Limits on fast radio bursts at 145 MHz with artemis, a real-time software backend

Abstract

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