TY - JOUR
T1 - Implications of the γ-ray polarization of GRB 021206
AU - Nakar, Ehud
AU - Piran, Tsvi
AU - Waxman, Eli
PY - 2003/10
Y1 - 2003/10
N2 - We compare two possible scenarios for the production of a high level of polarization within the prompt emission of a GRB: synchrotron emission from a relativistic jet with a uniform (in space and time) magnetic field and synchrotron emission from a jet with a random magnetic field in the plane of the shock. Somewhat surprisingly we find that both scenarios can produce a comparable level of polarization (∼ 45-50% for the uniform field and ∼ 30-35% for a random field). A uniform time independent field most naturally arises by expansion of the field from the compact object. It requires a 10 12 G field at the source and a transport of the field ∝ R -1. It does not imply Poynting flux domination of the energy of the wind. There is a serious difficulty however, within this scenario, in accounting for particle acceleration (which requires random magnetic fields) both for Poynting flux and non-Poynting flux domination. Significant polarization can also arise from a random field provided that the observer is located within a 1/Γ orientation from a narrow (θj ∼ 1/Γ) jet. While most jets are wider, the jet of GRB 021206 from which strong polarization was recently observed is most likely very narrow. GRB 021206 is among the strongest bursts ever. Adopting the energy-angle relation we find an estimated angle of < 1/40 rad or even smaller. Thus, for this particular burst the required geometry is not unusual. We conclude that the RHESSI observations suggest that the prompt emission results from synchrotron radiation. However, in view of the comparable levels of polarization predicted by both the random field and the homogeneous field scenarios, these observations are insufficient to rule out or confirm either one. An important difference between the two scenarios is that any observer will measure high polarization from a uniform magnetic field while only some of the observers will measure high polarization from a random magnetic field. Thus, future observations will enable us to distinguish between the two scenarios.
AB - We compare two possible scenarios for the production of a high level of polarization within the prompt emission of a GRB: synchrotron emission from a relativistic jet with a uniform (in space and time) magnetic field and synchrotron emission from a jet with a random magnetic field in the plane of the shock. Somewhat surprisingly we find that both scenarios can produce a comparable level of polarization (∼ 45-50% for the uniform field and ∼ 30-35% for a random field). A uniform time independent field most naturally arises by expansion of the field from the compact object. It requires a 10 12 G field at the source and a transport of the field ∝ R -1. It does not imply Poynting flux domination of the energy of the wind. There is a serious difficulty however, within this scenario, in accounting for particle acceleration (which requires random magnetic fields) both for Poynting flux and non-Poynting flux domination. Significant polarization can also arise from a random field provided that the observer is located within a 1/Γ orientation from a narrow (θj ∼ 1/Γ) jet. While most jets are wider, the jet of GRB 021206 from which strong polarization was recently observed is most likely very narrow. GRB 021206 is among the strongest bursts ever. Adopting the energy-angle relation we find an estimated angle of < 1/40 rad or even smaller. Thus, for this particular burst the required geometry is not unusual. We conclude that the RHESSI observations suggest that the prompt emission results from synchrotron radiation. However, in view of the comparable levels of polarization predicted by both the random field and the homogeneous field scenarios, these observations are insufficient to rule out or confirm either one. An important difference between the two scenarios is that any observer will measure high polarization from a uniform magnetic field while only some of the observers will measure high polarization from a random magnetic field. Thus, future observations will enable us to distinguish between the two scenarios.
KW - Gamma ray bursts
KW - Magnetic fields
UR - http://www.scopus.com/inward/record.url?scp=23244437636&partnerID=8YFLogxK
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AN - SCOPUS:23244437636
SN - 1475-7516
SP - 81
EP - 93
JO - Journal of Cosmology and Astroparticle Physics
JF - Journal of Cosmology and Astroparticle Physics
IS - 10
ER -