2014ApJ...785...27O

As a backend to the first station of the Long Wavelength Array (LWA1), the Prototype All Sky Imager has been imaging the sky > -26° declination during 34 gamma-ray bursts (GRBs) between 2012 January and 2013 May. Using this data, we were able to put the most stringent limits to date on prompt low-frequency emission from GRBs. While our limits depend on the zenith angle of the observed GRB, we estimate a 1σ rms sensitivity of 68, 65, and 70 Jy for 5 s integrations at 37.9, 52.0, and 74.0 MHz at zenith. These limits are relevant for pulses ≥ 5 s and are limited by dispersion smearing. For 5 s pulses, we are limited to dispersion measures (DMs) ≤ 220, 570, and 1600 pc/cm3 for the frequencies above. For pulses lasting longer than 5 s, the DM limits increase linearly with the duration of the pulse. We also report two interesting transients, which are, as of yet, of unknown origin and are not coincident with any known GRBs. For general transients, we give rate density limits of ≤ 7.5x10^–3, 2.9 x 10^–2, and 1.4x10^–2/yr/deg2 with pulse energy densities >1.3x10^–22, 1.1x10^–22, and 1.4x10^–22 J/m2/Hz and pulse widths of 5 s at the frequencies given above.