The Environment of FRB 121102

J. I. Katz

Published 2020-11-23Version 1

Variations of the dispersion (DM) and rotation (RM) measures of FRB 121102 indicate magnetic fields $\sim$ 3--17 mG in the dispersing plasma. The electron density may be $\sim 10^4\,$cm$^{-3}$. The observed time scales $\sim 1$ year 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 or changing electron density. The extraordinarily large RM of FRB 121102 implies an environment, and possibly also formation process and source, qualitatively different from those of other FRB. An appendix discusses the age of FRB 121102 in the context of a "Copernican Principle".