Diagnosing the Outflow from the SGR 1806-20 Giant Flare with Radio Observations
On 2004 Dec. 27, the soft gamma repeater (SGR) 1806-20 emitted the brightest giant flare (GF) ever detected from an SGR. This burst of energy, which resulted in an (isotropic) energy release {approx} 100 times greater than the only two other known SGR GFs, was followed by a very bright, fading radio afterglow. Extensive follow-up radio observations provided a wealth of information with unprecedented astrometric precision, revealing the temporal evolution of the source size, along with densely sampled light curves and spectra. Here we expand on our previous work on this source, by attempting to explain these observations within one self-consistent dynamical model. In this scenario, the early radio emission is due to the outflow ejected during the GF energizing a thin shell surrounding a pre-existing cavity, where the observed steep temporal decay is attributed to the adiabatic cooling of the electrons in the doubly shocked medium. The shocked ejecta and external shell move outward together, driving a forward shock into the ambient medium, and are eventually decelerated by a reverse shock. The radio emission from the shocked external medium naturally peaks when significant deceleration occurs, and then decays relatively slowly. The evolution of the source size is reproduced in our model, and suggests that most of the energy in the outflow was in mildly relativistic material, with v/c {approx} 0.4d{sub 15}, for a distance of 15d{sub 15} kpc to SGR 1806-20. An initially highly relativistic outflow would not have produce a long coasting phase at a mildly relativistic expansion velocity, as was observed.
- Research Organization:
- SLAC National Accelerator Lab., Menlo Park, CA (United States)
- Sponsoring Organization:
- USDOE Office of Science (SC)
- DOE Contract Number:
- AC02-76SF00515
- OSTI ID:
- 839903
- Report Number(s):
- SLAC-PUB-11061; TRN: US200516%%332
- Country of Publication:
- United States
- Language:
- English
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