LIMITS on FAST RADIO BURSTS from FOUR YEARS of the V-FASTR EXPERIMENT

The V-FASTR experiment on the Very Long Baseline Array was designed to detect dispersed pulses of milliseconds in duration, such as fast radio bursts (FRBs). We use all V-FASTR data through 2015 February to report V-FASTR’s upper limits on the rates of FRBs, and compare these with rederived rates from Parkes FRB detection experiments. V-FASTR’s operation at l = 20 cm allows direct comparison with the 20 cm Parkes rate, and we derive a power-law limit of g < -0.4 (95% confidence limit) on the index of FRB source counts, N (>S) μ Sg. Using the previously measured FRB rate and the unprecedented amount of survey time spent searching for FRBs at a large range of wavelengths (0.3 cm > l > 90 cm), we also place frequency-dependent limits on the spectral distribution of FRBs. The most constraining frequencies place two-point spectral index limits of a 20 cm < 5.8 4 cm and a > -7.6 90 cm 20 cm , where fluence F μ f a if we assume that the burst rate reported by Champion et al. of R(F ~ 0.6 Jy ms) = 7 ´ 103 sky-1 day-1 is accurate (for bursts of ∼3 ms duration). This upper limit on α suggests that if FRBs are extragalactic but noncosmological, on average they are not experiencing excessive free–free absorption due to a medium with high optical depth (assuming temperature ∼8000 K), which excessively inverts their low-frequency spectrum. This in turn implies that the dispersion of FRBs arises in either or both of the intergalactic medium or the host galaxy, rather than from the source itself.