Gamma-Ray Burst Afterglows in Pulsar-Wind Bubbles

We propose to identify pulsar-wind bubbles (PWBs) as the environment in which the afterglow emission in at least some gamma-ray bursts (GRBs) originates. Such bubbles could naturally account for the high fraction of the internal energy residing in relativistic in electrons/positrons $\epsilon_e$ and the high magnetic-to-internal energy ratio $\epsilon_B$ that have been inferred in a number of sources. GRBs might occur within PWBs under a number of scenarios: in particular, in the supranova model of GRB formation a prolonged (months to years) period of intense pulsar-type radiation by the GRB progenitor precedes the burst. Focusing on this scenario, we construct a simple model of the early-time structure of a plerionic supernova remnant (SNR). The model assumes near equipartition between the thermal and magnetic pressures and takes into account synchrotron-radiation cooling. We argue that the effective hydrogen number density of the shocked pulsar wind is $n_{H,equiv}=w_{tot}/m_p c^2$, where $w_{tot}$ is the total (particle and magnetic) enthalpy density and. We show that, for plausible parameter values, $n_{H,equiv}$ spans the range inferred from spectral fits to GRB afterglows and that its radial profile varies within the bubble and may resemble a uniform interstellar medium (ISM), a stellar wind, or a molecular cloud. We consider how the standard expressions for the characteristic synchrotron spectrum are modified in this picture and demonstrate that the predictions for the empirically inferred values of $\epsilon_e$ and $\epsilon_B$ are consistent with observations. Finally, we outline a self-consistent interpretation of the X-ray emission features detected in sources like GRB 991216 in the context of the supranova/PWB picture.