Persistent Counterparts to Gamma‐Ray Bursts

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Abstract
The recent discovery of persistent gamma-ray burst (GRB) counterparts at lower frequencies permits several important conclusions to be drawn. The spectrum of GRB 970508 is not consistent with an external shock origin for both the prompt GRB and the persistent emission, suggesting that at least the prompt radiation is produced by internal shocks. Comparisons among three GRBs with counterparts (or upper limits on them) establishes that GRBs are not all scaled versions of similar events. The angular size inferred from the apparent observation of self-absorption in the radio spectrum of GRB 970508 a week later implies that its expansion had slowed to semirelativistic speeds. This permits a remarkably low upper bound to be placed on its residual energy, suggesting either that radiation has been more than 99.7% efficient or that the initial outflow was strongly collimated. Observations of self-absorbed radio emission from future GRBs may permit direct measurement of their expansion and determination of their parameters and energetics. We estimate initial Lorentz factors of γ0 ~ 100 for GRB 970228 and GRB 970508, and present a solution for the evolution of a blast wave with instantaneous cooling.