The evolution of a jet ejection of the ultraluminous X-ray source
  Holmberg II X-1
  We present quasi-simultaneous, multi-epoch radio and X-ray measurements of
Holmberg II X-1 using the European VLBI Network (EVN), the Karl G. Jansky Very
Large Array (VLA), and the Chandra and Swift X-ray telescopes. The X-ray data
show apparently hard spectra with steady X-ray luminosities 4 months apart from
each other. In the high-resolution EVN radio observations, we have detected an
extended milli-arcsecond scale source with unboosted radio emission. The source
emits non-thermal, likely optically thin synchrotron emission and its
morphology is consistent with a jet ejection. The 9-GHz VLA data show an
arcsecond-scale triple structure of Holmberg II X-1 similar to that seen at
lower frequencies. However, we find that the central ejection has faded by at
least a factor of 7.3 over 1.5 years. We estimate the dynamical age of the
ejection to be higher than 2.1 years. We show that such a rapid cooling can be
explained with simple adiabatic expansion losses. These properties of Holmberg
II X-1 imply that ULX radio bubbles may be inflated by ejecta instead of
self-absorbed compact jets.
