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Afterglow, or long-lived emission, has now been detected from about a dozenwell-positioned gamma-ray bursts. Distance determinations made by measuringoptical emission lines from the host galaxy, or absorption lines in theafterglow spectrum, place the burst sources at significant cosmologicaldistances, with redshifts ranging from ~1--3. The energy required to producethe bright gamma-ray flashes is enormous: up to ~10^{53} erg or 10 percent ofthe rest mass energy of a neutron star, if the emission is isotropic. Here wepresent the discovery of the optical afterglow and the redshift of GRB 990123,the brightest well-localized GRB to date. With our measured redshift of >1.6,the inferred isotropic energy release exceeds the rest mass of a neutron starthereby challenging current theoretical models for the origin of GRBs. We arguethat the optical and IR afterglow measurements reported here may provide thefirst observational evidence of beaming in a GRB, thereby reducing the requiredenergetics to a level where stellar death models are still tenable.Comment: Revised version (27 Feb 99). 19 pages, 3 figures. Original version  submitted to Nature 2/11/99. Uses macros reforder.sty and citmac.sty  (supplied

The afterglow, redshift and extreme energetics of the γ-ray burst of 23 January 1999