A nebular origin for the persistent radio emission of fast radio bursts

Fast Radio Bursts (FRBs) are millisecond-duration, bright (∼Jy) extragalactic bursts, whose production mechanism is still unclear. Recently, a persistent radio source (PRS) of non-thermal origin was discovered to be physically associated to two of the repeating FRB sources. These two sources have unusually large Rotation Measure (RM) values likely tracing a dense magneto-ionic medium, consistent with a synchrotron radiation originating from a nebula surrounding the FRB source. Recent theoretical arguments predict that, if the observed RM mostly arises from the PRS region, there should be a simple relation between the luminosity of the PRS and the RM 8, 6. We report here the detection of a third, less luminous PRS associated with the nearby FRB 20201124A at a distance of 413 Mpc, significantly expanding the predicted relation into the low luminosity-low RM regime (<1000 rad/m2). At lower values of the RM, the radio luminosity falls below the limit of detection threshold for nowadays radio telescopes. These findings support the idea that the PRS is generated by a nebula in the FRB environment, and that most FRBs do not show a PRS because of a weaker magneto-ionic medium. This is generally consistent with models foreseeing a young magnetar as the central engine of the FRB, where the surrounding ionized nebula powers the PRS.