169 research outputs found
The Most Probable Cause for the High Gamma-Ray Polarization in GRB 021206
The exciting detection of a very high degree of linear polarization,
, in the prompt gamma-ray emission of GRB 021206, supports
synchrotron emission as being the dominant radiation mechanism. There were also
claims that this implies a magnetic field ordered on large scales within the
ejecta, that must therefore be produced at the source, which in turn was used
as an argument in favor magnetic fields playing an active role in the
production of GRB jets. However, an alternative explanation was suggested which
also works with a magnetic field that is generated in the internal shocks and
does not originate at the source: a very narrow jet, of opening angle
, where is the Lorentz factor during
the GRB, viewed at . We
calculate for these two scenarios, and find that it is significantly easier
to produce with an ordered field. More specifically, we obtain
for an ordered transverse magnetic field, , whereas a
random field within the plane of the shock, , produces
for a single pulse in the GRB light curve, but the
integrated emission over many pulses (as measured in GRB 021206) is a factor of
lower. A magnetic field normal to the shock front, ,
produces for the emission integrated over many pulses. However,
polarization measurements from GRB afterglows suggest a more isotropic
shock-produced field configuration that would reduce by a factor . Therefore, an ordered magnetic field, , that originates at the
source, can produce the observed polarization most naturally, while
is less likely, and is the least likely of the above.Comment: 4 pages, 4 figures; an improved assumption on the distribution of
cooled electrons simplifies the expressions for the polarization. ApJL in
pres
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