2025ApJ...993..221A

Fast radio bursts (FRBs) are millisecond-duration radio transients that serve as unique probes of ionizedextragalactic matter. We report the discovery and localization of two FRBs piercing the Andromeda galaxy (M31) with the realfast transient-detection system at the Very Large Array. These unique sightlines enable constraints on M31's electron density distribution. We localized FRB 20230930A to a host galaxy at redshift z = 0.0925 and FRB 20230506C to a host galaxy at redshift z = 0.3896. After accounting for the dispersion contributions from the Milky Way, the host galaxies, and the intergalactic medium, we estimate M31's contribution to be 26-239 pc cm^–3 toward FRB 20230930A and 51-366 pc cm^–3 toward FRB 20230506C, within the 90% credible interval (CI). By modeling the M31 disk's contribution, we isolate the halo component and find that M31's halo contributes 7-169 pc cm^–3 along FRB 20230930A (90% CI). The inferred values of DM_M31,halo from the FRBs are consistent with predictions from a modified Navarro-Frenk-White profile at the corresponding impact parameter. The cool and warm phase gas is unlikely to account for the DM_M31,halo unless the ionization fraction is as high as 90%. While limited to two sightlines, these results offer tentative evidence for the existence of a hot halo surrounding M31. We also discuss the potential contribution of other foreground structures, particularly in explaining the DM excess observed in FRB 20230506C. This work demonstrates how FRBs can be used to probe the circumgalactic medium of intervening galaxies.