Shock-driven synchrotron radio emission from the 2021 outburst of RS Ophiuchi

We present low-frequency radio observations of the Galactic symbiotic recurrent nova RS Ophiuchi during its 2021 outburst. The observations were carried out with the upgraded Giant Metrewave Radio Telescope spanning a frequency range of 0.15-1.4 GHz during 23-287 d post the outburst. The average value of the optically thin spectral index is alpha similar to -0.4 (F-nu proportional to nu(alpha)), indicating a non-thermal origin of the radio emission at the observed frequencies. The radio light curves are best represented by shock-driven synchrotron emission, initially absorbed by a clumpy ionized circumbinary medium. We estimate the mass-loss rate of the red giant companion star to be m(center dot)similar to 7.5 x 10(-8) M-circle dot yr(-1) for an assumed stellar wind velocity of 20 km s(-1). The 0.15-1.4 GHz radio light curves of the 2021 outburst are systematically brighter than those of the 2006 outburst. Considering similar shock properties between the two outbursts, this is indicative of a relatively higher particle number density in the synchrotron emitting plasma in the current outburst.