Fast Radio Bursts ( FRBs ) , are millisecond radio signals that exhibit dispersion larger than what the Galactic electron density can account for .
We have conducted a 1446 hour survey for Fast Radio Bursts ( FRBs ) at 145 MHz , covering a total of 4193 sq .
deg on the sky .
We used the UK station of the LOFAR radio telescope – the Rawlings Array – , accompanied for a majority of the time by the LOFAR station at Nançay , observing the same fields at the same frequency .
Our real-time search backend , ARTEMIS , utilizes graphics processing units to search for pulses with dispersion measures up to 320 cm ^ { -3 } pc .
Previous derived FRB rates from surveys around 1.4 GHz , and favoured FRB interpretations , motivated this survey , despite all previous detections occurring at higher dispersion measures .
We detected no new FRBs above a signal-to-noise threshold of 10 , leading to the most stringent upper limit yet on the FRB event rate at these frequencies : 29 sky ^ { -1 } day ^ { -1 } for 5 ms-duration pulses above 62 Jy .
The non-detection could be due to scatter-broadening , limitations on the volume and time searched , or the shape of FRB flux density spectra .
Assuming the latter and that FRBs are standard candles , the non-detection is compatible with the published FRB sky rate , if their spectra follow a power law with frequency ( \propto \nu ^ { \alpha } ) , with \alpha \gtrsim + 0.1 , demonstrating a marked difference from pulsar spectra .
Our results suggest that surveys at higher frequencies , including the low frequency component of the Square Kilometre Array , will have better chances to detect , estimate rates and understand the origin and properties of FRBs .