X-ray flash (astronomy)
In astronomy, an X-ray flash is a transient emission of X-rays originating in a distant galaxy, probably caused by a hypernova. They have been observed to last 90 to 200 seconds. Nearly all hypernovae are detected via (higher-energy) gamma-ray photons, at distances too great for any associated X-ray emissions from them to be observed; nevertheless, the two main theories of the nature of an X-ray flash each assume that a hypernova is involved: