2021MNRAS.502..521D

We present quasi-simultaneous radio, (sub-)millimetre, and X-ray observations of the Galactic black hole X-ray binary GX 339-4, taken during its 2017-2018 outburst, where the source remained in the hard X-ray spectral state. During this outburst, GX 339-4 showed no atypical X-ray behaviour that may act as an indicator for an outburst remaining within the hard state. However, quasi-simultaneous radio and X-ray observations showed a flatter than expected coupling between the radio and X-ray luminosities (with a best-fitting relation of L_radio ∝L_X^0.39 ±0.06^), when compared to successful outbursts from this system (L_radio ∝L_X^0.62 ±0.02^). While our 2017-2018 outburst data only span a limited radio and X-ray luminosity range (∼1 order of magnitude in both, where more than 2 orders of magnitude in L_X is desired), including data from other hard-only outbursts from GX 339-4 extends the luminosity range to ∼1.2 and ∼2.8 orders of magnitude, respectively, and also results in a flatter correlation (where L_radio ∝L_X^0.46 ±0.04^). This result is suggestive that for GX 339-4 a flatter radio-X-ray correlation, implying a more inefficient coupling between the jet and accretion flow, could act as an indicator for a hard-only outburst. However, further monitoring of both successful and hard-only outbursts over larger luminosity ranges with strictly simultaneous radio and X-ray observations is required from different single sources to explore if this applies generally to the population of black hole X-ray binaries, or even GX 339-4 at higher hard-state luminosities.