摘要：Aim.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 theKarl G. JanskyVery Large Array atXband andLband and with the European VLBI Network atLband in several epochs during 2015 and 2016.Results.We discovered radio emission atXband spatially coincident with the equal-mass M7.5 binary with a flux density of 60μJy. We determined a spectral indexα= −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 atLband where we set a 3σupper limit of 20μJy. 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 3σupper bound to the radio flux density of the L7 object of 9μJy at 10 GHz.
关键词：Keywordsenbrown dwarfsstars: magnetic fieldradiation mechanisms: generaltechniques: interferometric
