Colloquium: Probing the foreground and source-local medium with fast radio bursts

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Fast radio bursts (FRBs) are brief, energetic radio emissions originating predominantly from extragalactic sources. While their exact origins remain unknown, these low-frequency signals are excellent tools for studying the plasma along their propagation path. By monitoring the Faraday rotation of the repeating FRB 20190520B over 17 months, we constrained its local environment, shedding light on its potential sources. During this period, the FRB's Faraday rotation exhibited high variability, reversed sign twice, and the bursts showed depolarization toward low frequencies. These characteristics suggest changes in the parallel component of the integrated magnetic field along the line-of-sight, likely due to a turbulent, magnetized plasma screen situated 8 au to 100 parsecs from the FRB, potentially influenced by the winds of a binary companion.

We also utilized the frequency-dependent dispersion of FRB 20230930A, detected by the realfast transient detection system at the Very Large Array, to probe the halo of the Andromeda galaxy (M31). The unique line-of-sight geometry of this FRB enabled us to constrain the electron density distribution of M31. After accounting for dispersion contributions from the Milky Way, the host galaxy, and the intergalactic medium, we estimated M31’s contribution to be between 87- 305 pc cm^-3 with 90% confidence. By modeling the disk contribution of M31’s dispersion measure (DM), determined to be 139±70 pc cm^-3, we isolated the halo's contribution, which ranges between 28 - 219 pc cm^-3 with 90% confidence. This result aligns with predictions from the modified Navarro-Frenk-White profile for M31’s halo at the given impact parameter. The ions in the cool halo alone cannot account for the calculated DM of M31’s halo, suggesting indirect evidence of a hot halo component.