Radio observations of the Black Hole X-ray Binary EXO 1846-031 re-awakening from a 34-year slumber

We present radio [1.3 GHz MeerKAT, 4-8 GHz Karl G. Jansky Very Large Array (VLA), and 15.5 GHz Arcminute Microkelvin Imager Large Array (AMI-LA)] and X-ray (Swift and MAXI) data from the 2019 outburst of the candidate Black Hole X-ray Binary (BHXB) EXO 1846-031. We compute a Hardness-Intensity diagram, which shows the characteristic q-shaped hysteresis of BHXBs in outburst. EXO 1846-031 was monitored weekly with MeerKAT and approximately daily with AMI-LA. The VLA observations provide sub-arcsecond-resolution images at key points in the outburst, showing moving radio components. The radio and X-ray light curves broadly follow each other, showing a peak on ~MJD 58702, followed by a short decline before a second peak between ~MJD 58731-58739. We estimate the minimum energy of these radio flares from equipartition, calculating values of E_min ~ 4 × 10⁴¹ and 5 × 10⁴² erg, respectively. The exact date of the return to 'quiescence' is missed in the X-ray and radio observations, but we suggest that it likely occurred between MJD 58887 and 58905. From the Swift X-ray flux on MJD 58905 and assuming the soft-to-hard transition happened at 0.3-3 per cent Eddington, we calculate a distance range of 2.4-7.5 kpc. We computed the radio:X-ray plane for EXO 1846-031 in the 'hard' state, showing that it is most likely a 'radio-quiet' BH, preferentially at 4.5 kpc. Using this distance and a jet inclination angle of θ = 73°, the VLA data place limits on the intrinsic jet speed of β_int = 0.29c, indicating subluminal jet motion.