The ultra narrow FRB20191107B, and the origins of FRB scattering

TL;DR

This paper reports the detection of an ultra-narrow FRB with minimal scattering, analyzes survey sensitivities to such events, and constrains the turbulence in the intergalactic medium, suggesting local environments dominate FRB scattering.

Contribution

It presents the first detailed analysis of ultra-narrow FRB components, models survey sensitivities to narrow bursts, and constrains IGM turbulence using new high-resolution data.

Findings

Over 60% of similar narrow FRBs are missed by current surveys.

The IGM scattering measure is constrained to be very low, indicating minimal turbulence.

Local host galaxy environments likely dominate FRB scattering effects.

Abstract

We report the detection of FRB20191107B with the UTMOST radio telescope at a dispersion measure (DM) of 714.9 ${\rm pc~cm^{-3}}$. 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 FRB surveys to such narrow events. We find that $>60\%$ of FRBs like FRB20191107B are being missed, and that a significant population of very narrow FRBs probably exists and remains underrepresented in these surveys. The high DM and small scattering timescale of FRB20191107B allows us to place an upper limit on the strength of turbulence in the Intergalactic Medium (IGM), quantified as scattering measure (SM), of ${\rm SM_{IGM} < 8.4 \times 10^{-7} ~kpc~m^{-20/3}}$. 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 FRBs. Our 10.24 $\mu$s time resolution data yields accurately measured FRB scattering timescales. 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 which dominates the observed scattering of our FRBs.