High time resolution search for prompt radio emission from the long GRB 210419A with the Murchison Widefield Array

We present a low-frequency (170–200 MHz) search for prompt radio emission associated with the long GRB 210419A using the rapid-response mode of the Murchison Widefield Array (MWA), triggering observations with the Voltage Capture System for the first time. The MWA began observing GRB 210419A within 89 s of its detection by Swift, enabling us to capture any dispersion delayed signal emitted by this gamma-ray burst (GRB) for a typical range of redshifts. We conducted a standard single pulse search with a temporal and spectral resolution of 100 μs and 10 kHz over a broad range of dispersion measures from 1 to 5000 pc cm⁻³⁠, but none were detected. However, fluence upper limits of 77–224 Jy ms derived over a pulse width of 0.5–10 ms and a redshift of 0.6 < z < 4 are some of the most stringent at low radio frequencies. We compared these fluence limits to the GRB jet–interstellar medium interaction model, placing constraints on the fraction of magnetic energy (ϵ_B ≲ [0.05–0.1]). We also searched for signals during the X-ray flaring activity of GRB 210419A on minute time-scales in the image domain and found no emission, resulting in an intensity upper limit of 0.57 Jy beam⁻¹⁠, corresponding to a constraint of ϵ_B ≲ 10⁻³. Our non-detection could imply that GRB 210419A was at a high redshift, there was not enough magnetic energy for low-frequency emission, or the radio waves did not escape from the GRB environment.