Spectral evolution in gamma-ray bursts: Predictions of the internal shock model and comparison to observations

Abstract

Context. Several trends have been identified in the prompt gamma-ray burst (GRB) emission: e.g. hard-to-soft evolution, pulse width evolution with energy, time lags, and hardness-intensity and hardness-fluence correlations. Recently, Fermi has significantly extended the spectral coverage of GRB observations and improved the characterization of this spectral evolution. Aims. We want to study how internal shocks can reproduce these observations. In this model the emission comes from the synchrotron radiation of shock accelerated electrons, and the spectral evolution is governed by the evolution of the physical conditions in the shocked regions.Methods. We present a comprehensive set of simulations of a single pulse and investigate the impact of the model parameters, related to the shock microphysics and to the initial conditions in the ejecta. Results. We find general qualitative agreement between the model and the various observations used for the comparison. All these properties or relations are governed by the evolution of the peak energy and photon indices of the spectrum. In addition, we identify the conditions for quantitative agreement. We find that the best agreement is obtained for (i) steep electron slopes (p > ∼ 2.7); (ii) microphysics parameters varying with shock conditions so that more electrons are accelerated in stronger shocks; and (iii) steep variations in the initial Lorentz factor in the ejecta. When simulating short GRBs by contracting all timescales, all other parameters being unchanged, we show that the hardness-duration correlation is reproduced, as well as the evolution with duration of the pulse properties. Finally, we investigate the signature at high energy of these different scenarios and find distinct properties -delayed onset, longer emission, and flat spectrum in some cases -suggesting that internal shocks could have a significant contribution to the prompt LAT emission. Conclusions. Spectral evolution is an important property of GRBs that is not easily reproduced in most models for the prompt emission. We find that the main observed features can be accounted for in a quantitative way within the internal shock model. However, the current uncertainties on shock acceleration in the mildly relativistic regime and relativistic ejection by compact sources prevent us from deciding if one or several of the proposed scenario are viable. By combining observations over the whole spectral range of Fermi, it may be possible in the future to identify specific signatures imprinted by this uncertain underlying physics.

Is spectral width a reliable measure of GRB emission physics?

“…While model checking can provide a qualitative guide to the validity of a spectra fit, other information should be used to fully justify the use of a model. These information can include physically motivated priors and predictions from time-evolving models such as those used in Dermer et al (1999); Pe'er (2008); Bošnjak & Daigne (2014). For example, Burgess et al (2016) from Dermer et al (1999) combined with synchrotron spectral fits to argue for an external shock interpretation of GRB 141028A.…”

SWIFT OBSERVATIONS OF GAMMA-RAY BURST PULSE SHAPES: GRB PULSE SPECTRAL EVOLUTION CLARIFIED

“…Burgess et al (2011)), which thus far have not addressed the possibility that spectral evolution might be associated with specific light curve elements within an individual pulse. Bošnjak & Daigne (2014) have considered the radiation mechanisms of kinematically evolving synchrotron shocks, but find that the model better explains observed pulse spectral evolution if (i) the distribution of shock-accelerated electrons is steeper than generally assumed, (ii) the peak energy dependence is reduced by altering microphysics parameters, (iii) the Lorentz factor undergoes more pronounced initial variations than previously thought, and (iv) the relativistic ejection proceeds with a constant mass flux rather than a constant kinetic energy flux. Bošnjak & Daigne (2014) indicate that these conditions are hard to justify, but that their ability to test microphysics conditions are limited by existing data.…”

Gamma-Ray Burst Prompt Correlations

“…Many authors afterwards [254][255][256][257][258][259][260][261][262][263][264] have used the motivated pulse profile of Willingale et al [108] for various studies on the prompt emission properties of the pulses.…”