We point out that already existing literature on relativistic collisionless MHD shocks show that the parameter \sigma \equiv upstream proper magnetic energy density/upstream rest mass energy density , plays an important role in determining the structure and accelerating properties of such shocks .
By adopting the value of \sigma \approx 0.002 which corresponds to the relativistic shock associated with the Crab nebula , and by using appropriate relativistic shock jump conditions , we obtain here a generous upper-limit on the value of ( proper ) magnetic field , B _ { sh } \approx 1.5 \times 10 ^ { -3 } \eta n _ { 1 } ^ { 1 / 2 } G , for gamma ray burst ( GRB ) blast wave .
Here , \eta \equiv E / Mc ^ { 2 } , where E is the energy and M is the mass of the baryons entrained in the original fireball ( FB ) , and n _ { 1 } is the proper number density of the ambient medium .
Further , we point out that , in realistic cases , the actual value B _ { sh } could be as low as \sim 5 \times 10 ^ { -6 } \eta n _ { 1 } ^ { 1 / 2 } G. realistic cases .