LIMITS ON PROMPT, DISPERSED RADIO PULSES FROM GAMMA-RAY BURSTS

We have searched for prompt radio emission from nine gamma-ray bursts (GRBs) with a 12 m telescope at 1.4 GHz, with a time resolution of 64 [mu]s to 1 s. We detected single dispersed radio pulses with significances >6[sigma] in the few minutes following two GRBs. The dispersion measures of both pulses are well in excess of the expected Galactic values, and the implied rate is incompatible with known sources of single dispersed pulses. The arrival times of both pulses also coincide with breaks in the GRB X-ray light curves. A null trial and statistical arguments rule out random fluctuations as the origin of these pulses with >95% and ~97% confidence, respectively, although a simple population argument supports a GRB origin with confidence of only 2%. We caution that we cannot rule out radio frequency interference (RFI) as the origin of these pulses. If the single pulses are not related to the GRBs, we set an upper limit on the flux density of radio pulses emitted between 200 and 1800 s after a GRB of 1.27w super(-1/2) Jy, where 6.4 x 10 super(-5) s < w < 32 x 10 super(-3) s is the pulse width. We set a limit of less than 760 Jy for long timescale (> 1 s) variations. These limits are some of the most constraining at high time resolution and GHz frequencies in the early stages of the GRB phenomenon.