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the recent results obtained by the hst on the optical counterpart of grb 970228 [ 3,4 ] have given answer to important questions concerning its light decay , its proper motion and the nature of the underlying nebula .
this information must be taken into account in reanalysing all the previously published data .    in this paper
we aim to refine the values and lower limits ( particularly those obtained with the 1.5meter telescope of the bologna astronomical observatory ) of the points in the @xmath0 light curve of grb 970228 , taking into account the refined magnitude value for the underlying nebulosity given by fruchter et al .
moreover , we show for the first time and briefly discuss the @xmath1 light curve of this grb .
galama et al . [ 5 ] collected all the known @xmath0 points and lower limits referring to the grb 970228 optical light curve .
they however used a value for the luminosity of the nebulosity associated to the object ( @xmath2 ; groot et al . [ 6 ] ) which is overestimated according to the most recent hst observations ( @xmath3 ; fruchter et al . [ 4 ] ) .
this discrepancy is likely due to calibration errors which depend on the faintness of the object and not on an intrinsic variability ; fox et al .
[ 2 ] and fruchter et al . [
3 ] actually showed that the nebula is constant in brightness .
therefore , we refined the @xmath0 data collection by galama et al . [ 5 ] using the new value of the magnitude for the underlying fuzzy object ; in particular , since the ccd observations obtained at bologna observatory [ 7 ] measured the total flux received from the transient plus the nebula and a nearby k  type star whose magnitude is @xmath4 [ 9 ] , we obtained the new @xmath0 magnitudes or lower limits for the optical transient by subtracting the two latter contributions from the data .
we want to note that in the galama et al.s [ 5 ] data collection the @xmath0 point by guarnieri et al . [ 7 ] is wrongly plotted and must be corrected .
indeed , the quoted @xmath0 magnitude refers to the total contribution coming from the transient plus the nebulosity and the nearby star , and not from the optical transient alone . the right value on feb .
28.827 ( and not on feb .
28.76 ) is therefore @xmath5 .    in this revision
pedichini et al.s [ 10 ] observation of feb .
28.81 was not considered because of the difficulty of reliably converting their color system to the @xmath0 band .
anyway , the importance of pedichini et al.s [ 10 ] observations lies in the fact of providing a variation @xmath6 m @xmath7 between feb . 28 and
4 . their observations span from feb 28.795 to 28.827 ut . the bologna @xmath0 frame was taken between feb .
28.816 and 28.837 ut , so the two observations partially overlapped .
the magnitude could be quite different only if the luminosity of the transient during the first part of the observation was higher than in the last part . assuming that pedichini et al.s [ 10 ] magnitude on feb .
28 was the same as that found in bologna in the @xmath0 band the same day , the lower limit @xmath8 could be derived for the day mar
we consider this figure as representative of the magnitude variation in the @xmath0 band , too .    from the observation of margon et al .
[ 8 ] on mar .
3.1 , made with the astrophysical research consortium 3.5meter telescope at apache point in the apm @xmath9 photometric band , galama et al .
[ 5 ] derived an estimate of @xmath0 by interpolating between the color index @xmath10 measured on feb .
28 and on mar .
bartolini et al .
[ 1 ] found evidence of fast color variations near the maxima of the optical counterpart of both grb 970228 and grb 970508 , so it could be dangerous to extrapolate the @xmath0 magnitude from margon et al.s [ 8 ] observation and to assume @xmath9 = @xmath11 ; moreover , galama et al .
[ 5 ] used the color index of the optical transient alone on feb .
28 , and that of the optical transient plus the underlying extended source on mar . 9
for these reasons we did not include the value obtained by galama et al .
[ 5 ] from the apache point measurement .
the refined @xmath0 light curve of the optical transient associated to grb 970228 is shown in fig . 1
.    we also collected from the literature all the @xmath1 data points [ 3,7,11,12 ] and plotted them in fig .
2 . the @xmath1 band lower limits published by van paradijs et al .
[ 12 ] were slightly corrected with the use of the @xmath1 magnitude of the nebula ( @xmath12 , as reported by fruchter et al . [ 3 ] ) .
it should be noted that the value by guarnieri et al . [ 7 ] is interpolated from their @xmath11 and @xmath0 data acquired on feb .
the overall trends of the @xmath0 and @xmath1 light curves of the transient associated to grb 970228 are quite similar . indeed , we find that both decay following a power law with spectral indices @xmath13 and @xmath14 , i.e. they are coincident within the errors .
this similarity is however related to the long  term trend .
actually , in a short time scale the @xmath0 light curve ( fig .
1 ) deviates from a unique power law .
the decay from the light peak can indeed be divided into two phases : the first one , spanning from the optical maximum and lasting 34 days with @xmath15 and the second one , after march 4 , with @xmath16 .
these figures are in agreement with the findings by galama et al .
[ 5 ] and confirm the rapid light decay noticed by guarnieri et al . [ 7 ] during the first 3 days after the optical maximum .
we can not be sure of a similar trend in the @xmath1 since at that time the coverage of the light curve was very poor ( only two lower limits are available ; see fig .
2 ) ; we see however that the lower limit of mar .
4.9 [ 12 ] seems to suggest a possible rapid decay also in @xmath1 .    with the refined value of the @xmath0 magnitude of the underlying object , we can now correct the value for the ratio between the luminosity of the optical transient and that of the fuzzy object in the @xmath0 band reported by guarnieri et al . [ 7 ] .
we find that in @xmath0 the transient at maximum light was @xmath1760 times brighter than the underlying nebulosity .
if the nebula is a host galaxy , the optical transient associated to grb 970228 has been by far the brightest variable object known up to now .
we refined the @xmath0 light curve of the optical transient associated to grb 970228 with the use of a more correct value for the magnitude of its underlying nebula and compared this curve to the one in the @xmath1 band ( previously unpublished ) .
it is interesting that both the overall trends follow a power law decay and are almost identical within the errors .
the ratio between the @xmath0 luminosity of the optical transient at maximum and the nebulosity has been corrected : now we know that the transient was approximately 60 times brighter than the nebula at that time .
90 bartolini c. , beskin g.m . ,
guarnieri a. , masetti n. , piccioni a. , 1997 , did grb 970228 and grb 970508 present similar optical properties ? .
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we present the @xmath0 and @xmath1 light curves of the optical counterpart of grb 970228 . a critical analysis of all the available data is made in light of the results achieved in the recent grb symposium held in huntsville and by considering the latest information from the hst images on the underlying nebulosity .