Radio study of the colliding-wind binary HD 93129A near periastron and its surroundings

Context. HD 93129A is an O+O stellar system whose colliding-wind region (CWR) has been mapped by high angular resolution observations at centimetre wavelengths. The synchrotron nature of the radio emission confirms its particle accelerator status. Astrometric measurements since 1996 indicate that the system has an orbital period of ~120 yr and recently went through its periastron passage. Aims. We attempted to characterise the radio emission during the epoch of the periastron passage, when the particle density and the local magnetic energy density in the CWR increase. Methods. We monitored HD 93129A and its surroundings in bands centred at 2.1, 5.5, and 9.0 GHz, with the Australia Telescope Compact Array (ATCA) over a time span of 17 months, with an approximate 2-month cadence. Previous ATCA data at similar frequencies and data collected using other radio observatories were also included. Results. We obtained radio light curves in sub-bands for each band and epoch. The flux densities show an average increase of a factor of four between 2003 and 2018, though the 2009–2017 time-lapse lacks data, followed by a similar-magnitude decay between 2018 and 2020. We fit the spectral energy distribution (SED) of quasi-simultaneous data at three epochs and find that the magnetic-to-thermal pressure ratio η B does not remain constant along the orbit, possibly suggesting magnetic-field amplification close to periastron. In the 2019 epoch, we estimate a magnetic field strength of ≈1.1 G in the apex of the CWR (corresponding to η B ≈ 0.085). The evolution of the SED and spectral index along several epochs in 2019–2020 is also presented. By combining images from the ATCA and the Australian Square Kilometre Array Pathfinder telescope, a spectral index map was obtained in an area of 30′ size, including the surroundings of Eta Car, revealing positive and negative spectral indices. The radio emission in the direction of other massive binary systems in the fields of view (WR 22, WR 25, and HD 93250) was measured and briefly discussed. Conclusions. Intensive radio monitoring of a colliding-wind binary during key orbital phases allows us to track 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.