A flaring radio counterpart to a fast radio burst reveals a newborn magnetized engine

Fast Radio Bursts (FRBs) are energetic millisecond radio bursts at cosmological distances, whose underlying engine is not identified. Among a sub-population that emit repeated radio bursts, a handful were associated with a persistent radio source (PRS) whose origin is unknown. Here we report the discovery of a compact flaring radio source (FRS) associated with a newly-active repeating FRB within one month after the first radio burst was detected. Its temporal and spectral characteristics differ from those of the PRSs but are similar to those of engine-powered supernovae and low-luminosity active galactic nuclei. We detected a spectral peak around $1.6\pm0.2$ GHz that is consistent with synchrotron self-absorption. Assuming equipartition, the magnetic field strength in the FRS is larger than the line-of-sight component constrained from the FRB Faraday rotation, suggesting a highly magnetized engine. The radius of the FRS is constrained to be $\sim0.03$ pc and the minimum total energy is $\sim~6.2\times{10}^{47}~{\rm ergs~}$. This FRS reveals the birth of a highly magnetized FRB engine, and hints that PRSs associated with other active FRBs may be in the later stage of evolution.