Reconciling observed gamma-ray burst prompt spectra with synchrotron radiation?

Abstract

Context. Gamma-ray burst emission in the prompt phase is often interpreted as synchrotron radiation from high-energy electrons accelerated in internal shocks. Fast synchrotron cooling of a power-law distribution of electrons leads to the prediction that the slope below the spectral peak has a photon index α = −3/2 (N(E) ∝ E α ). However, this differs significantly from the observed median value α ≈ −1. This discrepancy has been used to argue against this scenario. Aims. We quantify the influence of inverse Compton (IC) and adiabatic cooling on the low energy slope to understand whether these processes can reconcile the observed slopes with a synchrotron origin. Methods. We use a time-dependent code developed to calculate the GRB prompt emission within the internal shock model. The code follows both the shock dynamics and electron energy losses and can be used to generate lightcurves and spectra. We investigate the dependence of the low-energy slope on the parameters of the model and identify parameter regions that lead to values α > −3/2. Results. Values of α between −3/2 and −1 are reached when electrons suffer IC losses in the Klein-Nishina regime. This does not necessarily imply a strong IC component in the Fermi/LAT range (GeV) because scatterings are only moderately efficient. Steep slopes require that a large fraction (10−30%) of the dissipated energy is given to a small fraction ( < ∼ 1%) of the electrons and that the magnetic field energy density fraction remains low ( < ∼ 0.1%). Values of α up to −2/3 can be obtained with relatively high radiative efficiencies (>50%) when adiabatic cooling is comparable with radiative cooling (marginally fast cooling). This requires collisions at small radii and/or with low magnetic fields. Conclusions. Amending the standard fast cooling scenario to account for IC cooling naturally leads to values α up to −1. Marginally fast cooling may also account for values of α up to −2/3, although the conditions required are more difficult to reach. About 20% of GRBs show spectra with slopes α > −2/3. Other effects, not investigated here, such as a thermal component in the electron distribution or pair production by high-energy gamma-ray photons may further affect α. Still, the majority of observed GRB prompt spectra can be reconciled with a synchrotron origin, constraining the microphysics of mildly relativistic internal shocks.

Time evolution of the spectral break in the high-energy extra component of GRB 090926A

“…Both values are higher than the theoretical prediction α = −3/2 for pure fast-cooling synchrotron (Sari et al 1998), whereas this regime is required to explain the high temporal variability and to reach a high radiative efficiency that is compatible with the huge observed luminosities. The value α ∼ −0.6 is difficult to reconcile with synchrotron radiation, except by invoking the marginally fastcooling regime (Daigne et al 2011;Beniamini & Piran 2013). The value α ∼ −0.9 found in the Band+CUTBPL model is in better agreement, as it is well below the synchrotron death line, α = −2/3, and very close to the limit α ∼ −1 that is expected in the fast-cooling regime affected by inverse Compton scatterings in the Klein Nishina regime (Daigne et al 2011).…”

“…The value α ∼ −0.6 is difficult to reconcile with synchrotron radiation, except by invoking the marginally fastcooling regime (Daigne et al 2011;Beniamini & Piran 2013). The value α ∼ −0.9 found in the Band+CUTBPL model is in better agreement, as it is well below the synchrotron death line, α = −2/3, and very close to the limit α ∼ −1 that is expected in the fast-cooling regime affected by inverse Compton scatterings in the Klein Nishina regime (Daigne et al 2011). At high energy, the CUTBPL component is slightly harder than the CUTPL component with a fitted photon index γ = −1.48 +0.09 −0.08 (resp.…”

Prompt gamma-ray burst emission from gradual magnetic dissipation

“…This problem is referred to as the synchrotron "line of death" (Preece et al 1998). Other problems with a synchrotron origin for the prompt phase involve the narrow peak energy distribution and the apparent sharp decline of the peak energy distribution above 1 MeV (Band et al 1993;Mallozzi et al 1995;Schaefer 2003;Beloborodov 2013), although this could also be due to a selection effect (Shahmoradi & Nemiroff 2010;Beniamini & Piran 2013), and the narrow spectral width of the observed "Band function" as compared with the synchrotron peak (Baring & Braby 2004;Burgess et al 2011;Daigne et al 2011;Beloborodov 2013;Yu et al 2016). The main alternative to synchrotron is photospheric emission.…”

Temporal Evolution of GRB Spectra: Leptonic and Hadronic

“…To discriminate between the emission models, detailed temporalspectral evolution studies for various situations (e.g. Pe'er 2008, Vurm & Poutanen 2009, Daigne et al 2011 are needed.…”