  The environs of massive, early-type stars have been inspected in recent years
in the search for sites where particles can be accelerated up to relativistic
energies. Wind regions of massive binaries that collide have already been
established as sources of high-energy emission; however, there is a different
scenario for massive stars where strong shocks can also be produced: the
bow-shaped region of matter piled up by the action of the stellar strong wind
of a runaway star interacting with the interstellar medium. We study the
bow-shock region produced by a very massive runaway star, BD+43 3654, to look
for nonthermal radio emission as evidence of a relativistic particle
population. We observed the field of BD+43 3654 at two frequencies, 1.42 and
4.86 GHz, with the Very Large Array (VLA), and obtained a spectral index map of
the radio emission. We have detected, for the first time, nonthermal radio
emission from the bow shock of a massive runaway star. After analyzing the
radiative mechanisms that can be at work, we conclude that the region under
study could produce enough relativistic particles whose radiation might be
detectable by forthcoming gamma-ray instruments, like CTA North.
