Complex sentence: February 28,1997, detected on 28 February 1997, at 02:58 UTC, was the first gamma-ray burst (GRB) for which an afterglow was observed.
Split into simpler sentences: February 28,1997 was a gamma-ray burst (GRB) detected on 28 February 1997, at 02:58 UTC. It was the first GRB for which an afterglow was observed.