A commensal Fast Radio Burst search pipeline for the Murchison Widefield Array

We present a demonstration version of a commensal pipeline for Fast Radio Burst (FRB) searches using a real-time incoherent beam from the Murchison Widefield Array (MWA). The main science target of the pipeline are bright nearby FRBs from the local Universe (including Galactic FRBs like from SGR 1935+2154) which are the best candidates to probe FRB progenitors and understand physical mechanisms powering these extremely energetic events. Recent FRB detections by LOFAR (down to 110 MHz), the Green Bank Telescope (at 350 MHz), and Canadian Hydrogen Intensity Mapping Experiment (CHIME) detections extending down to 400 MHz, prove that there is a population of FRBs that can be detected below 350 MHz. The new MWA beamformer, known as the 'MWAX multibeam beamformer', can form multiple incoherent and coherent beams (with different parameters) commensally to any ongoing MWA observations. One of the beams is currently used for FRB searches (tested in 10 kHz frequency resolution and time resolutions between 0.1 and 100 ms). A second beam (in 1 Hz and 1 s frequency and time resolutions, respectively) is used for the Search for Extraterrestrial Intelligence (SETI) project. This paper focuses on the FRB search pipeline and its verification on selected known bright pulsars. The pipeline uses the FREDDA implementation of the Fast Dispersion Measure Transform algorithm (FDMT) for single pulse searches. Initially, it was tested during standard MWA observations, and more recently using dedicated observations of a sample of 11 bright pulsars. The pulsar PSR J0835-4510 (Vela) has been routinely used as the primary probe of the data quality because its folded profile was always detected in the frequency band 200 - 230 MHz with typical signal-to-noise ratio $>$ 10, which agrees with the expectations. Similarly, the low dispersion measure pulsar PSR B0950+08 was always detected in folded profile in the frequency band 140-170 MHz and so far has been the only object for which single pulses were detected. We present the estimated sensitivity of the search in the currently limited observing bandwidth of a single MWA coarse channel (1.28 MHz) and for the upgraded, future system with 12.8 MHz (10 channels) of bandwidth. Based on expected sensitivity and existing FRB rate measurements, we project an FRB detection rate between a few and a few tens per year with large uncertainty due to unknown FRB rates at low frequencies.