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.^[1]

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:^[1]

• One theory assumes that an X-ray-flash hypernova does not differ inherently from gamma-ray-emitting hypernovae, but rather in the orientation, relative to the line of sight to the observer, of the narrow beam of gamma radiation from the hypernova.^[1] That is, gamma rays are assumed to be emitted, but in each case in a direction away from our instruments for observing them. Thus the only initially observable phenomenon is lower-energy x-rays emitted in a beam that diverges more than the gamma-ray beam does.
• A competing "dirty fireball" theory suggests that an X-ray flash comes from a hypernova that uses much of the available energy in expelling an unusually large amount of baryonic matter,^[1] thus limiting the energy available for electromagnetic radiation, and emitting a much "cooler" spectrum rich in X-rays and very poor in gamma rays.

Further reading

• Marc Türler, "Supernova follows X-ray flash", Cern Courier, Oct 5, 2006

References