Monitoring the extreme long-term variability of NGC 925 ULX-3

We report on the discovery and subsequent monitoring of NGC 925 ULX-3, a bright and highly-variable ultraluminous X-ray source detected by Chandra in November 2017 at a luminosity of (7.8±0.8) x 10³⁹ erg/s, as well as by XMM-Newton at a much lower luminosity of (3.8±0.5) x 10³⁸ erg/s. Combined with other non-detections of the source in archival data, it demonstrates a dynamic range in flux of at least a factor 26. The long-term light curve is sparsely sampled, so while it has been detected as bright on multiple occasions, it is as yet unclear whether ULX-3 is exhibiting the onset of the propeller effect, a high-amplitude super-orbital period, both of which have been observed in ULXs and particularly ULX pulsars, or some other variability behaviour. We have obtained further Swift monitoring of this source on a ~weekly cadence and report on the initial results of this monitoring and their implications for the causes of long-term variability in ULXs.