(b) *Jitter Radiation*

If the magnetic field coherence length is much smaller than the gyroradius of particles, then synchrotron radiation is not the correct description of the radiative mechanism by which relativistic electrons cool, as it assumes homogeneous fields. In this case, the particles experience small pitch-angle scattering where their motion is deflected by magnetic field inhomogeneities by angles that are smaller than the beaming cone of the emitted radiation (1/*γe*). This scenario has been proposed as a viable alternative to synchrotron radiation [94], where it has been shown to yield harder spectral slopes that cannot be obtained in opticallythin synchrotron emission. In addition, it can produce sharper spectral peaks as compared to synchrotron radiation, which agrees better with observations. The small-scale magnetic fields needed in this scenario may potentially be produced in relativistic collisionless shocks via the Weibel instability (although this may not be easy to achieve in practice; see e.g., [135]). The polarization when this small-scale field is confined to a slab normal to the local fluid velocity is calculated in [136,137], where it is shown that the maximum degree of polarization is obtained only at large viewing angles when the slab is viewed almost edge-on. For small viewing angles that apply to distant GRBs, the polarization is indeed very weak.
