Broad band observations of gamma-ray burst afterglows

We propose a broad band observation of three gammaray burst afterglows in orderto constrain the physics of the fireball that produce the burst, the densityprofile and composition of the surrounding environment of the burst, and deduceproperties about the stellar progenitor of the burst. This observation will beperformed in coordination with ESO APEX and VLT telescopes. We request threeobservations of 50 kiloseconds each, starting 70 kiloseconds after the start ofthe burst, using the EPIC MOS and PN cameras. truncated!, Please see actual data for full text [truncated!, Please see actual data for full text]