A fundamental test of the fireball model for GRBs

The most extreme GRBs radiate equivalent isotropic energies close to 10^54 erg;energies which cannot be accommodated in corecollapse models. Thus GRBs arethought to be narrowly beamed, which should give rise to an achromatic lightcurve break as the outflow slows and the Doppler beaming angle becomes of thesame order as the jet opening angle. However, observations with Swift suggestsuch jet breaks are absent, leading to concerns about the total energetics whichmay undermine the entire GRB paradigm. We seek XMMNewton observations to testfor the absence of breaks in the Xray light curves of two highly energeticGRBs. If even one is found with no break at late times, then the whole class ofcorecollapse models for GRBs will be thrown into serious doubt. truncated!, Please see actual data for full text [truncated!, Please see actual data for full text]