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

We present a high-quality hard X-ray spectrum of the ultraluminous X-ray source (ULX) NGC 5643 X-1 measured with NuSTAR in 2014 May-June. We have obtained this spectrum by carefully separating the signals from the ULX and from the active nucleus of its host galaxyNGC 5643 located 0.8 arcmin away. Together with long XMM-Newton observations performed in 2009 July and 2014 August, the NuSTAR data confidently reveal a high-energy cutoff in the spectrum of NGC 5643 X-1 above ∼10 keV, which is a characteristic signature of ULXs. The NuSTAR and XMM-Newton data are consistent with the source having a constant luminosity ∼1.5 × 10⁴⁰ erg s^-1 (0.2-12 keV) in all but the latest observation (2014 August) when it brightened to ∼3 × 10⁴⁰ erg s^-1. This increase is associated with the dominant, hard spectral component (presumably collimated emission from the inner regions of a supercritical accretion disc), while an additional, soft component (with a temperature ∼0.3 keV if described by multicolour disc emission), possibly associated with a massive wind outflowing from the disc, is also evident in the spectrum but does not exhibit significant variability.