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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 ergs cm-2 is only surpassed by GRB 990123. In terms of the isotropic 20-2000 keV energy output of 7.8×1053 ergs, 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 nonrelativistic regime quickly (within a fraction of day) by a dense (~106 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

BeppoSAXMeasurements of the Bright Gamma‐Ray Burst 010222