  Radio observations are an effective tool to discover particle acceleration
regions 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 of
massive binaries that contain both thermal and non-thermal radio emission;
however, key constraints for models come from high-resolution imaging. Only
five 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 I
stars, is a promising target for study: recently, a second massive, early-type
star about 50 mas away was discovered, and a non-thermal radio source detected
in the region. Preliminary long-baseline array data suggest that a significant
fraction of the radio emission from the system comes from a putative WCR. We
sought evidence that HD 93129A is a massive binary system with colliding
stellar winds that produce non-thermal radiation, through spatially resolved
images of the radio emitting regions. We completed observations with the
Australian Long Baseline Array (LBA) to resolve the system at mas angular
resolutions and reduced archival Australia Telescope Compact Array (ATCA) data
to derive the total radio emission. We also compiled optical astrometric data
of the system in a homogeneous way. We reduced historical Hubble Space
Telescope data and obtained absolute and relative astrometry with milliarcsec
accuracy. The astrometric analysis leads us to conclude that the two stars in
HD 93129A form a gravitationally bound system. The LBA data reveal an extended
arc-shaped non-thermal source between the two stars, indicative of a WCR. The
wind momentum-rate ratio of the two stellar winds is estimated. The ATCA data
show a point source with a change in flux level ...
