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

Long-lived emission, known as afterglow, has now been detected from about a dozen gamma-ray bursts. Distance determinations place the bursts at cosmol ogical distances, with redshifts,z, ranging from similar to 1 to 3, The ene rgy required to produce these bright gamma-ray flashes is enormous: up to s imilar to 10(53) erg, or to per cent of the rest-mass energy of a neutron s tar, if the emission is isotropic. Here we present optical and near-infrare d observations of the afterglow of GRB990123, and we determine a redshift o f z greater than or equal to 1.6, This is to date the brightest gamma-ray b urst with a well-localized position and if the gamma-rays were emitted isot ropically, the energy release exceeds the rest-mass energy of a neutron st ar, so challenging current theoretical models of the sources. We argue, how ever, that our data may provide evidence of beamed (rather than isotropic) radiation, thereby reducing the total energy released to a lever where stel lar-death models are still tenable.