The V-FASTR experiment on the Very Long Baseline Array was designed to detect dispersed pulses of milliseconds duration , such as fast radio bursts ( FRBs ) .
We use all V-FASTR data through February 2015 to report V-FASTR ’ s upper limits on the rates of FRBs , and compare these with re-derived rates from Parkes FRB detection experiments .
V-FASTR ’ s operation at \lambda = 20 cm allows direct comparison with the 20 cm Parkes rate , and we derive a power-law limit of \gamma < -0.4 ( 95 % confidence limit ) on the index of FRB source counts , N ( > S ) \propto S ^ { \gamma } .
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 } > \lambda > 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 \alpha _ { 20 cm } ^ { 4 cm } < 5.8 and \alpha _ { 90 cm } ^ { 20 cm } > -7.6 , where fluence F \propto f ^ { \alpha } if we assume true the burst rate reported by of R ( F \sim 0.6 { Jy ms } ) = 7 \times 10 ^ { 3 } { sky ^ { -1 } day ^ { -1 } } ( for bursts of \sim 3 ms duration ) .
This upper limit on \alpha suggests that if FRBs are extragalactic but non-cosmological , that on average they are not experiencing excessive free-free absorption due to a medium with high optical depth ( assuming temperature \sim 8,000 K ) , which excessively invert 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 .