Implications of the \U0001d6c4-ray polarization of GRB 021206

Abstract

We compare two possible scenarios for the production of a high levelof polarization within the prompt emission of a GRB: synchrotronemission from a relativistic jet with a uniform (in space andtime) magnetic field and synchrotron emission from a jet with arandom magnetic field in the plane of the shock. Somewhatsurprisingly we find that both scenarios can produce a comparablelevel of polarization (~45–50% for the uniform field and~30–35% for a random field). A uniform time independentfield most naturally arises by expansion of the field from thecompact object. It requires a 1012 G field at the source anda transport of the field ∝R−1. It does notimply Poynting flux domination of the energy of the wind. There isa serious difficulty however, within this scenario, in accounting forparticle acceleration (which requires random magnetic fields) bothfor Poynting flux and non-Poynting flux domination. Significantpolarization can also arise from a random field provided thatthe observer is located within a 1/Γ orientation from anarrow (θj ~ 1/Γ) jet. While most jets are wider,the jet of GRB 021206 from which strong polarization was recentlyobserved is most likely very narrow. GRB 021206 is among thestrongest bursts ever. Adopting the energy–angle relation we findan estimated angle of <1/40 rad or even smaller. Thus, for thisparticular burst the required geometry is not unusual. We concludethat the RHESSI observations suggest that the prompt emissionresults from synchrotron radiation. However, in view of thecomparable levels of polarization predicted by both the randomfield and the homogeneous field scenarios, these observations areinsufficient to rule out or confirm either one. An importantdifference between the two scenarios is that any observer will measure high polarization from a uniform magnetic field while onlysome of the observers will measure high polarization from a random magnetic field. Thus, future observations will enable us to distinguishbetween the two scenarios.