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Context. FU Orionis is the archetypal FUor star, a subclass of young stellarobject (YSO) that undergo rapid brightening events, often gaining 4-6magnitudes on timescales of days. This brightening is often associated with amassive increase in accretion; one of the most ubiquitous processes inastrophysics from planets and stars to super-massive black holes. We presentmulti-band interferometric observations of the FU Ori circumstellarenvironment, including the first J-band interferometric observations of a YSO.Aims. We investigate the morphology and temperature gradient of the inner-mostregions of the accretion disk around FU Orionis. We aim to characterise theheating mechanisms of the disk and comment on potential outburst triggeringprocesses. Methods. Recent upgrades to the MIRC-X instrument at the CHARA arrayallowed the first dual-band J and H observations of YSOs.Using baselines up to331 m, we present high angular resolution data of a YSO covering thenear-infrared bands J, H, and K. The unprecedented spectral range of the dataallows us to apply temperature gradient models to the innermost regions of FUOri. Results. We spatially resolve the innermost astronomical unit of the diskand determine the exponent of the temperature gradient of the inner disk to$T=r^{-0.74\pm0.02}$. This agrees with theoretical work that predicts $T =r^{-0.75}$ for actively accreting, steady state disks, a value only obtainablethrough viscous heating within the disk. We find a disk which extends down tothe stellar surface at $0.015\pm0.007$ au where the temperature is found to be$5800\pm700$ K indicating boundary layer accretion. We find a disk inclined at$32\pm4^\circ$ with a minor-axis position angle of $34\pm11^\circ$.

Viscous Heating and Boundary Layer Accretion in the Disk of Outbursting Star FU Orionis