Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Comments on the paper ``Polarimetric Constraints on the Optical
  Afterglow Emission from GRB 990123'' by Hjorth et al. (Science, 26 1999)   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' <= 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 <= \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 ~ 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.

3
The categories of this paper are: physics
22
The categories of this paper are: chao-dyn
33
The categories of this paper are: plasm-ph
28
The categories of this paper are: alg-geom
7
The categories of this paper are: hep-th
20
The categories of this paper are: acc-phys
10
The categories of this paper are: q-bio
4
The categories of this paper are: cond-mat
15
The categories of this paper are: nucl-ex
2
The categories of this paper are: math
19
The categories of this paper are: eess
25
The categories of this paper are: patt-sol
18
The categories of this paper are: econ
1
The categories of this paper are: cs
30
The categories of this paper are: q-alg
24
The categories of this paper are: dg-ga