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

We report on the discovery of an apparent triple radio structure hidden inside the radio bubble of the ultraluminous X-ray source Holmberg II X-1. The morphology is consistent with a collimated jet structure, which is observed to emit optically thin synchrotron radiation. The central component has a steep radio spectrum and is brighter than the outer components indicating a renewed radio activity. We estimate a minimum time-averaged jet power of ∼2 × 10³⁹ erg s^-1 that is associated with a time-averaged isotropic X-ray luminosity of at least 4 × 10³⁹ erg s^-1. Our results suggest that Holmberg II X-1 is powered by a black hole of M_BH ≥ 25 M_⊙, that is inferred to be accreting at a high Eddington rate with intermittent radio activity.