Implications of Fast Radio Burst Pulse Widths

Understand

The pulse widths, dispersion measures and dispersion indices of Fast Radio Bursts (FRB) impose coupled constraints that all models must satisfy.

• We show that if the dispersion measures resulted from propagation through the intergalactic medium at cosmological distances and the pulse widths were a consequence of scattering by single thin screens, then the screens' electron densities were $\gtrsim 20$/cm$^3$, $10^8$ times the mean intergalactic density.
• This problem is resolved if the radiation scattered close to its source, where high densities are possible.
• Observation of dispersion indices close to their low density limit of $-2$ sets a model-independent upper bound on the electron density and a lower bound on the size of the dispersive plasma cloud, excluding terrestrial or Solar System origin.