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Vela X-1 is a runaway X-ray binary system hosting a massive donor star, whosestrong stellar wind creates a bow shock as it interacts with the interstellarmedium. This bow shock has previously been detected in H$\alpha$ and IR, but,similar to all but one bow shock from a massive runaway star (BD+43$^{\rmo}$3654), has escaped detection in other wavebands. We report on the discoveryof $1.3$ GHz radio emission from the Vela X-1 bow shock with the MeerKATtelescope. The MeerKAT observations reveal how the radio emission closelytraces the H$\alpha$ line emission, both in the bow shock and in thelarger-scale diffuse structures known from existing H$\alpha$ surveys. The VelaX-1 bow shock is the first stellar-wind-driven radio bow shock detected aroundan X-ray binary. In the absence of a radio spectral index measurement, weexplore other avenues to constrain the radio emission mechanism. We find thatthermal/free-free emission can account for the radio and H$\alpha$ properties,for a combination of electron temperature and density consistent with earlierestimates of ISM density and the shock enhancement. In this explanation, thepresence of a local ISM over-density is essential for the detection of radioemission. Alternatively, we consider a non-thermal/synchrotron scenario,evaluating the magnetic field and broad-band spectrum of the shock. However, wefind that exceptionally high fractions ($\gtrsim 13$%) of the kinetic windpower would need to be injected into the relativistic electron population toexplain the radio emission. Assuming lower fractions implies a hybrid scenario,dominated by free-free radio emission. Finally, we speculate about thedetectability of radio bow shocks and whether it requires exceptional ISM orstellar wind properties.

MeerKAT discovery of radio emission from the Vela X-1 bow shock