A search for prompt radio emission of gamma-ray bursts

A conducting fireball expanding at relativistic speed into an ambient magnetic field generates a rapidly changing electric current which emits coherent electromagnetic radiation at radio frequencies. The critical frequency (upper limit of the emission) strongly depends on the Lorentz factor of the expansion. We have searched for simultaneous radio emission in the data of solar radio spectrographs at times when BATSE reported a non-solar gamma-ray burst (GRB) within the beams of the radio telescopes. Solar spectrographs are less sensitive than single frequency receivers, but yield a broad overview on the spectrum ideal for discriminating against atmospheric and man-made interference. In 7 well-observed cases no radio emission was found. This puts upper and lower limits on the Lorentz factor of the fireball expansion in GRBs if the source distance is less than 1 kpc, i.e. if GRBs are local. The coherent expansion radiation is not observable with current instruments if the GRB sources are at >= 1 kpc.