Broad band observation of gamma-ray burst afterglows to constrain the fireball p

We propose a broad band observation of one gammaray burst afterglow in order toconstrain 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 optical telescopes. We request oneobservations of 50 kiloseconds, starting 70 kiloseconds after the start of theburst, using the EPIC MOS and PN cameras. This proposal is the continuation ofthe proposal 60140, accepted during AO08. truncated!, Please see actual data for full text [truncated!, Please see actual data for full text]