Fast radio bursts (FRBs) are millisecond-duration, bright (approximately Jy) extragalactic bursts, whose production mechanism is still unclear¹. Recently, two repeating FRBs were found to have a physically associated persistent radio source of non-thermal origin^2,3. These two FRBs have unusually large Faraday rotation measure values^2,3, probably tracing a dense magneto-ionic medium, consistent with synchrotron radiation originating from a nebula surrounding the FRB source^4,5,6,7,8. Recent theoretical arguments predict that, if the observed Faraday rotation measure mostly arises from the persistent radio source region, there should be a simple relation between the persistent radio source luminosity and the rotation measure itself^7,9. Here we report the detection of a third, less luminous persistent radio source associated with the repeating FRB source FRB 20201124A at a distance of 413 Mpc, substantially expanding the predicted relation into the low luminosity–low Faraday rotation measure regime (<1,000 rad m⁻²). At lower values of the Faraday rotation measure, the expected radio luminosity falls below the limit-of-detection threshold for present-day radio telescopes. These findings support the idea that the persistent radio sources observed so far are generated by a nebula in the FRB environment and that FRBs with low Faraday rotation measure may not show a persistent radio source because of a weaker magneto-ionic medium. This is generally consistent with models invoking a young magnetar as the central engine of the FRB, in which the surrounding ionized nebula—or the interacting shock in a binary system—powers the persistent radio source.

快速射电暴（FRB）是一种持续时间为毫秒、明亮（约 Jy）的河外爆发，其产生机制尚不清楚¹ 。最近，发现两个重复的快速射电暴具有物理关联的非热源持久射电源^2,3 。这两个 FRB 具有异常大的法拉第旋转测量值^2,3 ，可能追踪致密磁离子介质，与源自 FRB 源周围星云的同步加速器辐射^{一致 4,5,6,7,8} 。最近的理论论证预测，如果观察到的法拉第旋转测量主要来自持续无线电源区域，则持续无线电源光度和旋转测量本身之间应该存在简单的关系^7,9 。在这里，我们报告了在 413 Mpc 距离处检测到与重复 FRB 源 FRB 20201124A 相关的第三个亮度较低的持续射电源，从而将预测关系大大扩展为低光度 - 低法拉第旋转测量范围 (<1,000 rad) m ^-2 )。在法拉第旋转测量值较低时，预期射电光度低于当今射电望远镜的检测极限阈值。这些发现支持这样的观点，即迄今为止观察到的持久射电源是由快速射电暴环境中的星云产生的，并且由于磁离子介质较弱，法拉第旋转测量值较低的快速射电暴可能不会显示出持久射电源。这通常与调用年轻磁星作为快速射电暴中央引擎的模型一致，其中周围的电离星云或双星系统中相互作用的激波为持久射电源提供动力。