ECLAIRs: A microsatellite to observe the prompt optical and x-ray emission of gamma-ray bursts

The prompt gamma-ray emission of Gamma-Ray Bursts (GRBs) is currently interpreted in terms of radiation from electrons accelerated in internal shocks in a relativistic fireball. On the other hand, the origin of the prompt (and early afterglow) optical and X-ray emission is still debated, mostly because very few data exist for comparison with theoretical predictions. It is however commonly agreed that this emission hides important clues on the GRB physics and can be used to constrain the fireball parameters, the acceleration and emission processes and to probe the surroundings of the GRBs. ECLAIRs is a microsatellite devoted to the observation of the prompt optical and X-ray emission of GRBs. For about 100 GRBs yr(-1), independent of their duration, ECLAIRs will provide high time resolution high sensitivity spectral coverage from a few eV up to similar to 50 keV and localization to similar to 5" in near real time. This capability is achieved by combining wide field optical and X-ray cameras sharing a common field of view (greater than or similar to 2.2 steradians) with the coded-mask imaging telescopes providing the triggers and the coarse localizations of the bursts. Given the delays to start ground-based observations in response to a GRB trigger, ECLAIRs is unique in its ability to observe the early phases (the first similar to 20 sec) of all GRBs at optical wavelengths. Furthermore, with its mode of operation, ECLAIRs will enable to search for optical and X-ray precursors expected from theoretical grounds. Finally ECLAIRs is proposed to operate simultaneously with GLAST on a synchronous orbit. This combination will ensure broad band spectral coverage from eV to GeV energies for the GRBs detected by the two satellites, ECLAIRs further providing their accurate localization to enable follow-up studies. ECLAIRs relies upon an international collaboration involving theoretical and hardware groups from Europe and the United States. In particular, it builds upon the extensive knowledge and expertise that is currently being gained with the HETE-2 mission.