Particle acceleration in bow shocks around massive runaway stars
When massive stars move supersonically through the Milky Way, a bow shock can form at the location where the pressure from their strong stellar wind and from the surrounding interstellar medium balance. Hundreds of such bow shocks are known from infra-red telescopes, that observed heated dust in these shocks. With MeerKAT, we recently discovered the second example of radio emission from such a runaway stellar bow shock, around the famous X-ray binary Vela X-1.
Vela X-1, indicated by the black cross in the image above, is a binary system containing a neutron star and a massive normal star in a tight orbit. The system travels relative to its surrounding at a velocity over 50 km/second, while the massive star sheds mass at a rate approximately one millionth of a solar mass per year. This gas rams into surrounding low-density gaseous space at a speed of hundreds of km/second, creating the shock seen above the cross.
While this bow shock was discovered over two decades ago, this is the first time it is seen with a radio telescope: MeerKAT. Our paper reporting this discovery discusses in great detail why radio emission is seen from this bow shock, as well as one other example, but not the hundreds of remaining ones. We conclude that the relatively high density surrounding Vela X-1 creates ideal circumstances for thermal radio emission to be detectable. In that scenario, the same population of electrons is responsible for both optical and radio emission in the shock (and further surroundings). This is clearly highlighted below, in the figure showing radio contours overlaid on an optical image: the two observing bands traces the same structures. However, non-thermal emission from accelerated particles is also likely to contribute.
Building on this discovery, we cross-matched the E-BOSS bow shock catalogue by Peri et al. (2012, 2015) with the Rapid ASKAP Continuum Survey. Surprisingly, we found nine more bow shocks with associated radio emission. In three cases, the emission clearly constitutes the bow shock's radio emission. In the other six, this connection is suspected but future observations will shed more light on this connection. Below, I show the comparison of the radio, IR, and optical images of the best candidate.