No Low-frequency Emission from Extremely Bright Fast Radio Bursts

We present the results of a coordinated campaign conducted with the Murchison Widefield Array (MWA) to shadow fast radio bursts (FRBs) detected by the Australian Square Kilometre Array Pathfinder (ASKAP) at 1.4 GHz, which resulted in simultaneous MWA observations of seven ASKAP FRBs. We de-dispersed the 24 ×1.28 MHz MWA images across the 170-200 MHz band taken at 0.5 s time resolution at the known dispersion measures (DMs) and arrival times of the bursts and searched both within the ASKAP error regions (typically ∼10′ ×10′), and beyond (4° ×4°). We identified no candidates exceeding a 5σ threshold at these DMs in the dynamic spectra. These limits are inconsistent with the mean fluence scaling of α = -1.8 ±0.3 (F_ν ∝ ν ^α, where ν is the observing frequency) that is reported for ASKAP events, most notably for the three high-fluence (F_1.4 GHz ≳ 100 Jy ms) FRBs 171020, 180110, and 180324. Our limits show that pulse broadening alone cannot explain our non-detections, and that there must be a spectral turnover at frequencies above 200 MHz. We discuss and constrain parameters of three remaining plausible spectral break mechanisms: free-free absorption, intrinsic spectral turnover of the radiative processes, and magnification of signals at ASKAP frequencies by caustics or scintillation. If free-free absorption were the cause of the spectral turnover, we constrain the thickness of the absorbing medium in terms of the electron temperature, T, to <0.03 (T/10⁴ K)^-1.35 pc for FRB 171020.