A Photometric Investigation of the GRB 970228 Afterglow and the Associated Nebulosity

Abstract

We carefully analyze the WFPC2 and STIS images of GRB 970228. We measure magnitudes for the GRB 970228 point-source component in the WFPC2 images of (V = 26.20, I_c = 23.94) and (V = 26.52, I_c = 24.31) on March 26 and April 7, respectively; and R_c = 27.09 on September 4 in the STIS image. For the extended component, we measure magnitudes of R_c = 25.48 in the combined WFPC2 images and R_c = 25.54 in the STIS image, which are consistent with no variation. This value is fainter than previously reported (Galama et al. 1998) and modifies the previously assumed magnitudes for the optical transient when it faded to a level at which the extended-source component contribution was not negligible, alleviating the discrepancy with a power-law temporal behavior. We also measure a color of (V₆₀₆ - I₈₁₄)_ST = -0.18 for the extended-source component. Taking into account the extinction measured in this field, this color implies that the extended source is most likely a galaxy with ongoing star formation.