We report low-frequency radio observations of the 2021 outburst of the
recurrent nova RS Ophiuchi. These observations include the lowest frequency
observations of this system to date. Detailed light curves are obtained by
MeerKAT at 0.82 and 1.28 GHz and LOFAR at 54 and 154 MHz. These low-frequency
detections allow us to put stringent constraints on the brightness temperature
that clearly favour a non-thermal emission mechanism. The radio emission is
interpreted and modelled as synchrotron emission from the shock interaction
between the nova ejecta and the circumbinary medium. The light curve shows a
plateauing behaviour after the first peak, which can be explained by either a
non-uniform density of the circumbinary medium or a second emission component.
Allowing for a second component in the light curve modelling captures the steep
decay at late times. Furthermore, extrapolating this model to 15 years after
the outburst shows that the radio emission might not fully disappear between
outbursts. Further modelling of the light curves indicates a red giant mass
loss rate of ∼5⋅10−8 M⊙​ yr−1. The spectrum cannot
be modelled in detail at this stage, as there are likely at least four emission
components. Radio emission from stellar wind or synchrotron jets are ruled out
as the possible origin of the radio emission. Finally, we suggest a strategy
for future observations that would advance our understanding of the physical
properties of RS Oph.Comment: submitted to MNRA