The decay of optical emission from the γ-ray burst GRB 970228.


The optical counterpart of the γ-ray burst GRB 970228 appears to be a transient point source embedded in a region of extended nebulosity, the latter having been tentatively identified as a high-redshift galaxy. This would seem to favour models that place γ-ray bursts at cosmological distances, although a range of mechanisms for producing the bursts is still allowed. A crucial piece of information for distinguishing between such models is how the brightness of the optical counterpart evolves with time. Here the authors re-evaluate the existing photometry of the optical counterpart of GRB 970228 to construct an optical light curve for the transient event. They find that between 21 hours and six days after the burst, the R-band brightness decreased by a factor of ≡40, with any subsequent decrease in brightness occurring at a much slower rate. As the point source faded, it also became redder. The initial behaviour of the source appears to be consistent with the ``fireball’’ model, but the subsequent decrease in the rate of fading may prove harder to explain.