Magnetic Field Reversal around an Active Fast Radio Burst
  The environment of actively repeating fast radio bursts (FRBs) has been shown
to be complex and varying. The recently localized FRB 20190520B is extremely
active, has the largest confirmed host dispersion measure, and is only the
second FRB source associated with a compact, persistent radio source (PRS). The
main tracer of the magneto-ionic environments is the rotation measure (RM), a
path-integral of the line-of-sight component of magnetic field strength (B) and
electron density, which does not allow a direct probe of the B-field
configuration. Here we report direct evidence for a B-field reversal based on
the observed sign change and extreme variation of FRB 20190520B's RM, which
changed from $\sim10000$ rad m$^{-2}$ to $\sim-16000$ rad m$^{-2}$ between June
2021 and January 2022. Such extreme RM reversal has never been observed before
in any FRB nor in any astronomical object. The implied short-term change of the
B-field configuration in or around the FRB could be due to the vicinity of
massive black holes, or a magnetized companion star in binary systems, or a
young supernova remnant along the line of sight.
