GRB 100621A was a long gamma-ray burst observed on June 21, 2010, by the Swift spacecraft. It is the third brightest gamma-ray burst yet observed, after GRB 221009A and GRB 130427A. However when it was discovered, it was the brightest GRB in terms of X-ray brightness. GRB 100621A is approximately five billion light years from Earth.
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