Synchrotron emission in the fast cooling regime: which spectra can be explained?

Abstract

We consider the synchrotron emission from relativistic shocks assuming that the radiating electrons cool rapidly (either through synchrotron or any other radiation mechanism). It is shown that the theory of synchrotron emission in the fast cooling regime can account for a wide range of spectral shapes. In particular, the magnetic field, which decays behind the shock front, brings enough flexibility to the theory to explain the majority of gamma-ray burst spectra even in the parameterfree fast cooling regime. Also, we discuss whether location of the peak in observed spectral energy distributions of gamma-ray bursts and active galactic nuclei can be made consistent with predictions of diffusive shock acceleration theory, and find that the answer is negative. This result is a strong indication that a particle injection mechanism, other than the standard shock acceleration, works in relativistic shocks.

Detection of Low-energy Breaks in Gamma-Ray Burst Prompt Emission Spectra

“…These studies have assumed a homogeneous magnetic field and an instantaneous, one-shot acceleration. Other scenarios leading to a similar spectral shape invoke a magnetic field that decays downstream with a strength that depends on the distance from the shock front (Derishev 2007 …”

“…Among the first class of models, we recall scenarios invoking Comptonization and/or thermal components Blinnikov et al 1999;Ghisellini & Celotti 1999;Lazzati et al 2000;Mészáros & Rees 2000;Stern & Poutanen 2004;Rees & Mészáros 2005;Ryde & Pe'er 2009;Guiriec et al 2011Guiriec et al , 2015aGuiriec et al ,b, 2016aGhirlanda et al 2013;Burgess et al 2014). For the second class of models (studies that consider synchrotron radiation) effects producing a hardening of the low-energy spectral index have been invoked, such as Klein-Nishina effects, marginally fast cooling regime, and anisotropic pitch angle distributions (Lloyd & Petrosian 2000;Derishev 2001Derishev , 2007Bosnjak et al 2009;Nakar et al 2009;Daigne et al 2011;Uhm & Zhang 2014). In spite of all theoretical efforts, there is still no consensus on the origin of the prompt emission.…”

Radiative Signatures of Relativistic Shocks

“…As we show below (see Equation (6)) the resulting mean free path of a particle of Lorentz factor γ is proportional to γ 2 , rather than the linear dependence expected in Bohm diffusion. This modifies the maximum energy of the synchrotron photons (Derishev 2007), but the radiation emitted when a particle is deflected through very small angles differs significantly from synchrotron radiation (Landau & Lifshitz 1975), and can, in principle, produce more energetic photons. This has led to the suggestion that "jitter" radiation from shock-accelerated particles is responsible for both the prompt emission (Medvedev 2006) and the afterglow (Medvedev et al 2007) from gamma-ray bursts.…”

TheEp−Eisorelation and the internal shock model

“…Many predictions on the light curves and spectra of GRBs can be made from internal shocks (Bosnjak & Daigne 2014). Several agree well with observations, but the shape of the expected synchrotron spectrum does not fit well, because it is too soft at low energy (Preece et al 1998;Ghisellini et al 2000; see however Derishev 2007;Daigne et al 2011). Moreover, the necessary efficient transfer of dissipated energy to electrons has been disputed for a moderate magnetization of the flow σ > 0.1, where σ is the ratio of the Poynting flux to the particle rest energy flux (Mimica & Aloy 2010;Narayan et al 2011).…”

“…The low-and high-energy spectral indices were fixed to α = −1 and β = −2.5, which corresponds to the mean values of the observed distributions (Preece et al 2000). Synchrotron emission in the fast-cooling regime instead predicts α = −1.5, but including the inverse-Compton process and a decreasing magnetic field behind the shocks can help to reduce the discrepancy (Derishev 2007;Daigne et al 2011). Adopting α = −1.5 or −1 for the present study leads to very similar results.…”