On the synchrotron spectrum of GRB prompt emission [HEAP]


The prompt emission spectrum of Gamma-ray bursts (GRB) is characterized by a smoothly joint broken power-law spectrum known as the Band function. The typical low-energy photon index is $\sim -1$, which poses challenge to the standard synchrotron radiation models. We investigate the electron energy spectrum as a result of the interplay among adiabatic stochastic acceleration (ASA), particle injection, and synchrotron cooling. In the ASA-dominated low-energy range, ASA enables an efficient hardening of the injected energy spectrum to approach a spectral index $-1$. In the synchrotron cooling-dominated high-energy range, the injected high-energy electrons undergo fast synchrotron cooling and have a softer photon spectrum. With the energy range of the injected electrons broadly covering both the ASA- and synchrotron cooling-dominated ranges, the resulting photon number spectrum has the low- and high-energy indices as $\alpha_s \sim -1$ and $\beta_s \sim -p/2-1$, respectively. The break energy is of the order of $\sim 100 $ keV, depending on the turbulence properties.

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S. Xu, Y. Yang and B. Zhang
Mon, 13 Nov 17

Comments: 9 pages, 3 figures, submitted to ApJ on Oct. 23, 2017