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Variability is a powerful tool to investigate properties of X-ray binaries(XRB), in particular for Ultraluminous X-ray sources (ULXs) that are mainlydetected in the X-ray band. For most ULXs the nature of the accretor isunknown, although a few ULXs have been confirmed to be accreting atsuper-Eddington rates onto a neutron star (NS). Monitoring these sources isparticularly useful both to detect transients and to derive periodicities,linked to orbital and super-orbital modulations. Here we present the results ofour monitoring campaign of the galaxy NGC 925, performed with the Neil GehrelsSwift Observatory. We also include archival and literature data obtained withChandra, XMM-Newton and NuSTAR. We have studied spectra, light-curves andvariability properties on days to months time-scales. All the three ULXsdetected in this galaxy show flux variability. ULX-1 is one of the mostluminous ULXs known, since only 10% of the ULXs exceed a luminosity of$\sim$5$\times$10$^{40}$ erg s$^{-1}$, but despite its high flux variability wefound only weak spectral variability. We classify it as in a hard ultraluminousregime of super-Eddington accretion. ULX-2 and ULX-3 are less luminous but alsovariable in flux and possibly also in spectral shape. We classify them as inbetween the hard and the soft ultraluminous regimes. ULX-3 is a transientsource: by applying a Lomb-Scargle algorithm we derive a periodicity of $\sim$126 d, which could be associated with an orbital or super-orbital origin.

Investigating the nature of the ultraluminous X-ray sources in the galaxy NGC 925