2025MNRAS.543.1935W

We present the detection of the binary system 2MASS J02132062+3648506 AB using the Karl G. Jansky Very Large Array archive data observed at 4-8 GHz. The system is a triple consisting of a tight binary (${\sim} 0.2\,\mathrm{ arcsec}$) of two M dwarfs of spectral types M4.5 and M6.5 and a wide T3 brown dwarf companion ($\sim$16.4 arcsec). The binary displays coronal and chromospheric activity as traced by previously measured X-ray flux and H $\alpha$ emission. We detect the unresolved binary at a peak flux density of ${\sim} 356\, \mu \mathrm{Jybeam}^{-1}$ at a signal-to-noise ratio (SNR) of ${\sim} 36$ and determine a radio luminosity of $\mathrm{log}L_\mathrm{ R}/\mathrm{log}L_\mathrm{bol}\sim -7.76$. The radio emission is quiescent, polarized at a mean circular polarization fraction $f_\mathrm{c}=45.20 \pm 1.58$% and exhibits a spectral index $\alpha =-0.44\pm 0.07$. We probe the binary using the Enhanced Multi-Element Remotely Linked Interferometer Network (e-MERLIN) with an angular resolution of ${\sim} 40$ mas at 5 GHz and detect a component at a peak flux density of ${\sim} 90\, \mu$Jy $\mathrm{beam}^{-1}$ at a SNR $\sim 5$. We propose a gyrosynchrotron origin for the radio emission and estimate a magnetic field strength $B< 174.86$ G, an emitting region of size $L< 1.54$ times the radius of the M4.5 primary and a plasma number density $n_\mathrm{e}< 2.91\times 10^5\ \mathrm{cm}^{-3}$. The brown dwarf companion is not detected. Additionally, we have analysed observations of 2MASS J04183483+213127, a chromospherically active L5 brown dwarf which is also not detected and can only place $3\sigma$ flux density upper limits at $36.9\, \mu$Jy $\mathrm{beam}^{-1}$ and $42.3\, \mu$Jy $\mathrm{beam}^{-1}$ for Stokes I and V, respectively.