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We present a high-quality hard X-ray spectrum of the ultraluminous X-raysource (ULX) NGC 5643 X-1 measured with NuSTAR in May-June 2014. We haveobtained this spectrum by carefully separating the signals from the ULX andfrom the active nucleus of its host galaxy NGC 5643 located 0.8 arcmin away.Together with long XMM-Newton observations performed in July 2009 and August2014, the NuSTAR data confidently reveal a high-energy cutoff in the spectrumof NGC 5643 X-1 above ~10 keV, which is a characteristic signature of ULXs. TheNuSTAR and XMM-Newton data are consistent with the source having a constantluminosity ~1.5E40 erg/s (0.2-12 keV) in all but the latest observation (August2014) when it brightened to ~3E40 erg/s. This increase is associated with thedominant, hard spectral component (presumably collimated emission from theinner regions of a supercritical accretion disc), while an additional, softcomponent (with a temperature ~0.3 keV if described by multicolor diskemission), possibly associated with a massive wind outflowing from the disk, isalso evident in the spectrum but does not exhibit significant variability.

NuSTAR and XMM-Newton observations of the ultraluminous X-ray source NGC 5643 X-1