On April 12, 2011, Fermi's Large Area Telescope detected a six-day long outburst from the famous Crab Nebula supernova remnant. This enormous flare was five times more powerful than any yet seen from the object. In this figure, the inset image is a Hubble Space Telescope mosaic while the background shows the gamma-ray sky as seen by Fermi. The bright horizontal band is the plane of our Milky Way galaxy, and the location of the Crab Nebula is indicated on the right.
The nebula, which is the wreckage of an exploded star whose light first reached Earth in the year 1054, is one of the most studied objects in the sky. At the heart of an expanding gas cloud lies what's left of the original star's core, a superdense neutron star that spins 30 times a second. With each rotation, the star swings intense beams of radiation toward Earth, creating the pulsed emission characteristic of spinning neutron stars (also known as pulsars).
Scientists think that the flares occur as the intense magnetic field near the pulsar undergoes sudden restructuring. Such changes can accelerate particles like electrons to velocities near the speed of light. As these high-speed electrons interact with the magnetic field, they emit gamma-rays.