2025ApJ...993..208S

Fast radio bursts (FRBs) are unique probes of extragalactic ionized baryonic structure as each signal, through its burst properties, holds information about the ionized matter it encounters along its sightline. FRB 20200723B is a burst with a scattering timescale of τ_400 MHz_ > 1 s at 400 MHz and a dispersion measure of DM ∼244 pc cm^–3. Observed across the entire Canadian Hydrogen Intensity Mapping Experiment (CHIME)/FRB frequency band, it is the single-component burst with the largest scattering timescale yet observed by CHIME/FRB. The combination of its high scattering timescale and relatively low dispersion measure present an uncommon opportunity to use FRB 20200723B to explore the properties of the cosmic web it traversed. With an ∼arcminute-scale localization region, we find the most likely host galaxy is NGC 4602 (with PATH probability P(O∣x) = 0.985), which resides ∼30 Mpc away within a cosmic sheet structure on the outskirts of the Virgo Cluster. We place an upper limit on the average free electron density of this cosmic structure of $\langle {n}_{e}\rangle \,\lt \,4.{6}_{-2.0}^{+9.6}\times 1{0}^{-5}$ cm^–3, broadly consistent with expectations from cosmological simulations. We investigate whether the source of scattering lies within the same galaxy as the FRB, or at a farther distance from an intervening structure along the line of sight. Comparing with Milky Way pulsar observations, we suggest the scattering may originate from within the host galaxy of FRB 20200723B.