Limits on prompt, dispersed radio pulses from gamma-ray bursts

TL;DR

This study searched for prompt dispersed radio pulses from nine gamma-ray bursts at 1.4 GHz, detecting two pulses with high significance that may be related to the GRBs, setting strict upper limits on radio emission in early stages.

Contribution

First high-time-resolution search for prompt radio pulses from GRBs at GHz frequencies, with detection of two candidate pulses and new constraints on early radio emission.

Findings

Detected two high-significance dispersed radio pulses following two GRBs.

Pulses have dispersion measures exceeding Galactic expectations, suggesting extragalactic origin.

Established upper limits on radio flux density from GRBs at early times.

Abstract

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 us 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 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 to 1800 s after a GRB of 1.27 w^{-1/2} Jy, where 64 us < w < 32 ms 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.