GRBÂ 110918A is the brightest long \gamma -ray burst ( GRB ) detected by Konus- WIND during its almost 19 years of continuous observations and the most luminous GRB ever observed since the beginning of the cosmological era in 1997 .
We report on the final Interplanetary Network localization of this event and its detailed multi-wavelength study with a number of space-based instruments .
The prompt emission is characterized by a typical duration , a moderate peak energy of the time-integrated spectrum , and strong hard-to-soft evolution .
The high observed energy fluence yields , at z=0.984 , a huge isotropic-equivalent energy release E _ { \mathrm { iso } } = ( 2.1 \pm 0.1 ) \times 10 ^ { 54 } Â erg .
The record-breaking energy flux observed at the peak of the short , bright , hard initial pulse results in an unprecedented isotropic-equivalent luminosity L _ { \mathrm { iso } } = ( 4.7 \pm 0.2 ) \times 10 ^ { 54 } erg s ^ { -1 } .
A tail of the soft \gamma -ray emission was detected with temporal and spectral behavior typical of that predicted by the synchrotron forward-shock model . Swift /XRT and Swift /UVOT observed the bright afterglow from 1.2 to 48 days after the burst and revealed no evidence of a jet break .
The post-break scenario for the afterglow is preferred from our analysis , with a hard underlying electron spectrum and ISM-like circum-burst environment implied .
We conclude that , among multiple reasons investigated , the tight collimation of the jet must have been a key ingredient to produce this unusually bright burst .
The inferred jet opening angle of 1.7 \arcdeg -3.4 \arcdeg results in reasonable values of the collimation-corrected radiated energy and the peak luminosity , which , however , are still at the top of their distributions for such tightly collimated events .
We estimate a detection horizon for a similar ultraluminous GRB of z \sim 7.5 for Konus- WIND and z \sim 12 for Swift /BAT , which stresses the importance of \gamma -ray bursts as probes of the early Universe .