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Ultraluminous X-ray Sources (ULXs) provide a unique opportunities to probethe geometry and energetics of super-Eddington accretion. The radiativeprocesses involved in super-Eddington accretion are not well understood, and sostudying correlated variability between different energy bands can provideinsights into the causal connection between different emitting regions. Wepresent a spectral-timing analysis of NGC 1313 X-1 from a recent XMM-Newtoncampaign. The spectra can be decomposed into two thermal-like components, thehotter of which may originate from the inner accretion disc, and the coolerfrom an optically thick outflow. We find correlated variability between hard(2-10 keV) and soft (0.3-2 keV) bands on kilosecond timescales, and find a softlag of ~150 seconds. The covariance spectrum suggests that emissioncontributing to the lags is largely associated with the hotter of the twothermal-like components, likely originating from the inner accretion flow. Thisis only the third ULX to exhibit soft lags. The lags range over three orders ofmagnitude in amplitude, but all three are ~5 to ~20 percent of thecorresponding characteristic variability timescales. If these soft lags can beunderstood in the context of a unified picture of ULXs, then lag timescales mayprovide constraints on the density and extent of radiatively-driven outflows.

Discovery of a soft X-ray lag in the Ultraluminous X-ray Source NGC 1313 X-1