We present B-, V -, J-, H-, K-, and K@-band observations of the optical transient (OT) associ-R C -, I C -, ated with GRB 970228, based on a reanalysis of previously used images and unpublished data. In order to minimize calibration di †erences, we have collected and analyzed most of the photometry and consistently determined the magnitude of the OT relative to a set of secondary Ðeld stars. We conÐrm our earlier Ðnding that the early decay of the light curves (before 1997 March 6) was faster than that at intermediate times (between 1997 March 6 and April 7). At late times the light curves resume a fast decay (after 1997 April 7). The early-time observations of GRB 970228 are consistent with relativistic blast-wave models, but the intermediate-and late-time observations are hard to understand in this framework. The observations are well explained by an initial power-law decay with a \ [1.51^0.06 modiÐed at later times by a Type Ic supernova light curve. Together with the evidence for GRB 980326 and GRB 980425, this gives further support for the idea that at least some c-ray bursts are associated with a possibly rare type of supernova.
We analyse the spatially resolved colours of distant galaxies of known redshift in the Hubble Deep Field, using a new technique based on matching resolved four‐band colour data to the predictions of evolutionary synthesis models. Given some simplifying assumptions, we demonstrate how our technique is capable of probing the evolutionary history of high‐redshift systems, noting the specific advantage of observing galaxies at an epoch closer to the time of their formation. We quantify the relative age, dispersion in age, on‐going star formation rate and star formation history of distinct components. We explicitly test for the presence of dust and quantify its effect on our conclusions. To demonstrate the potential of the method, we study the spirals and ellipticals in the near‐complete sample of 32 I814<21.9 mag galaxies with z∼0.5 studied by Bouwens, Broadhurst & Silk. The dispersion of the internal colours of a sample of 0.4
We report on the results of optical follow-up observations of the counterpart of GRB 970508, starting 7 hours after the event. Multi-color U, B, V, R c and I c band observations were obtained during the first three consecutive nights. The counterpart was monitored regularly in R c until ∼ 4 months after the burst. The light curve after the maximum follows a decline that can be fitted with a power law with exponent α = -1.141 ± 0.014. Deviations from a smooth power law decay are moderate (r.m.s. = 0.15 magnitude). We find no flattening of the light curve at late times. The optical afterglow fluence is a significant fraction, ∼ 5%, of the GRB fluence. The optical energy distribution can be well represented by a power law, the slope of which changed at the time of the maximum (the spectrum became redder).
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