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NASA鈥檚 Fermi telescope has found a dozen pulsars that can be detected only by the gamma rays they emit, and not by lower-energy radio waves characteristic of most pulsars.

The new clutch of spinning stars rules out a long-held idea that pulsars beam gamma-rays from the stars鈥� magnetic poles, astronomers say. Instead, gamma rays seem to be produced close to the equator, where a pulsar鈥檚 spin whips charged particles up to near-light speeds.

This model could explain why astronomers see a menagerie of pulsars, some of which blast both radio and gamma rays and others just one of the two types of radiation. The star鈥檚 geometry and the Earth鈥檚 viewing angle play important parts in determining which type of pulsar astronomers observe.

Pulsars form when a massive star explodes at the end of its life, leaving behind a dense, fast-spinning ball of neutrons. These stellar cinders blast tight beams of radio waves from their magnetic poles.

If a pulsar鈥檚 magnetic pole is not perfectly aligned with the star鈥檚 axis of rotation, the radio beam will sweep around as the star spins and could cast light towards Earth like a lighthouse beacon.

Strange star

Some 1800 pulsars have been found that beam radio waves. But until the Fermi telescope鈥檚 launch in June 2008, only a half dozen or so such stellar corpses had been found to produce gamma rays as well.

An even stranger star turned up four months after the launch, when Fermi researchers announced they had discovered a pulsar that seems to emit only gamma rays.

Some suggested the new pulsar was consistent with a long-held notion that the stars produce gamma rays in the same way they do radio waves, which originate close to the surface of the star and are blasted into space at the pulsar鈥檚 magnetic poles. If the gamma-ray beam were wider than the radio wave beam, it could be visible even though the radio waves were not.

Trumpet-shaped sheets

But models of the 12 new pulsars rule out this scenario. 鈥淭he classical, near-surface polar cap idea of where gamma rays come from has been killed,鈥� physicist Roger Romani of Stanford University said on Tuesday at the American Astronomical Society meeting in Long Beach, California.

Instead, gamma rays seem to be produced in vast, trumpet-shaped sheets along the equator high above the star鈥檚 surface (see video). There, the star鈥檚 spin pulls magnetic fields 鈥� as well as electrons and their antimatter counterparts, positrons 鈥� to nearly the speed of light. The fast-moving particles radiate gamma rays as they swirl around in space.

Pulsars thus blast radiation from two regions 鈥� their equator and their magnetic poles 鈥� but 鈥渨hat you see depends on where you look鈥�, Romani says.

A pulsar seen along its equator may boast gamma rays. Radio waves will be seen if one of the star鈥檚 magnetic poles aligns temporarily with astronomers鈥� line of sight. But both types of radiation may be visible if the star鈥檚 magnetic poles are aligned close enough to the pulsar鈥檚 equator.

The young and the restless

While the model may apply to most pulsars, only a fraction of the stars may produce gamma rays, Romani says. The phenomenon may be restricted to young, especially fast-rotating pulsars that are capable of accelerating particles to the energies needed to emit gamma rays.

Since pulsars release 10,000 times more energy in gamma rays than they do in radio wavelengths, the high-energy radiation could offer insight into how these stellar particle accelerators work, Romani says.

Fermi has also found the first millisecond pulsars known to produce gamma rays. While most pulsars spin several times per second, millisecond pulsars spin hundreds of times faster.

These stars are thought to have once been ordinary pulsars that got a boost in energy by gobbling up material from ageing, bloated stellar companions. But it is not yet clear whether these extra-energetic pulsars produce gamma rays in the same way as other pulsars.