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

Abstract

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 × 10^(40) erg s^(−1) (0.2–12 keV) in all but the latest observation (2014 August) when it brightened to ∼3 × 10^(40) 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.