1 Introduction

The Cannonball Model is based on the hypothesis that GRBs and their afterglows are made in supernova explosions by the jetted ejection of relativistic plasmoids: ``cannonballs'' made of ordinary baryonic matter (Dar & De Rújula 2000a), similar to the ones observed in quasars and microquasars (e.g., Mirabel & Rodriguez 1994, 1999 and references therein). The name cannonball (CB) originates in the contention that - due to a mechanism that we have explicitly discussed in Dado et al. (2001) - the ejected plasmoids stop expanding very early in the afterglow (AG) phase.

The CB paradigm gives a good description of the properties of the $\gamma$-rays in a GRB, that we modelled in simple approximations in Dar & De Rújula (2000b). It suggests an alternative (Dar & De Rújula 2001a), which is rather promising (Dado et al. 2002), to the ``Fe-line'' interpretation of the spectral lines observed in some X-ray afterglows (GRB 970508: Piro et al. 1998; GRB 970828: Yoshida et al. 1999, 2001; GRB 991216: Piro et al. 2000; GRB 000214: Antonelli et al. 2000). The model also provides an extremely simple and successful description of the spectrum, and of the shape and absolute magnitude of the light curves of the optical and X-ray afterglows of all GRBs of known redshift, at all observed times (Dado et al. 2001, hereafter called DDD 2001). This description is universal, it encompasses the early optical flash of GRB 990123, the very peculiar optical and X-ray AG of GRB 970508, and all of the properties of GRB 980425, associated with SN1998bw.

In this paper we derive the CB model's predictions for radio afterglows, and compare them to all radio observations in GRBs of known redshift. We also study the evolution of the spectral index of AGs as a function of time. The CB model - in parameter-thrifty and very simple terms - passes these tests with flying colours.