Detection of Low-energy Breaks in Gamma-Ray Burst Prompt Emission Spectra
- SISSA, via Bonomea 265, I-34136 Trieste (Italy)
- INAF—Osservatorio Astronomico di Brera, via Bianchi 46, I-23807 Merate (Italy)
The radiative process responsible for gamma-ray burst (GRB) prompt emission has not been identified yet. If dominated by fast-cooling synchrotron radiation, the part of the spectrum immediately below the νF{sub ν} peak energy should display a power-law behavior with slope α{sub 2}=−3/2, which breaks to a higher value α{sub 1}=−2/3 (i.e., to a harder spectral shape) at lower energies. Prompt emission spectral data (usually available down to ∼10--20 keV) are consistent with one single power-law behavior below the peak, with typical slope 〈α〉=−1, higher than (and then inconsistent with) the expected value α{sub 2}=−3/2. To better characterize the spectral shape at low energy, we analyzed 14 GRBs for which the Swift X-ray Telescope started observations during the prompt. When available, Fermi-GBM observations have been included in the analysis. For 67% of the spectra, models that usually give a satisfactory description of the prompt (e.g., the Band model) fail to reproduce the 0.5–1000 keV spectra: low-energy data outline the presence of a spectral break around a few keV. We then introduce an empirical fitting function that includes a low-energy power law α{sub 1}, a break energy E{sub break}, a second power law α{sub 2}, and a peak energy E{sub peak}. We find 〈α{sub 1}〉=−0.66 (σ=0.35), 〈log(E{sub break}/ keV)〉=0.63 (σ=0.20), 〈α{sub 2}〉=−1.46 (σ=0.31), and 〈log(E{sub peak}/ keV)〉=2.1 (σ=0.56). The values 〈α{sub 1}〉 and 〈α{sub 2}〉 are very close to expectations from synchrotron radiation. In this context, E{sub break} corresponds to the cooling break frequency. The relatively small ratio E{sub peak}/E{sub break}∼30 suggests a regime of moderately fast cooling, which might solve the long-lasting problem of the apparent inconsistency between measured and predicted low-energy spectral index.
- OSTI ID:
- 22875818
- Journal Information:
- Astrophysical Journal, Vol. 846, Issue 2; Other Information: Country of input: International Atomic Energy Agency (IAEA); ISSN 0004-637X
- Country of Publication:
- United States
- Language:
- English
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