The Crab Nebula has erupted in an enormous flare five times more powerful than its previous outbursts and 30 times more energetic than its normal gamma-ray output, shaking the idea that it is a constant source of high energy radiation.

The Crab Nebula, located 6,500 light years away in the constellation Taurus, is the remnant of a stellar explosion that occurred in 1054. At the heart of the expanding debris cloud lies a superdense neutron star that spins 30 times a second, sending pulses of radiation towards the Earth, rather like the sweeping beam of a lighthouse.

Several short-lived gamma-ray flares emanating from the Crab Nebula remnant had been detected by various space observatories since 2009 – and since the start of this year changes in X-ray brightness had also been noted – but on 12 April, NASA's Fermi and the Italian Space Agency's AGILE satellite watched as a "superflare" reached 30 times the background value of the remnant's gamma-ray output. A second, even brighter, flare erupted on 16 April, with activity subsiding just two days later.

"These superflares are the most intense outbursts we've seen to date, and they are all extremely puzzling events," says Alice Harding at NASA's Goddard Space Flight Center. "We think they are caused by sudden rearrangements of the magnetic field not far from the neutron star, but exactly where that's happening remains a mystery."

During this restructuring, particles accelerated to nearly the speed of light interact with the magnetic field to produce gamma rays. Based on the observations of the April outbursts, scientists estimate that the size of the region emitting the gamma-rays must be comparable to the size of our own Solar System.

“The Crab Nebula hosts high-energy variability that we’re only now fully appreciating,” adds Fermi scientist Rolf Buehler of the Kavli Institute for Particle Astrophysics and Cosmology.