Extended Power‐Law Decays in BATSE Gamma‐Ray Bursts: Signatures of External Shocks?

We present a temporal and spectral analysis of a subset of BATSE GRBs with smooth extended emission tails to search for signatures of the “early high‐energy” afterglow, i.e., the onset of the afterglow emission that initially peaks in gamma‐rays and subsequently evolves into X‐Ray, optical, and radio emission according to the internal/external shock model. From our sample of 40 GRBs we find that the temporal decays are best described with a power‐law ∼ tβ, rather than an exponential, with a mean index 〈β〉 ≈ −2. Spectral analysis shows that ∼ 20% of these events are consistent with a fast‐cooling synchrotron spectrum for an adiabatic blast wave; three of which are consistent with the blast wave evolution of a jet, with Fv ∼ t−p. This behavior suggests that, in some cases, the emission may originate from a narrow jet, possibly consisting of “nuggets” whose angular size are less than 1/Γ, where Γ is the bulk Lorentz factor.