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We report on the discovery of an apparent triple radio structure hiddeninside the radio bubble of the ultraluminous X-ray source Holmberg II X-1. Themorphology is consistent with a collimated jet structure, which is observed toemit optically thin synchrotron radiation. The central component has a steepradio spectrum and is brighter than the outer components indicating a renewedradio activity. We estimate a minimum time-averaged jet power of 2 x 10^{39}erg/s that is associated with a time-averaged isotropic X-ray luminosity of atleast 4 x 10^{39} erg/s. Our results suggest that Holmberg II X-1 is powered bya black hole of M_BH \geq 25 M_sun, that is inferred to be accreting at a highEddington rate with intermittent radio activity.

Unveiling recurrent jets of the ULX Holmberg II X-1: evidence for a massive stellar-mass black hole?