2023MNRAS.518.4278T

We present a targeted search for low-frequency (144-215 MHz) fast radio burst (FRB) emission from five repeating FRBs using 23.3 h of archival data taken with the Murchison Widefield Array (MWA) voltage capture system (VCS) between 2014 September and 2020 May. This is the first time that the MWA VCS has been used to search for FRB signals from known repeaters, which enables much more sensitive FRB searches than previously performed with the standard MWA correlator mode. We performed a standard single-pulse search with a temporal and spectral resolution of $400\, \mu$s and 10 kHz, respectively, over a $100\, \text{pc}\, \text{cm}^{-3}$ dispersion measure (DM) range centred at the known DM of each studied repeating FRB. No FRBs exceeding a 6σ threshold were detected. The fluence upper limits in the range of 32-1175 and 36-488 Jy ms derived from 10 observations of FRB 20190711A and four observations of FRB 20201124A, respectively, allow us to constrain the spectral indices of their bursts to ≳-1 if these two repeaters were active during the MWA observations. If free-free absorption is responsible for our non-detection, we can constrain the size of the absorbing medium in terms of the electron temperature T to ${\lt} 1.00\times (T/10^4\,\text{K})^{-1.35}\, \text{pc}$, ${\lt} 0.92\times (T/10^4\,\text{K})^{-1.35}\, \text{pc}$, and ${\lt} [0.22\!-\!2.50]\times (T/10^4\,\text{K})^{-1.35}\, \text{pc}$ for FRB 20190117A, FRB 20190711A, and FRB 20201124A, respectively. However, given that the activities of these repeaters are not well characterized, our non-detections could also suggest they were inactive during the MWA observations.