AGILE discovery that the Crab Nebula is variable

The Crab Nebula is certainly one of the most famous astronomical objects. It is at the center of a bright supernova that exploded in 1054 and was recorded by Chinese astronomers. It encloses now at its very center one of the most powerful pulsars. The Crab has been considered for decades as one of the strongest persistent X-ray and gamma-ray source in the sky and was then used as a standard calibration source in astrophysics. Therefore, when between Sept. 19 and Sept.21, 2010 the AGILE team detected a strongly enhanced gamma-ray emission from the Crab Nebula region (see ATel # 2855) it provoked a sort of shock in the community. Following the AGILE discovery and the confirmation by Fermi/LAT on the following day (see Atel #2861), many telescopes (Swift, INTEGRAL, Hubble, Chandra) have then pointed and are still pointing at the Crab, gathering precious information for precisely identifying the phenomenon. The AGILE team reported the surprising gamma-ray flares of the Crab Nebula in the paper by Tavani et al., Science, 331, 736, 2011.


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