Spectral Lags Explained as Scattering from Accelerated Scatterers
  A quantitative theory of spectral lags for $\gamma$-ray bursts (GRBs) is given. The underlying hypothesis is that GRB subpulses are photons that are scattered into our line of sight by local concentrations of baryons that are accelerated by radiation pressure. For primary spectra that are power laws with exponential cutoffs, the width of the pulse and its fast rise, slow decay asymmetry is found to increase with decreasing photon energy, and the width near the exponential cutoff scales approximately as $E_{ph}^{-\eta}$, with $\eta\sim 0.4$, as observed. The spectral lag time is naturally inversely proportional to luminosity, all else being equal, also as observed. 
David Eichler, Hadar Manis