2021MNRAS.501L..76K

Variations of the dispersion measures (DM) and rotation measures (RM) of fast radio bursts (FRBs) 121102 indicate magnetic fields ∼3-17 mG in the dispersing plasma. The electron density may be ∼10⁴ cm^–3. The observed time scales ∼1 yr constrain the size of the plasma cloud. Increasing DM excludes simple models involving an expanding supernova remnant, and the non-zero RM excludes spherical symmetry. The varying DM and RM may be attributable to the motion of plasma into or out of the line of sight to or changing electron density within slower moving plasma. The extraordinarily large RM of FRB 121102 implies an environment, and possibly also a formation process and source, qualitatively different from those of other FRB. The comparable and comparably varying RM of SGR/PSR J1745-2900 suggests it as a FRB candidate. Appendix A discusses the age of FRB 121102 in the context of a 'Copernican Principle'.