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We fit the UV/optical lightcurves of the Seyfert 1 galaxy Mrk 817 to producemaps of the accretion disk temperature fluctuations $\delta T$ resolved in timeand radius. The $\delta T$ maps are dominated by coherent radial structuresthat move slowly ($v \ll c$) inwards and outwards, which conflicts with theidea that disk variability is driven only by reverberation. Instead, theseslow-moving temperature fluctuations are likely due to variability intrinsic tothe disk. We test how modifying the input lightcurves by smoothing andsubtracting them changes the resulting $\delta T$ maps and find that most ofthe temperature fluctuations exist over relatively long timescales ($\sim$100sof days). We show how detrending AGN lightcurves can be used to separate theflux variations driven by the slow-moving temperature fluctuations from thosedriven by reverberation. We also simulate contamination of the continuumemission from the disk by continuum emission from the broad line region (BLR),which is expected to have spectral features localized in wavelength, such asthe Balmer break contaminating the $U$ band. We find that a disk with a smoothtemperature profile cannot produce a signal localized in wavelength and thatany BLR contamination should appear as residuals in our model lightcurves.Given the observed residuals, we estimate that only $\sim$20% of the variableflux in the $U$ and $u$ lightcurves can be due to BLR contamination. Finally,we discus how these maps not only describe the data, but can make predictionsabout other aspects of AGN variability.

AGN STORM 2. VI. Mapping Temperature Fluctuations in the Accretion Disk of Mrk 817