2015A&A...580L...6D

MWC 656 is the recently discovered first binary system case composed of a Be-type star and an accreting black hole. Its low X-ray luminosity indicates that the system is in a quiescent X-ray state. The aim of our investigation is to establish if the MWC 656 system has detectable radio emission and if the radio characteristics are consistent with those of quiescent black hole systems. We used three archived VLA data sets, one hour each at 3GHz, and seven new VLA observations, two hours each at 10GHz, to produce very high sensitivity images down to ∼1µJy. We detected the source twice in the new observations: in the first VLA run, at apastron passage, with a flux density of 14.2±2.9µJy and by combining all together the other six VLA runs, with a flux density of 3.7±1.4µJy. The resulting combined map of the archived observations has the sensitivity of 1σ=6.6µJy, but no radio emission is detected there. The radio and X-ray luminosities agree with the behaviour of accreting binary black holes in the hard and quiescent state. In particular, MWC 656 in the L_X, L_R plane occupies the same region as A0620-00 and XTE J1118+480, the faintest known black holes up to now.