Radio emission from the Be/black hole binary MWC 656

Abstract

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. Aims. 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. Methods. We used three archived VLA data sets, one hour each at 3 GHz, and seven new VLA observations, two hours each at 10 GHz, to produce very high sensitivity images down to ~1μJy. Results. 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 = 6.6 μJy, but no radio emission is detected there. Conclusions. 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.