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Inverse-Compton cooling in Klein-Nishina regime and gamma-ray burst prompt spectrum

Synchrotron radiation mechanism, when electrons are accelerated in a relativistic shock, is known to have serious problems in explaining the observed gamma-ray spectrum below the peak for most gamma- ray bursts (GRBs) ; the synchrotron spectrum below the peak is much softer than observed spectra. Recently, the possibility that electrons responsible for the radiation cool via inverse Compton, but in the Klein-Nishina regime, has been proposed as a solution to this problem. We provide an analytical study of this effect and show that it leads to a hardening of the low-energy spectrum but not enough to make it consistent with the observed spectra for most GRBs (this is assuming that electrons are injected continuously over a time-scale comparable to the dynamical time-scale, as is expected for internal shocks of GRBs). In particular, we find that it is not possible to obtain a spectrum with α > -0.1 (fν ∝ να), where the typical observed value is α ˜ 0. Moreover, extreme values for a number of parameters are required in order that α ˜ -0.1: the energy fraction in magnetic field needs to be less than about 10-4, the thermal Lorentz factor of electrons should be larger than 106 and the radius where gamma- rays are produced should be not too far away from the deceleration radius. These difficulties suggest that the synchrotron radiation mechanism in internal shocks does not provide a self-consistent solution when α ≳ -0.2.

radiation mechanisms: non-thermal, methods: analytical, gamma-ray burst: general, gamma-ray burst: individual: GRB 080916C

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