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To date, 20 radio afterglows have been detected.
The first radio afterglow was detected in the case of GRB970508
([Frail et al., 1997]); the fluctuations shown by this radio afterglow
for about one month have been interpreted as interstellar scintillation
effects, connected with the small angular dimension of the source.
Since the distance to this burst was known by the optical afterglow
observation, it has been possible to estimate the size required to
ease this effect, and hence the expansion velocity of the fireball,
that came out to be relativistic. Also because this particular burst
showed the longest-lived afterglow, it is one of the most famous
and studied GRB afterglows.
The cause of non-detection of radio afterglows is most likely due to lack
of sensitivity ([Kulkarni et al., 2000b]).