London, August 29 : Scientists have pinpointed a powerful cosmic particle accelerator in the Crab Nebula, which is a doughnut-shaped magnetic field surrounding the stellar corpse at the nebula's heart.
According to a report in New Scientist, the finding is based on a tricky measurement showing that high-energy radiation near the star is polarized, with its electric field lining up neatly with the star's spin axis.
The Crab Nebula is the expanding remnant of a supernova that was observed by Chinese and Arab astronomers in 1054 CE. When the star exploded, it left behind a dense corpse called a pulsar.
The pulsar spins about 30 times per second, but is gradually slowing down as it emits a wind of particles and electromagnetic fields.
Some of these particles - mainly electrons - emit high-energy radiation, in the form of X-rays and gamma rays, when they are accelerated by magnetic fields in the region. ut it has been unclear where this acceleration is taking place.
Now, researchers led by Tony Dean of the University of Southampton in the UK say it is occurring quite close to the pulsar.
They have based that conclusion on observations from Europe's INTEGRAL satellite, which showed that 46 per cent of the pulsar's gamma-ray emission was polarized, with the photons' electromagnetic fields lined up in a common orientation.
"That's very high for anything in astrophysics," said David Thompson of NASA's Goddard Space Flight Center in Greenbelt, Maryland. "Such a high percentage of polarization means you really have to have very good conditions - a magnetic field that is very ordered," he added.
Such a well-ordered magnetic field is thought to occur close to the pulsar, whose surface is thought to boast a magnetic field a trillion times as strong as that on Earth.
The pulsar's field is like a bar magnet. So close to the pulsar, its magnetic field lines emerge from one pole and curve around, forming a doughnut shape, before returning at the other pole.
"But once you get away from the pulsar, then you get a much more complex situation because the field begins to break up into little patches and knots," Thompson told New Scientist.
The new result suggests that the particles in the pulsar's wind are being accelerated close to the pulsar, before the magnetic field has become tangled.
According to Thompson, that fits in with theoretical predictions, which are difficult to verify observationally.
The new study's insights about the magnetic field "engine" that accelerates particles will help researchers infer what powers more distant, dimmer objects.
"Pulsars and their surrounding regions are examples of physics under extreme conditions. Anything you can do to learn about how they work helps us understand the basic physics of particle acceleration and magnetic field generation," said Thompson.