Towards the origin of the radio emission in AR Scorpii, the first radio-pulsing white dwarf binary

Context. The binary system AR Sco contains an M star and the only known radio-pulsing white dwarf. The system shows emission from radio to X-rays, likely dominated by synchrotron radiation. The mechanism that produces most of this emission remains unclear. Two competing scenarios have been proposed: collimated outflows, and direct interaction between the magnetospheres of the white dwarf and the M star. Aims. The two proposed scenarios can be tested via very long baseline interferometric radio observations. Methods. We conducted a radio observation with the Australian Long Baseline Array (LBA) on 20 Oct. 2016 at 8.5 GHz to study the compactness of the radio emission. Simultaneous data with the Australian Telescope Compact Array (ATCA) were also recorded for a direct comparison of the obtained flux densities. Results. AR Sco shows radio emission compact on milliarcsecond angular scales (less than or similar to 0.02 AU, or 4 R-circle dot). The emission is orbitally modulated, with an average flux density of approximate to 6.5 mJy. A comparison with the simultaneous ATCA data shows that no flux is resolved out on mas scales, implying that the radio emission is produced in this compact region. Additionally, the obtained radio light curves on hour timescales are consistent with the optical light curve. Conclusions. The radio emission in AR Sco is likely produced in the magnetosphere of the M star or the white dwarf, and we see no evidence for a radio outflow or collimated jets significantly contributing to the radio emission.