Radio emission in ultracool dwarfs: the nearby substellar triple system VHS 1256-1257

Aims. With the purpose of investigating the radio emission of new ultracool objects, we carried out a targeted search in the recently discovered system VHS J125601.92-125723.9 (hereafter VHS 1256-1257); this system is composed by an equal-mass M7.5 binary and a L7 low-mass substellar object located at only 15.8 pc. Methods. We observed in phase-reference mode the system VHS 1256-1257 with the Karl G. Jansky Very Large Array at X band and L band and with the European VLBI Network at L band in several epochs during 2015 and 2016. Results. We discovered radio emission at X band spatially coincident with the equal-mass M7.5 binary with a flux density of 60 mu Jy. We determined a spectral index alpha = -1.1 +/- 0.3 between 8 and 12 GHz, suggesting that non-thermal, optically thin, synchrotron, or gyrosynchrotron radiation is responsible for the observed radio emission. Interestingly, no signal is seen at L band where we set a 3 sigma upper limit of 20 mu Jy. This might be explained by strong variability of the binary or self-absorption at this frequency. By adopting the latter scenario and gyrosynchrotron radiation, we constrain the turnover frequency to be in the interval 5-8.5 GHz, from which we infer the presence of kG-intense magnetic fields in the M7.5 binary. Our data impose a 3 alpha upper bound to the radio flux density of the L7 object of 9 mu Jy at 10 GHz.EVN project EG092 Spanish MINECO projects AYA2012-38491-C02-01 AYA2015-63939-C2-2-P Generalitat Valenciana projects PROMETEO/2009/104 PROM-ETEOII/2014/057 CONICYT through FONDECYT Fellowship 3170513 Spanish MINECO AYA2015-63939-C2-1-