Magnetic fields in relativistic collisionless shocks
- Department of Astronomy, University of Texas at Austin, Austin, TX 78712 (United States)
We present a systematic study on magnetic fields in gamma-ray burst (GRB) external forward shocks (FSs). There are 60 (35) GRBs in our X-ray (optical) sample, mostly from Swift. We use two methods to study ε {sub B} (fraction of energy in magnetic field in the FS): (1) for the X-ray sample, we use the constraint that the observed flux at the end of the steep decline is ≥ X-ray FS flux; (2) for the optical sample, we use the condition that the observed flux arises from the FS (optical sample light curves decline as ∼t {sup –1}, as expected for the FS). Making a reasonable assumption on E (jet isotropic equivalent kinetic energy), we converted these conditions into an upper limit (measurement) on ε {sub B} n {sup 2/(p+1)} for our X-ray (optical) sample, where n is the circumburst density and p is the electron index. Taking n = 1 cm{sup –3}, the distribution of ε {sub B} measurements (upper limits) for our optical (X-ray) sample has a range of ∼10{sup –8}-10{sup –3} (∼10{sup –6}-10{sup –3}) and median of ∼few × 10{sup –5} (∼few × 10{sup –5}). To characterize how much amplification is needed, beyond shock compression of a seed magnetic field ∼10 μG, we expressed our results in terms of an amplification factor, AF, which is very weakly dependent on n (AF∝n {sup 0.21}). The range of AF measurements (upper limits) for our optical (X-ray) sample is ∼1-1000 (∼10-300) with a median of ∼50 (∼50). These results suggest that some amplification, in addition to shock compression, is needed to explain the afterglow observations.
- OSTI ID:
- 22357198
- Journal Information:
- Astrophysical Journal, Vol. 785, Issue 1; Other Information: Country of input: International Atomic Energy Agency (IAEA); ISSN 0004-637X
- Country of Publication:
- United States
- Language:
- English
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