Figure 1. Two ways of modeling the spectrum of GRB prompt emission (Courtesy of F. Ryde)
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The spectrum during this phase (see, e.g., figure 1 above), is often modeled with a "broken power law" function (after Band et. al.). This phenomenological model was found to be consistent with the majority of the obtained spectra. The break in the spectra often occurs at ~200-300 keV, which is above the energy band seen by the Swift satellite, that was launched in 2004. This is the reason why still today old BATSE data is used to model GRB prompt emission spectra. In spite of its phenomenological success, the "broken power law" model encounters some theoretical difficulties. The main theoretical problem concerns the source of the emission. Efficient non-thermal emission can be produced by Synchrotron emission. However, it is difficult to understand why the peak in the energy is clustered in a narrow energy band; In addition, in up to 1/3 of the bursts, the low energy spectral slopes are too hard to account for in this model. Due to these difficulties, it was suggested by several authors, as of the late 90's that the source of the observed radiation may not be purely synchrotron, but in addition contain a Thermal component. These ideas were put forward by Felix Ryde , who studied in 2004 the evolution of the temperature of the thermal emission, in a small sample of GRBs with hard spectra (and good time coverage). Felix found a repetitive behavior, which encourages further investigation. |
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This analysis enables us first to determine what is the fraction of energy that is carried in the thermal component. we found that on the average up to, ~30%-50% of the energy emitted during the prompt phase is in the form of thermal photons. This is an important result, since it can help explaining why the prompt emission is so efficient, and why the breaks in the spectra are so narrowly clustered.
We continued the analysis by studying the temporal behavior of the
thermal flux. Here, too, we found a broken power law behavior:
The flux first rises with an index ~1/3, and then it decay with an
index -2. The break time is within the errors of the break time in the
temperature.
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We further quantified the temporal behavior of both the
temperature and the flux of the thermal emission component. We found
that the temperature show a repetitive behavior: broken power law in
time, (similar to the results reported earlier by Ryde, albeit
at a much larger sample). We found that the temperature is ~constant
at the first few seconds, after which is decays as a power law in time
with power law index (average) of -2/3. Examples of this analysis is
shown in figure 2, to the right.
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Thus, thermal photons carry an important information on the radius of the photosphere. Study of this component thus enables direct measurement of the properties of the photosphere, hence of the relativistic outflow. Below is a connection to further explanations on the theory and implications of these results. The paper by Ryde & Pe'er where we publish our findings can be found here. |
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 Study of thermal emission in GRBs: II- Theory |
Asaf Pe'er, Created: 30-Nov-2008