  Photons of energy larger than 100 MeV from long-GRBs arrive a few seconds
after <10 MeV photons do. We show that this delay is a natural consequence of a
magnetic dominated relativistic jet. The much slower acceleration of a magnetic
jet with radius (compared with a hot baryonic outflow) results in high energy
gamma-ray photons to be converted to electron-positron pairs out to a larger
radius whereas lower energy gamma-rays of energy less than ~10 MeV can escape
when the jet crosses the Thomson-photosphere. The resulting delay for the
arrival of high energy photons is found to be similar to the value observed by
the Fermi satellite for a number of GRBs. A prediction of this model is that
the delay should increase with photon energy (E) as E^{0.17} for E>100 MeV. The
delay depends almost linearly on burst redshift, and on the distance from the
central compact object where the jet is launched (R_0). Therefore, the delay in
arrival of >10^2 MeV photons can be used to estimate burst redshift if the
magnetic jet model for gamma-ray generation is correct and R_0 is roughly the
same for long-GRBs.
