Late-time radio observations of the short GRB200522A: constraints on the magnetar model

GRB200522A is a short duration gamma-ray burst (GRB) at redshift $z$=0.554 characterized by a bright infrared counterpart. A possible, although not unambiguous, interpretation of the observed emission is the onset of a luminous kilonova powered by a rapidly rotating and highly-magnetized neutron star, known as magnetar. A bright radio flare, arising from the interaction of the kilonova ejecta with the surrounding medium, is a prediction of this model. Whereas the available dataset remains open to multiple interpretations (e.g. afterglow, r-process kilonova, magnetar-powered kilonova), long-term radio monitoring of this burst may be key to discriminate between models. We present our late-time upper limit on the radio emission of GRB200522A, carried out with the Karl G. Jansky Very Large Array at 288 days after the burst. For kilonova ejecta with energy $E_{rm ej} approx 10^{53} rm erg$, as expected for a long-lived magnetar remnant, we can already rule out ejecta masses $M_{rm ej} lesssim0.03 mathrm{M}_odot$ for the most likely range of circumburst densities $ngtrsim 10^{-3}$ cm$^{-3}$. Observations on timescales of $approx$3-10 yr after the merger will probe larger ejecta masses up to $M_{rm ej} sim 0.1 mathrm{M}_odot$, providing a robust test to the magnetar scenario.