Fast radio bursts (FRBs) are millisecond-duration radio pulses of extragalactic origin with largely unknown progenitors. Some FRBs show mild variations in their magneto-ionic environment, possibly due to plasma turbulence or changes in the geometry of a binary system. Here, Li et al. report a sudden and significant shift in the magneto-ionic environment of FRB 20220529, a repeating FRB from a disk galaxy at redshift 0.1839. For 17 months, its Faraday rotation measure (RM) remained stable at 21 ± 96 rad m⁻². However, in December 2023, the RM spiked to 1976.9 rad m⁻², exceeding 20 times the standard deviation of its previous variations, before returning to typical values within two weeks. This abrupt RM fluctuation suggests the passage of a dense, magnetised clump across the line of sight—potentially linked to a coronal mass ejection (CME) from a stellar flare. These findings imply that the FRB likely has a companion star responsible for the observed stellar activity.

The figure illustrates the temporal variations of key parameters of FRB 20220529. Panels (A–C) show the daily observing time, number of detected bursts, and burst rate, with vertical grey dotted lines marking observation days, including non-detections. Panel (D) presents the dispersion measure (DM) of detected bursts, with blue and cyan dots representing FAST and Parkes observations, respectively. Panel (E) shows the linear polarization fraction, while panel (F) displays the rotation measure (RM) evolution. A distinct RM flare is observed, with the horizontal dotted line indicating the Milky Way’s contribution and the filled blue region highlighting the low RM state.