While the X-ray monitoring of GRB970228 was going on, an observational campaign of the same object was simultaneously started with the most important optical telescopes. This campaign led to the discovery ([van Paradijs et al. 1997]) of an optical transient associated with the X-ray afterglow. This new optical source was discovered in the images taken the same night of the GRB and about one week later with the Isaac Newton Telescope (INT) and the William Herschel Telescope (WHT) at the Canary Islands. The source showed a brightness decline from the first to the second observation of about 2.5 magnitudes (V band), corresponding to a flux decrease of about a factor of 300. This was the first discovery of an optical afterglow of a GRB.
The source was continuously monitored up to September 1997 from the most important world-wide telescopes. As in the X-ray domain, the optical flux of the source showed a decrease well described by a power law with index -1.12 ([Garcia et al. 1997]), again in agreement with the general predictions of the fireball model. But there was still another information to gather from the optical observations. Very important in this respect are the two pointings of the Hubble Space Telescope (HST) ([Sahu et al. 1997,Fruchter et al. 1997]), the optical telescope orbiting around the Earth and therefore not suffering for the disturbances due to the Earth atmosphere. In the images taken with HST the presence of a nebulosity emerges around the point source located by the ground-based telescopes. An indication of this was also derived from the first images taken at the WHT and INT and other on-ground telescopes. Since both the source and the nebulosity were very weak, there is still today no clear agreement among the scientists if the intensity of this fuzziness is constant with time and what is its interpretation. Many people thinks that the nebulosity was actually constant, and in this case the easiest interpretation would be that it is the host galaxy of the object. If this interpretation will be someday confirmed, this would be the first experimental evidence for an extra-galactic origin of GRB970228.
Confirmation of the positional consistency between GRB970228, the X-ray afterglow and the optical counterpart also comes from other measurements. In fact, the German satellite ROSAT, that is able to locate a X-ray source with an accuracy of about 10 arc-sec, pointed the GRB970228 field on 10 March 1997 ([Frontera et al. 1997]). It detected the X-ray source 1SAX J0501.7+1146 at a flux level consistent with the extrapolation, in the assumption of a constant spectrum, of the decay law derived from the BeppoSAX measurements and refined its position at few arc-sec accuracy, still coincident with the location of the optical transient.
Furthermore, using the GRB arrival times at the BeppoSAX GRBM and at the Ulysses (an interplanetary mission at that time about 2.000 light seconds away from the Earth) GRB detector, Hurley et al. ([Hurley et al. 1997]) were able to derive an annulus in the sky of the possible arrival directions of GRB970228. This annulus crosses the WFC, NFI error boxes. The X-ray/optical afterglow lies just within the sky area derived from the overlap of all of these error boxes. In figure 5.6 the final location of the source is shown together will all the error boxes.