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During a 2018 outburst, the black hole X-ray binary MAXI J1820+070 wascomprehensively monitored at multiple wavelengths as it underwent a hard tosoft state transition. During this transition a rapid evolution in X-ray timingproperties and a short-lived radio flare were observed, both of which werelinked to the launching of bi-polar, long-lived relativistic ejecta. We providedetailed analysis of two Very Long Baseline Array observations, using both timebinning and a new dynamic phase centre tracking technique to mitigate theeffects of smearing when observing fast-moving ejecta at high angularresolution. We identify a second, earlier ejection, with a lower proper motionof $18.0\pm1.1$ mas day$^{-1}$. This new jet knot was ejected $4\pm1$ hoursbefore the beginning of the rise of the radio flare, and $2\pm1$ hours before aswitch from type-C to type-B X-ray quasi-periodic oscillations (QPOs). We showthat this jet was ejected over a period of $\sim6$ hours and thus its ejectionwas contemporaneous with the QPO transition. Our new technique locates theoriginal, faster ejection in an observation in which it was previouslyundetected. With this detection we revised the fits to the proper motions ofthe ejecta and calculated a jet inclination angle of $(64\pm5)^\circ$, and jetvelocities of $0.97_{-0.09}^{+0.03}c$ for the fast-moving ejecta ($\Gamma>2.1$)and $(0.30\pm0.05)c$ for the newly-identified slow-moving ejection($\Gamma=1.05\pm0.02$). We show that the approaching slow-moving component ispredominantly responsible for the radio flare, and is likely linked to theswitch from type-C to type-B QPOs, while no definitive signature of ejectionwas identified for the fast-moving ejecta.

The Varying Kinematics of Multiple Ejecta from the Black Hole X-ray Binary MAXI J1820+070