A fast radio burst localized at detection to an edge-on galaxy using very-long-baseline interferometry

Fast radio bursts (FRBs) are millisecond-duration radio transients whose origins remain unknown. As the vast majority of bursts are one-off events, it is necessary to pinpoint FRBs precisely within their host galaxies at the time of detection. Here we use two purpose-built outrigger telescopes to localize FRB 20210603A at the time of its detection by the Canadian Hydrogen Intensity Mapping Experiment (CHIME). Our very-long-baseline interferometry stations localized the burst to a 0.2" × 2" final ellipse in the disk of its host galaxy SDSS J004105.82+211331.9. A spatially resolved spectroscopic follow-up revealed recent star formation (Hα emission) on kiloparsec scales near the burst position. The excess dispersion measure is consistent with expectations from the nearly edge-on disk of the host galaxy, demonstrating the utility of FRBs as probes of the interstellar medium in distant galaxies. The excess dispersion measure, rotation measure and scattering are consistent with expectations for a pulse travelling from deep within its host galactic plane, strengthening the link between the local environment of FRB 20210603A and the disk of its host galaxy. Finally, this technique demonstrates a way to overcome the trade-off between angular resolution and field of view in FRB instrumentation, paving the way towards plentiful and precise FRB localizations. Two ‘outrigger’ telescopes were added to CHIME, leading to the localization at detection of FRB 20210603A. The burst originated from deep within the star-forming disk of its host galaxy.