Bepposax Measurements of the Bright Γ - Ray Burst 010222

We analyze the BeppoSAX measurements of the prompt and afterglow emission of the γ-ray burst GRB 010222. Among 45 GRBs detected with the Wide Field Cameras on BeppoSAX, the 40-700 keV fluence of (9.3 ± 0.3) × 10 −5 erg cm −2 is only surpassed by GRB 990123. In terms of the isotropic 20-2000 keV energy output of 7.8 × 10 53 erg, it ranks third of all GRBs with measured distances. Since this burst is so bright, the data provide complete and valuable coverage up to 65 hr after the event, except for a gap between 3.5 and 8.0 hr. The 2-10 keV flux history shows clear signs of a break which is consistent with a break seen in the optical, and provides supporting evidence for the achromatic nature of the break. An explanation for the break in the context of a collimated expansion is not straightforward. Rather, a model is favored whereby the fireball is braked to the non-relativistic regime quickly (within a fraction of day) by a dense (∼ 10 6 cm −3) circumburst medium. This implies that, after a mild beaming correction, GRB 010222 may be the most energetic burst observed thus far. The X-ray decay index after the break is 1.33 ± 0.04, the spectral index 0.97 ± 0.05. The decay is, with unprecedented accuracy, identical to that observed in the optical.