Target of opportunity: an ultraluminous X-ray source in the Magellanic Clouds

Supercritical accretion flows launch radiatively driven outflows, detected in a few ULXs with the highest quality X-ray datasets. However, due to large ULX distances, any outflow detections are very challenging. We commonly observe transient X-ray binaries in the nearby Magellanic Clouds temporarily accreting right on the 10^39 erg/s ULX threshold. A brighter (~3*10^39 erg/s) transient ULX may eventually appear in SMC/LMC, similar to NGC 300 ULX-1, the only ULX pulsar with evidence for fast winds. The resulting HETG spectrum will be of very high quality. We will detect or place strong limits on any ionised absorption. We propose to observe the ULX twice (1 week delay) to study the short- and long-term variability of the outflow, determine if it is multiphase and measure its abundances.