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If ultraluminous X-ray sources (ULXs) are powered by accretion onto stellarremnant black holes, then many must be accreting at super-Eddington rates. Itis predicted that such high accretion rates should give rise to massive,radiatively-driven winds. However, observational evidence of a wind, in theform of absorption or emission features, has remained elusive. As such, thereported detection of X-ray spectral residuals in XMM-Newton spectra of NGC5408 X-1, which could be related to absorption in a wind is potentially veryexciting. However, it has previously been assumed by several authors that thesefeatures simply originate from background diffuse plasma emission related tostar-formation in the ULX's host galaxy. In this work we utilise the spatialresolving power of Chandra to test whether we can rule out this latterinterpretation. We demonstrate that the majority of the luminosity in thesespectral features is emitted from a highly localised region close to the ULX,and appears point-like even with Chandra. It is therefore highly likely thatthe spectral features are associated with the ULX itself, and little of theflux in this spectral component originates from spatially extended emission inthe host galaxy. This may be consistent with the suggestion of absorption in anoptically thin phase of a super-Eddington wind. Alternatively, we could beseeing emission from collisionally ionised material close to the black hole,but critically this would be difficult to reconcile with models where thesource inclination largely determines the observed X-ray spectral and timingproperties.

X-ray spectral residuals in NGC 5408 X-1: diffuse emission from star formation, or the signature of a super-Eddington wind?