Compton Cooling in the Afterglows of Gamma‐Ray Bursts: Application to GRB 980923 and GRB 971214

Abstract

The role of Compton cooling in the standard model for the afterglows of gamma-ray bursts is con- sidered. When electrons cool by scattering o† their own synchrotron photons, three cooling regimes are identiÐed in which the observed synchrotron radiation exhibits qualitatively di†erent characteristics. Depending on the values of the source parameters, an afterglow may evolve through one, two, or all three of these regimes. Since synchrotron radiation can be regarded as Compton scattering of the virtual photons due to the magnetic Ðeld, in one of these regimes the instantaneous synchrotron spectrum has properties identical to those when Compton cooling is negligible. During this phase in the evolution, the synchrotron radiation falls mainly in the near-infrared/optical/UV spectral range. In order to break this degeneracy, good temporal coverage is needed. Alternatively, the importance of Compton scattering can be determined for those afterglows that are observed outside this degeneracy phase. It is suggested that the afterglows of GRB 980923 and GRB 971214 are two such cases. In the prompt afterglow of GRB 980923, the Klein-Nishina limit suppresses Compton scattering, and the cooling is due to synchrotron radiation. However, the derived values of the source parameters are such that Compton cooling is expected to have been important in its subsequent evolution. The observed properties of GRB 971214 indicate cooling to be dominated by Compton scattering rather than synchrotron radiation. If Compton cooling is generally important in the afterglows of gamma-ray bursts, the likelihood of the shock becom- ing radiative is increased. It is suggested that this e†ect contributes to the low frequency of detected afterglows.