  The supersonic winds produced by massive stars carry a large amount of
kinetic power. In numerous scenarios such winds have been proven to produce
shocks in which relativistic particles are accelerated emitting non-thermal
radiation. Here, we report the first detection of non-thermal emission from a
single stellar bubble, G2.4+1.4, associated with a WO star. We observed this
source with the uGMRT in Band 4 ($550-850$ MHz) and Band 5 ($1050-1450$ MHz).
We present intensity and spectral index maps for this source that are
consistent with synchrotron emission (average spectral index, $\alpha = -0.83
\pm 0.10$). The fraction of the available kinetic wind power that is converted
into cosmic ray acceleration is estimated to be of the order of a few per cent.
This finding constitutes an observational breakthrough and gives new insight on
the non-thermal physical processes taking place in the environments of isolated
massive stars. In particular, our results show that non-runaway isolated
massive stars are capable of accelerating relativistic particles and are
therefore confirmed as sources of Galactic cosmic rays.
