We follow the bright , highly energetic afterglow of Swift -discovered GRB 080721 at z = 2.591 out to 36 days or 3 \times 10 ^ { 6 } s since the trigger in the optical and X-ray bands .
We do not detect a break in the late-time light curve inferring a limit on the opening angle of \theta _ { j } \geq 7.3 ^ { \circ } and setting tight constraints on the total energy budget of the burst of E _ { \gamma } \geq 9.9 \times 10 ^ { 51 } erg within the fireball model .
To obey the fireball model closure relations the GRB jet must be expanding into a homogeneous surrounding medium and likely lies behind a significant column of dust .
The energy constraint we derive can be used as observational input for models of the progenitors of long gamma-ray bursts : we discuss how such high collimation-corrected energies could be accommodated with certain parameters of the standard massive star core-collapse models .
We can , however , most probably rule out a magnetar progenitor for this GRB which would require 100 % efficiency to reach the observed total energy .