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Highly polarized light from stable ordered magnetic fields in GRB 120308A
C G Mundell1, D Kopač, D M Arnold
1Astrophysics Research Institute, Liverpool John Moores University, IC2, Liverpool Science Park, 146 Brownlow Hill, Liverpool L3 5RF, UK.
Nature
|December 6, 2013
Summary
High polarization in gamma-ray burst afterglows suggests magnetized jets with stable fields. This finding supports models where large-scale magnetic fields in jets survive explosions, ruling out plasma instabilities.
Area of Science:
- Astrophysics
- High-energy astrophysics
- Plasma physics
Background:
- Gamma-ray bursts (GRBs) are powerful cosmic explosions.
- Afterglows are produced by shocks in GRB ejecta colliding with surrounding matter.
- Polarization of light from reverse shocks can reveal properties of the GRB jet's magnetic field.
Purpose of the Study:
- To investigate the polarization of light from the early afterglow of a GRB.
- To test competing models of GRB jet structure and magnetic field behavior.
- To determine if GRB jets possess globally ordered magnetic fields.
Main Methods:
- Observed optical polarization of Swift GRB 120308A four minutes after its gamma-ray detection.
- Measured polarization levels and position angles over the subsequent ten minutes.
- Analyzed polarization data to infer properties of the GRB jet's magnetic field.
Main Results:
- Detected high linear polarization (P = 28% decreasing to 16%) in the immediate afterglow.
- Observed a stable polarization position angle with minimal variation (≤15 degrees).
- These polarization properties are inconsistent with random magnetic fields generated by plasma instabilities.
Conclusions:
- GRBs feature magnetized baryonic jets with large-scale, uniform magnetic fields.
- These ordered magnetic fields can persist long after the initial GRB explosion.
- The findings support models of advected, ordered magnetic fields in GRB jets.
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