We demonstrate that the rapid spectral variability of prompt GRBs is an inherent property of radiation emitted from shock-generated , highly anisotropic small-scale magnetic fields .
We interpret the hard-to-soft evolution and the correlation of the soft index \alpha with the photon flux observed in GRBs as a combined effect of temporal variation of the shock viewing angle and relativistic aberration of an individual thin , instantaneously illuminated shell .
The model predicts that about a quarter of time-resolved spectra should have hard spectra , violating the synchrotron \alpha = -2 / 3 limit .
The model also naturally explains why the peak of the distribution of \alpha is at \alpha \sim - 1 .
The presence of a low-energy break in the jitter spectrum at oblique angles also explains the appearance of a soft X-ray component in some GRBs and their paucity .
We emphasize that our theory is based solely on the first principles and contains no ad hoc ( phenomenological ) assumptions .