Astrophile: Split personality tarnishes pulsars' rep

Astrophile is our weekly column on curious cosmic objects, from the solar system to the far reaches of the multiverse

Object: Pulsar (pulsing star)
Behaviour: Erratic

A split personality can be disturbing, especially when it affects someone you least expected it to. Now it seems stars, too, can have disturbingly fractured personas. The discovery of the first pulsar that shines at different frequencies at different times is tarnishing the reputation of this class of pulsing dead star. Until now, pulsars have been relied upon as cosmic timekeepers, but perhaps we can't take their regularity for granted any longer.

We had thought of pulsars ? the dense remnants of exploded stars ? as emitting radio waves, X-rays or gamma rays in bursts of such regularity that you could practically set your watch by them. Pulsars are spinning neutron stars, with strong magnetic fields that emit beams of radiation from their poles. The poles are offset from the axis of rotation ? just as the magnetic poles on Earth do not line up with true north and south. This offset means the beams sweep around like a lighthouse as the pulsar rotates, giving the star's radiation a distinctive pulse when viewed from a distance.

Individual pulsars can emit radiation at a range of frequencies, from radio waves to gamma rays, and some radio pulsars have been seen to dim or even stop broadcasting all together. That finding had previously cast some doubt on pulsars' otherwise impeccable reputation for dependability.

The latest discovery takes that to a new level, though. When Wim Hermsen of the Netherlands Institute for Space Research in Utrecht and colleagues took the first simultaneous measurements of both the radio and gamma radiation from B0943+10, a pulsar known to emit at both frequencies, they got a shock.

Radio flip

Rather than shining continuously as the team expected, B0943+10 flips at the drop of a hat, toning down its radio emissions for a few hours while pumping out extra X-rays, before switching back as if nothing had happened ? a totally new type of pulsar behaviour. "This variability has never been seen before," says Hermsen.

The pulsar's first personality is similar to that of its more stable siblings. In this mode, the pulsar emits a beam of radio waves from its pole and rotates every 1.1 seconds, while also emitting low-level background X-rays in every direction.

Every few hours though ? exactly when seems random ? the pulsar switches to its second personality. The main radio beam strength halves, though its timing remains the same, and is accompanied by another, even weaker radio signal just before the main pulse. Meanwhile the X-ray output doubles and pulses in time with the star's spin. Some time later, it switches back.

Astronomers already disagree over whether pulsar emissions are created by annihilation of particles close to the surface or further out but still within the pulsar's magnetic field. B0943+10's quirks throw a further spanner in the works because they don't fit either picture. What could be going on?

Cosmic act

One explanation is that what looks a radio-to-X-ray pulsar personality transplant is actually just an act. The object basically remains a radio pulsar, but whenever the radio beam halves in strength, the pulsar's magnetic field switches on a second radio beam in a different direction, which does not sweep past Earth.

In this scenario, the second radio beam is accompanied by increased X-ray emissions. Since they are produced in all directions, they are visible from Earth. If that's true, it would be rather like ascribing a friend's odd behaviour to a personality problem ? only to discover that there was in fact an explanation for the behaviour that was consistent with the person as you normally think of them.

Even this explanation raises obvious questions, such as why a second radio beam is triggered and how that boosts the pulsar's X-ray emissions. That's a problem, given that pulsars are used to search for the effects of gravitational waves - these ripples in spacetime should change pulsars' apparent frequency - and have even been touted as replacing satellites in a kind of GPS for deep space. "Whatever is creating this beam in one mode stops in the other mode, and for that you need something drastic," says Hermsen. "None of the known models can explain it."

Journal reference: Science, DOI: 10.1126/science.1230960

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