission components comma such as the broad line region (BLR). Here we propose a high-cadence spectroscopic-RM campaign of Mrk279 using STIS. Data will be used to constrain moments of the transfer function (between the driving continuum signal and its echo) in addition to the lag comma thereby breaking the degeneracy between competing models for the origin of continuum time-delays in the UV-to-NIR spectral range. RM of the broad emission line signal will also be pursued to constrain physical models for the BLR and test claims for a tight connection between the BLR and the AD. The proposed observations are extemely timely as HST-STIS is the only operating telescope that meets our scientific requirements comma and could shed light on the theoretical uncertainties that are currently plaguing the field.