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Although more than 2,000 astronomical gamma-ray bursts (GRBs) have beendetected, and numerous models proposed to explain their occurrence, they haveremained enigmatic owing to the lack of an obvious counterpart at otherwavelengths. The recent ground-based detection of a transient source in thevicinity of GRB 970228 may therefore have provided a breakthrough. The opticalcounterpart appears to be embedded in an extended source which, if a galaxy ashas been suggested, would lend weight to those models that place GRBs atcosmological distances. Here we report the observations using the Hubble SpaceTelescope of the transient counterpart and extended source 26 and 39 days afterthe initial gamma-ray outburst. We find that the counterpart has faded sincethe initial detection (and continues to fade), but the extended source exhibitsno significant change in brightness between the two dates of observationsreported here. The size and apparent constancy between the two epochs of HSTobservations imply that it is extragalactic, but its faintness makes adefinitive statement about its nature difficult. Nevertheless, the decayprofile of the transient source is consistent with a popular impulsive-fireballmodel, which assumes a merger between two neutron stars in a distant galaxy.Comment: 11 pages + 2 figures. To appear in Nature (29 May 1997 issue

The optical counterpart to γ-ray burst GRB970228 observed using the Hubble Space Telescope