THE ENERGETIC AFTERGLOW OF THE GAMMA-RAY BURST OF 14 DECEMBER 1997

The discovery of fading but relatively long-lived X-ray emission(1) ac companying gamma-ray bursts has revolutionized the study of these obje cts. This 'afterglow' is most easily explained by models(2-4) similar to those describing supernovae, but with relativistic ejecta. And as w ith supernovae, afterglow measurements should in principle provide imp ortant constraints on burst properties, permitting, for example, estim ates of the amount of energy released, the geometry of the emitting su rface and the density of the ambient medium, Here we report infrared o bservations of the fading optical transient(5) associated with the bur st of 14 December 1997 (GRB971214;ref. 6). We detect a 'break' in the broad-band spectrum, as predicted by afterglow models, which constrain s the total energy in the burst to be > 10(51) erg. Combining the flue nce of optical afterglow with the redshift (z = 3.42; ref. 7), we esti mate that the energy released in the afterglow alone was 2 x 10(51) er g. Estimates of afterglow energetics are less likely to be subject to geometric effects-such as beaming-that render uncertain estimates of t he total burst energy, but it nevertheless appears from our measuremen ts that gamma-ray bursts may be much more energetic than the 10(51) er g usually assumed.