In one widely accepted model, the stage is set for any type of gamma-ray emission, constant or short-lived, when electrons launched from the central Crab pulsar encounter strong magnetic fields in the surrounding debris. Electrons spin around magnetic fields and are accelerated to energies high enough to emit gamma.
But the Crab’s recently detected outbursts appear to pose problems for that acceleration model. The brevity of the flares indicates that the electrons could not have spun long enough to produce the energetic radiation, Buehler said. Another problem: Because electrons accelerated to very high energies lose that energy quickly, the nebula’s magnetic field might have to be three to 10 times stronger (3 to 10 milliGauss) than commonly assumed. (For comparison, the Earth’s surface magnetic field is about 500 milliGauss.)
The short duration suggests that the gamma rays originate from a relatively small part of the inner nebula. Buehler suggested that the pulsar’s own electric field helped accelerate electrons in the inner part of the nebula to energies high enough to emit gamma.
Wlodek Bednarek and a colleague at the University of Lodz in Poland offered another explanation. In a paper published on www.arXiv.org on November 19, they suggest that the pulsar’s wind of charged particles collides with and compresses the nebula’s magnetic field. As the disrupted field breaks like a rubber band and reconfigures itself, it releases an enormous amount of energy that accelerates the electrons, the researchers propose.
While researchers sift through the details, astronomers are also trying to determine the exact region where the September burst originated. As revealed in visible light and X-ray images, the nebula contains a complex series of wisps and jets. A series of portraits taken by the Chandra X-ray Observatory from a few weeks after the September flare shows that the base of one of the jets has illuminated. This could be where the gamma-ray flare originated, Tavani says.
Solving the Crab Nebula’s puzzle is likely to shed new light on the nature of its pulsar, said Jonathan Arons of the University of California, Berkeley. “All these particles come out screaming [of the pulsar] and become detained in the nebula,” which acts as the capture vessel for the pulsar, Arons said. “Studying what happens in the inner nebula is the closest thing to a laboratory experiment” for probing the pulsar, he added.
It may also help elucidate the physics of a number of other astronomical systems that feature a central compact object, Arons said. These include black holes whose jets of charged particles crash into surrounding interstellar space, or collisions between clumps of material within such jets that are believed to create the most energetic explosions in the universe: events called gamma-ray bursts.
Image: A composite photograph of the Crab Nebula showing x-ray light (light blue), visible light (green and dark blue), and infrared light (red). Credit: NASA, ESA, CXC, JPL-Caltech, J. Hester and A. Loll (Arizona State Univ.), R. Gehrz (Univ. Minn.) and STScI
See also:
