And then they were two: Detection of non-thermal radio emission from the bow shocks of two runaway stars

Context. In recent years, winds from massive stars have been considered promising sites for investigating relativistic particle acceleration. In particular, the resulting bow-shaped shocks from the interaction of the supersonic winds of runaway stars with interstellar matter have been intensively observed at many different wavelengths, from radio to gamma-rays. Aims. In this study we investigate the O4If star, BD+43 degrees 3654, the bow shock of which is, so far, the only one proven to radiate both thermal and non-thermal emission at radio frequencies. In addition, we consider NGC 7635, the Bubble Nebula, as a bow shock candidate and examine its apex for indications of thermal and non-thermal radio emission. Methods. We observed both bow shocks in radio frequencies with the Very Large Array (VLA) in the C and X bands (4-8 GHz and 8-12 GHz) and with the Effelsberg telescope at 4-8 GHz. We analysed single-dish and interferometric results individually, in addition to their combined emission, obtained spectral index maps for each source, and calculated their spectral energy distributions. Results. We find that both sources emit non-thermal emission in the radio regime, with the clearest evidence for NGC 7635, whose radio emission has a strongly negative spectral index along the northern rim of the bubble. We present the first high-resolution maps of radio emission from NGC 7635, finding that the morphology closely follows the optical nebular emission. Our results are less conclusive for the bow shock of BD+43 degrees 3654, as its emission becomes weaker and faint at higher frequencies in VLA data. Effelsberg data show a much larger emitting region (albeit a region of thermal emission) than is detected with the VLA for this source. Conclusions. Our results extend the previous radio results from the BD+43 degrees 3654 bow shock to higher frequencies, and with our NGC 7635 results we double the number of bow shocks around O stars with detected non-thermal emission, from one to two. Modelling of the multi-wavelength data for both sources shows that accelerated electrons at the wind termination shock are a plausible source for the non-thermal radio emission, but energetics arguments suggest that any non-thermal X-ray and gamma-ray emission could be significantly below existing upper limits. Enhanced synchrotron emission from compressed galactic cosmic rays in the radiative bow shock could also explain the radio emission from the BD+43 degrees 3654 bow shock, but not from NGC 7635. The non-detection of point-like radio emission from BD+43 degrees 3654 puts an upper limit on the mass-loss rate of the star that is lower than values quoted in the literature.