(c) *Synchrotron Self-Compton*

Synchrotron self-Compton (SSC) emission has been considered in some works as a mechanism that could yield low-energy spectral slopes with photon indices as hard as *α*ph = 0, a change of ∆*α*ph = 2/3 over the synchrotron line of death [138]. This is facilitated by the fact that for typical values of the model parameters in the internal shock scenario, optically thin synchrotron emission peaks at much lower energies (at a few eV when the SSC peak is at ∼100 keV) and is mostly self-absorbed. One of the major drawbacks of this radiation mechanism is that it requires the synchrotron emission in the optical, which is the seed for the harder inverse-Compton emission, to be much (by a factor of &10<sup>3</sup> ) brighter than observed (or upper limits [139]). Otherwise, it requires the Compton-*y* parameter to

exceed unity by the same factor, which is hard to accommodate while not strongly violating the total energy budget of the burst [140,141]. The energy-dependent local polarization for SSC in an ultrarelativistic spherical flow for two different ordered B-field configurations, one parallel and the other transverse to the local velocity vector, is calculated in [142], where they found that the local polarization can be as high as Π . 25% under simplifying assumptions.
