Discovery of a recurrent spectral evolutionary cycle in the ULXs Holmberg II X-1 and NGC 5204 X-1

While the discovery of evolutionary cycles have marked a breakthrough in our understanding of the accretion flow changes in the sub-Eddington regime in Galactic Black Hole Binaries, their evidence in the super-Eddington regime has remained so far elusive. Ultraluminous X-ray sources are known to display jets, outflows and clear evidence of interaction with the local environment up to hundreds of parsec scales, much like the Galactic microquasar SS433, suggesting that super-Eddington accretion powers most of them. Combining data from XMM-Newton, Chandra and Swift, we have been able to determine a clear recurrent evolutionary spectral cycle in two archetypcal ULXs, Holmberg II X-1 and NGC 5204 X-1, allowing for the first time to constrain the timescale of the spectral transitions observed in the super-Eddington regime. We discuss the nature of the spectral changes in the framework of super-Eddington accretion by invoking changes in the accretion rate and in the geometry of the accretion flow. These findings make Holmberg II X-1 and NGC 5204 X-1 ideal laboratories to test theories of super-Eddington accretion, given their predictive behaviour, and could potentially be extended to similar transitions seen in other ULXs