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Radio observations are an effective tool to discover particle accelerationregions in colliding-wind binaries, through detection of synchrotron radiation;these regions are natural laboratories for the study of relativistic particles.Wind-collision region (WCR) models can reproduce the radio continuum spectra ofmassive binaries that contain both thermal and non-thermal radio emission;however, key constraints for models come from high-resolution imaging. Onlyfive WCRs have been resolved to date at radio frequencies at milliarcsec (mas)angular scales. The source HD 93129A, prototype of the very few known O2 Istars, is a promising target for study: recently, a second massive, early-typestar about 50 mas away was discovered, and a non-thermal radio source detectedin the region. Preliminary long-baseline array data suggest that a significantfraction of the radio emission from the system comes from a putative WCR. Wesought evidence that HD 93129A is a massive binary system with collidingstellar winds that produce non-thermal radiation, through spatially resolvedimages of the radio emitting regions. We completed observations with theAustralian Long Baseline Array (LBA) to resolve the system at mas angularresolutions and reduced archival Australia Telescope Compact Array (ATCA) datato derive the total radio emission. We also compiled optical astrometric dataof the system in a homogeneous way. We reduced historical Hubble SpaceTelescope data and obtained absolute and relative astrometry with milliarcsecaccuracy. The astrometric analysis leads us to conclude that the two stars inHD 93129A form a gravitationally bound system. The LBA data reveal an extendedarc-shaped non-thermal source between the two stars, indicative of a WCR. Thewind momentum-rate ratio of the two stellar winds is estimated. The ATCA datashow a point source with a change in flux level ...

A radio-map of the colliding winds in the very massive binary system HD 93129A