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We present accretion-disk structure measurements from UV-opticalreverberation mapping observations of a sample of eight quasars at 0.24<z<0.85.Ultraviolet photometry comes from two cycles of Hubble Space Telescopemonitoring, accompanied by multi-band optical monitoring by the Las CumbresObservatory network and Liverpool Telescopes. The targets were selected fromthe Sloan Digital Sky Survey Reverberation Mapping (SDSS-RM) project samplewith reliable black-hole mass measurements from Hbeta reverberation mappingresults. We measure significant lags between the UV and various optical grizbands using JAVELIN and CREAM methods. We use the significant lag results fromboth methods to fit the accretion-disk structure using a Markov chain MonteCarlo approach. We study the accretion disk as a function of disknormalization, temperature scaling, and efficiency. We find direct evidence fordiffuse nebular emission from Balmer and FeII lines over discrete wavelengthranges. We also find that our best-fit disk color profile is broadly consistentwith the Shakura \& Sunyaev disk model. We compare our UV-optical lags to thedisk sizes inferred from optical-optical lags of the same quasars and find thatour results are consistent with these quasars being drawn from a limitedhigh-lag subset of the broader population. Our results are therefore broadlyconsistent with models that suggest longer disk lags in a subset of quasars,for example, due to a nonzero size of the ionizing corona and/or magneticheating contributing to the disk response.

The Sloan Digital Sky Survey Reverberation Mapping Project: UV-Optical Accretion Disk Measurements with Hubble Space Telescope