The odyssey of the black hole low-mass X-ray binary GX 339−4: five years of dense multiwavelength monitoring

ABSTRACT We present the longest and the densest quasi-simultaneous radio, X-ray, and optical campaign of the black hole low-mass X-ray binary GX 339−4, covering five years of weekly GX 339−4 monitoring with MeerKAT, Swift/XRT, and MeerLICHT, respectively. Complementary high-frequency radio data with the Australia Telescope Compact Array are presented to track in more detail the evolution of GX 339−4 and its transient ejecta. During the five years, GX 339−4 has been through two ‘hard-only’ outbursts and two ‘full’ outbursts, allowing us to densely sample the rise, quenching, and re-activation of the compact jets. Strong radio flares were also observed close to the transition between the hard and the soft states. Following the radio flare, a transient optically thin ejection was spatially resolved during the 2020 outburst, and was observed for a month. We also discuss the radio/X-ray correlation of GX 339−4 during this five year period, which covers several states in detail from the rising phase to the quiescent state. This campaign allowed us to follow ejection events and provide information on the jet proper motion and its intrinsic velocity. With this work, we publicly release the weekly MeerKAT L-band radio maps from data taken between 2018 September and 2023 October.