Black holes in binary systems execute patterns of outburst activity where two characteristic X-ray states are associated with different behaviours observed at radio wavelengths. The hard state is associated with radio emission indicative of a continuously replenished, collimated, relativistic jet, whereas the soft state is rarely associated with radio emission, and never continuously, implying the absence of a quasi-steady jet. Here we report radio observations of the black hole transient MAXI J1820+070 during its 2018 outburst. As the black hole transitioned from the hard to soft state, we observed an isolated radio flare, which, using high-angular-resolution radio observations, we connect with the launch of bipolar relativistic ejecta. This flare occurs as the radio emission of the core jet is suppressed by a factor of over 800. We monitor the evolution of the ejecta over 200 days and to a maximum separation of 10″, during which period it remains detectable due to in situ particle acceleration. Using simultaneous radio observations sensitive to different angular scales, we calculate an accurate estimate of energy content of the approaching ejection. This energy estimate is far larger than that derived from the state transition radio flare, suggesting a systematic underestimate of jet energetics.

双星系统中的黑洞执行爆发活动的模式，其中两种特征 X 射线状态与在无线电波长下观察到的不同行为相关联。硬态与无线电发射相关，表明持续补充、准直、相对论射流，而软态很少与无线电发射相关，并且从不连续，这意味着不存在准稳态射流。在这里，我们报告了黑洞瞬态 MAXI J1820+070 在 2018 年爆发期间的无线电观测。随着黑洞从硬状态转变为软状态，我们观察到了一个孤立的射电耀斑，使用高分辨率射电观测，我们将其与双极相对论喷射物的发射联系起来。当核心射流的无线电发射被抑制超过 800 倍时，就会发生这种耀斑。我们在 200 天内监测喷射物的演变，最大间隔为 10 英寸，在此期间，由于原位粒子加速，它仍然可以检测到。使用对不同角度尺度敏感的同时无线电观测，我们计算了接近弹射的能量含量的准确估计。这种能量估计远大于从状态转换射电耀斑中得出的估计，表明系统地低估了射流能量学。