2011MNRAS.418L..64L

Recent analytical and numerical work argue that successful relativistic Fermi acceleration requires a weak magnetization of the unshocked plasma, all the more so at high Lorentz factors. The present Letter tests this conclusion by computing the afterglow of a gamma-ray burst outflow propagating in a magnetized stellar wind using `ab initio' principles regarding the microphysics of relativistic Fermi acceleration. It is shown that in magnetized environments, one expects a drop-out in the X-ray band on subday scales as the synchrotron emission of the shock-heated electrons exits the frequency band. At later times, Fermi acceleration becomes operative when the blast Lorentz factor drops below a certain critical value, leading to the recovery of the standard afterglow light curve. Interestingly, the observed drop-out bears resemblance with the fast decay found in gamma-ray bursts early X-ray afterglows.