Reconciling observed gamma-ray burst prompt spectra with synchrotron radiation?
By F. Daigne, Ž. Bošnjak, G. D. I. D. D. Paris, U. Cnrs, Aim IrfuService d'Astrophysique, L. D. D. Grenoble, Umr 5571 Universit'e Joseph Fourier-CNRS. 2010
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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/ 2a nd−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 ( 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.
Subfields: Astronomy and Astrophysics, Nuclear and High Energy Physics
View Paper: https://www.semanticscholar.org/paper/733213fb6fbf05033baf2ad3ffb4ed62e2e8bebc
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Citations in the corpus
- The physics of gamma-ray bursts & relativistic jets (2014, background): “This poses some intrinsic constraint on ∆L/L with respect to ∆t/t (e.g. Nakar et al., 2003; Ioka et al., 2005), making the bumps very smooth.”
- Relativistic Jets Shine through Shocks or Magnetic Reconnection (2015, background): “Relativistic jets are ubiquitous in the Universe, with gamma-ray bursts (GRBs) and active galactic nuclei (AGNs) being two representative examples of high-energy sources powered by jets.”
- A Decade of Gamma-Ray Bursts Observed by Fermi-LAT: The Second GRB Catalog (2019, background): “SSC emission from late internal shocks, with much longer variability time at GeV than in MeV, can explain delayed LAT onset in some cases (see, e.g., Toma et al. 2009; Daigne et al. 2011).”