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 a 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 fit relation of $L_{rm radio} propto L_{rm X}^{0.39 pm 0.06}$), when compared to successful outbursts from this system ($L_{rm radio} propto L_{rm X}^{0.62 pm 0.02}$). While our 2017--2018 outburst data only span a limited radio and X-ray luminosity range ($sim$1 order of magnitude in both, where more than 2-orders of magnitude in $L_{rm X}$ is desired), including data from other hard-only outbursts from GX 339-4 extends the luminosity range to $sim$1.2 and $sim$2.8 orders of magnitude, respectively, and also results in a flatter correlation (where $L_{rm radio} propto L_{rm X}^{0.46 pm 0.04}$). 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.
تحميل البحث