GRB 990123 is the most luminous event detected so far , and in an important paper , Hjorth et al .
( [ 1999 ] ) reported an upper limit on the degree of linear polarization of the optical afterglow for this burst ( P < 2.3 \% ) .
One of the interprtations for this small value of P was that the emission was probably due a relativistic jet with ordered magnetic field , and the viewing angle in the lab frame \theta ^ { \prime } \la \Gamma ^ { -1 } , where \Gamma is the bulk Lorentz factor of the jet at the time of the optical emission .
We point out that this conclusion resulted from a confusion between the angles measured in the lab frame and in the plasma rest frame .
For ordered magnetic field , one would actually obtain a large value of P because the above mentioned angle would correspond to a very large angle , \theta \la \pi / 2 , in the plasma rest frame .
And this is probably the case with the blazars .
On the other hand , it is indeed possible to have P \approx 0 if the magnetic field of GRB 990123 was completely chaotic and the viewing angle was considerably smaller than the semi-angle of the jet .