Early detection of the Optical Transient following the Gamma-Ray Burst GRB 970228

Abstract

The optical counterpart of the Gamma-Ray Burst GRB 970228, discovered by Groot et al. (IAU Circ. 6584), is also detected in the B and R frames obtained about 4 hours earlier at the Bologna Observatory. Our observations indicate a very likely rise of the optical emission within these 4 hours. The R luminosity of the transient at maximum was about 15 times that of an underlying extended object. Follow-up data show that the maximum optical emission was delayed of not less than 0.7 days with respect to the gamma-ray peak and that no new big flares were seen after the main one. The optical transient became significantly redder once it has reverted to quiescence.