Massive, early-type stars, belonging to the OB and Wolf-Rayet classes, exhibit strong winds that transfer material and energy to their surroundings. A large proportion of these stars are part of binary or higher multiplicity systems. In such systems, if the individual stars are sufficiently close, their stellar winds interact in a colliding-wind region (CWR) and the systems are referred to as colliding-wind binaries (CWBs). Since the 1970s, these stars have been identified as sources of thermal and non-thermal (NT) radio emission. The former can be attributed to the free–free emission of extended envelopes, while the latter is due to synchrotron radiation.

Benaglia et al. present a study of HD 93129A, an O+O stellar system that has an orbital period of ∼120 yr with a predicted periastron passage in late 2018. They monitored HD 93129A and its surroundings in bands centred at 2.1, 5.5, and 9.0 GHz, with the ATCA over a time span of 17 months. Archival ATCA data at similar frequencies and data collected using other radio observatories were also included. The synchrotron nature of the radio emission confirms its particle accelerator status. Radio light show an average rise in flux density by a factor of four from 2003 to 2018, and a similar decay between 2018 to 2020. This intensive radio monitoring of a CWB during key orbital phases allowed the tracking of the evolution of physical conditions in the shocks. The general trend of decreasing emission of HD 93129A in the high-frequency bands in 2019–2020 suggests that the system is at post-periastron, consistent with model predictions. The image above shows the 0.89 GHz (ASKAP RACS) continuum emission centred at the position of HD 93129A. The synthesised beam of 15 x 11 arcseconds is shown in the bottom left corner, and a zoom of the source is displayed at the top right corner.