Fast Radio Bursts: Constraints on the Dispersing Medium
  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, leading to the inference that the sources of the bursts are at Gpc distances. This then implies that the bursts are extremely energetic events, originating from quite small volumes (due to the millisecond burst durations). To circumvent the energetic difficulties, Loeb, Shvartzvald, & Maoz (2014) propose that the bursts are produced by flares near the surfaces of M stars or contact binaries within a local volume of the galaxy. Most of the dispersion would then occur in the overlying stellar coronae. With the dispersion concentrated in a relatively 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. Because any model invoking local galactic sources would require a concentrated high density dispersing medium, it 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. Optical and X-ray observations, including redshifts and combined with cosmological dispersion measures, can probe the distribution of the IGM and determine how much of it lies outside the X-ray luminous concentrations seen in clusters and groups. 
Brian Dennison
astro-ph.HE