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 x 10^{39}
erg/s that is associated with a time-averaged isotropic X-ray luminosity of at
least 4 x 10^{39} erg/s. Our results suggest that Holmberg II X-1 is powered by
a black hole of M_BH \geq 25 M_sun, that is inferred to be accreting at a high
Eddington rate with intermittent radio activity.
