TITLE: GCN GRB OBSERVATION REPORT NUMBER: 1459 SUBJECT: GRB020331: Late-time Optical Light Curve DATE: 02/07/24 16:15:14 GMT FROM: Alicia Soderberg at Caltech A. M. Soderberg, D. W. Fox, and S. R. Kulkarni (Caltech) report on behalf of the larger Caltech-NRAO-CARA GRB collaboration. "We have observed the optical afterglow associated with GRB 020331 (GCN 1334) with HST+STIS (Clear) at four epochs spanning the period 25-50 days following the burst (one orbit per epoch). Registration of the STIS data against ground-based images allows us to identify the afterglow amidst a complex of at least three galaxies, including the presumed host galaxy which lies under the afterglow itself. Aperture photometry on the optical transient + host yields the following R-equivalent magnitudes for the first three epochs: HST Epoch T (days since burst) R (mag) 1 23.9 24.54 +/- 0.07 2 36.6 25.01 +/- 0.10 3 42.7 24.76 +/- 0.09 We have derived our R-equivalent magnitudes by assuming (R-Clear)=-0.67 as given by the STIS Exposure Time Calculator for a source with power-law spectrum (index=-1). These values imply a power-law flux decay index over this interval with index alpha=0.75, compatible with previous ground-based estimates (Ricker et al., GCN 1382). Preliminary analysis of the fourth epoch, T=50 days, indicates a rebrightening of the optical afterglow by 0.2 +/- 0.1 mag as compared with our Epoch 3. We have requested additional HST observations to help us clarify whether this rebrightening is statistically significant. A finding chart of the field can be found at: http://www.astro.caltech.edu/~ams/grb020331.html This message may be cited."