Fast radio bursts: constraints on the dispersing medium

Abstract

Abstract Fast radio bursts appear to exhibit large dispersion measures, typically exceeding any expected Galactic interstellar contribution, especially along the moderate to high Galactic latitude directions in which such events have been most often observed. The dispersions have been therefore interpreted as extragalactic, with the sources of the bursts at Gpc distances. This then implies that the bursts are extremely energetic events, originating from quite small volumes. To circumvent the energetic difficulties, Loeb et al. propose that the bursts are produced by flares near the surfaces of M stars or contact binaries within a local volume of the Galaxy, with the observed dispersion occurring in the overlying stellar coronae. With the dispersion concentrated in a high-density region, the quadratic dispersion approximation breaks down as the plasma frequency is comparable to (although less than) the propagation frequency. The observed dispersion curves are closely quadratic, however, consistent with a low-density medium, ruling out this model. It thus appears highly likely that the dispersions occur in the intergalactic medium. This medium, probably containing most of the baryon content of the Universe, is expected to be highly structured on large scales. Hot gas within clusters and especially groups of galaxies may contribute significantly to the observed dispersion.