The ultranarrow FRB20191107B, and the origins of FRB scattering

We report the detection of FRB20191107B with UTMOST radio telescope at a dispersion measure (DM) of 714.9 pc cm(-3) . The burst consists of three components, the brightest of which has an intrinsic width of only 11.3 mu s and a scattering tail with an exponentially decaying time-scale of 21.4 mu s measured at 835 MHz. We model the sensitivity of UTMOST and other major fast radio burst (FRB) surveys to such narrow events. We find that > 60 per cent of FRBs like FRB20191107B are being missed, and that a significant population of very narrow FRB5 probably exists and remains underrepresented in these surveys. The high DM and small scattering time-scale of FRB20191107B allows us to place an upper limit on the strength of turbulence in the intergalactic medium, quantified as scattering measure (SM), of SMIGM < 8.4 x 10(-7) kpc m(-20/3) . Almost all UTMOST FRBs have full phase information due to real-time voltage capture, which provides us with the largest sample of coherently dedispersed single burst FRB5. Our 10.24 mu s time resolution data yields accurately measured FRB scattering time-scales. We combine the UTMOST FRBs with 10 FRBs from the literature and find no obvious evidence for a DM-scattering relation, suggesting that IGM is not the dominant source of scattering in FRBs. We support the results of previous studies and identify the local environment of the source in the host galaxy as the most likely region that dominates the observed scattering of our FRBs.