Article

The origin of the prompt GRB spectrum

12/2009;
Source: arXiv

ABSTRACT Using a detailed model of the internal shock phase, we discuss the origin of the prompt emission in gamma-ray bursts. We focus on the identification of the dominant radiative process (Fermi-GBM range) and propose an explanation for some features observed by Fermi-LAT at high energy in some GRB lightcurves. Comment: 2009 Fermi Symposium, eConf Proceedings C091122;

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    ABSTRACT: Detailed information on the physical parameters in the sources of cosmological Gamma-Ray Bursts (GRBs) is obtained from few plausible assumptions consistent with observations. Model-independent requirements posed by these assumptions on the emission mechanism in GRBs are formulated. It is found that the observed radiation in sub-MeV energy range is generated by the synchrotron emission mechanism, though about ten per cent of the total GRB energy should be converted via the inverse Compton process into ultra-hard spectral domain (above 100 GeV). We estimate the magnetic field strength in the emitting region, the Lorentz factor of accelerated electrons, and the typical energy of IC photons. We show that there is a "line-of-death" relation for GRBs and derive from this relation the lower limits on both GRB duration and GRB variability timescale. The upper limit on the Lorentz factor of GRB fireballs is also found. We demonstrate that steady-state electron distribution consistent with the Compton losses may produce different spectral indices, e.g., 3/4 as opposed to the figure 1/2 widely discussed in the literature. It is suggested that the changes in the decline rate observed in the lightcurves of several GRB afterglows may be due to the time evolution of spectral break, which appears in the synchrotron emission generated by steady-state self-consistent electron distribution. Comment: Journal reference added, introduction extended, minor changes in notations
    Astronomy and Astrophysics 06/2000; · 5.08 Impact Factor
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    ABSTRACT: If the emission of gamma-ray bursts were due to the synchrotron process in the standard internal shock scenario, then the typical observed spectrum should have a slope F(nu) \propto nu^{-1/2}, which strongly conflicts with the much harder spectra observed. This directly follows from the cooling time being much shorter than the dynamical time. Particle re-acceleration, deviations from equipartition, fastly changing magnetic fields and adiabatic losses are found to be inadequate to account for this discrepancy. We also find that in the internal shock scenario the relativistic inverse Compton scattering is always as important as the synchrotron process, and faces the same problems. This indicates that the burst emission is not produced by relativistic electrons emitting synchrotron and inverse Compton radiation. Comment: 5 pages, accepted for publications in MNRAS, pink pages
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    ABSTRACT: Context. Gamma-ray bursts (GRB) are powerful, short duration events with a spectral luminosity peaking in the keV-MeV (BATSE) range. The prompt emission is thought to arise from electrons accelerated in internal shocks propagating within a highly relativistic outflow.Aims. The launch of Fermi offers the prospect of observations with unprecedented sensitivity in high-energy (HE, $>$100 MeV) gamma-rays. The aim is to explore the predictions for HE emission from internal shocks, taking into account both dynamical and radiative aspects, and to deduce how HE observations constrain the properties of the relativistic outflow.Methods. The prompt GRB emission is modelled by combining a time-dependent radiative code, solving for the electron and photon distributions, with a dynamical code giving the evolution of the physical conditions in the shocked regions of the outflow. Synthetic lightcurves and spectra are generated and compared to observations.Results. The HE emission deviates significantly from analytical estimates, which tend to overpredict the IC component, when the time dependence and full cross-sections are included. The exploration of the parameter space favors the case where the dominant process in the BATSE range is synchrotron emission. The HE component becomes stronger for weaker magnetic fields. The HE lightcurve can display a prolonged pulse duration due to IC emission, or even a delayed peak compared to the BATSE range. Alternatively, having dominant IC emission in the BATSE range requires most electrons to be accelerated into a steep power-law distribution and implies strong second order IC scattering. In this case, the BATSE and HE lightcurves are very similar.Conclusions. The combined dynamical and radiative approach allows a firm appraisal of GRB HE prompt emission. A diagnostic procedure is presented to identify from observations the dominant emission process and derive constrains on the bulk Lorentz factor, particle density and magnetic field of the outflow.
    Astronomy and Astrophysics 01/2009; · 5.08 Impact Factor

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