We present results from a new incoherent-beam Fast Radio Burst ( FRB ) search on the Canadian Hydrogen Intensity Mapping Experiment ( CHIME ) Pathfinder .
Its large instantaneous field of view ( FoV ) and relative thermal insensitivity allow us to probe the ultra-bright tail of the FRB distribution , and to test a recent claim that this distribution ’ s slope , \alpha \equiv - \frac { \partial \log N } { \partial \log S } , is quite small .
A 256-input incoherent beamformer was deployed on the CHIME Pathfinder for this purpose .
If the FRB distribution were described by a single power-law with \alpha = 0.7 , we would expect an FRB detection every few days , making this the fastest survey on sky at present .
We collected 1268 hours of data , amounting to one of the largest exposures of any FRB survey , with over 2.4 \times 10 ^ { 5 } deg ^ { 2 } hrs .
Having seen no bursts , we have constrained the rate of extremely bright events to < 13 sky ^ { -1 } day ^ { -1 } above \sim 220 \sqrt { ( \tau / ms ) } Jy ms for \tau between 1.3 and 100 ms , at 400–800 MHz .
The non-detection also allows us to rule out \alpha \lesssim 0.9 with 95 \% confidence , after marginalizing over uncertainties in the GBT rate at 700–900 MHz , though we show that for a cosmological population and a large dynamic range in flux density , \alpha is brightness-dependent .
Since FRBs now extend to large enough distances that non-Euclidean effects are significant , there is still expected to be a dearth of faint events and relative excess of bright events .
Nevertheless we have constrained the allowed number of ultra-intense FRBs .
While this does not have significant implications for deeper , large-FoV surveys like full CHIME and APERTIF , it does have important consequences for other wide-field , small dish experiments .