Speedy spinner: An artist's impression depicting a binary pulsar system. Pulses of radio waves are seen emanating from the poles of the pulsar.

Credit: CSIRO

Related articles

• The stars say Einstein was 99.95 per cent right
• At last, a cannibal star discovered
• Milky Way is much bigger than we thought
• Mysterious radio signal from deep space
• Huge star partners huge black hole

SYDNEY: An outsized pulsar that spins 465 times a second and seems to be breaking all the rules could overturn our theories of how these rotating stars form.

The object is a rapidly rotating millisecond pulsar (MSP) and is part of a binary star system. Nothing unusual there, but its orbit is highly irregular and the star it circles is not a typical red giant, but instead a smaller Sun-like star.

"It's a weird object on many levels," said astrophysicist and pulsar hunter David Champion, based at the CSIRO Australia Telescope National Facility (ATNF) in Epping, New South Wales. Champion is a co-author of a paper on the find published today in the U.S. journal Science Express.

Hyperactive state

Pulsars, short for 'pulsating stars', are rotating neutron stars, the remnants of massive stars that have collapsed into extremely dense objects. Rather like the beam from a lighthouse, they emit pulses of radio waves as they rotate that can be detected from Earth. Most produce regular pulses in time periods ranging from one millisecond (ms) to eight seconds in duration. Those that spin very fast – with pulse periods shorter than 10 ms – are called millisecond pulsars or MSPs.

This MSP, bearing the name PSR J1903+0327, was found during a survey of our own galaxy, using the Arecibo Observatory in Puerto Rico, the world's largest radio telescope.

Conventional theory suggests that MSPs begin life as slower-spinning pulsars, but are later "recycled" into their hyperactive state. But it was thought this could only happen if the pulsar has a red giant companion star – something not seen in this example.

Normally, as the red giant companion swells, the super-dense pulsar starts to suck material away from it – a process called accretion. As this extra material is pulled in it speeds up the pulsar up until the star completes an entire revolution in less than 10 ms. "We talk about it being like when an ice-skater pulls in their arms to go faster," said Champion.

Highly eccentric

This process is also what makes a typical pulsar's orbit around its companion completely circular, and therefore "have low eccentricity".

"All MSPs found up until now have a low eccentricity, but now we have this unusual system which sticks out like a sore thumb," said Champion. The eccentricity of most MSPs is around 0.001. This new one comes in at 0.44, giving it an elliptical orbit.

But perhaps this binary system is not a total enigma. While no similar systems have been spotted in our own galaxy, similar binary pairs have been detected in globular star clusters, said astrophysicist Edward van den Heuvel, who penned an accompanying commentary in Science Express.

Globular clusters contain extremely dense population of stars, much denser than the arrangement of stars in our galaxy. This means there is more chance of stars colliding, which could potentially lead to strange phenomena.

"To explain the eccentric orbit [of an MSP in our own galaxy] … something must have disturbed the initially circular orbit of the pulsar system," said van den Heuvel, who is based at the University of Amsterdam in the Netherlands. This is likely to have been an interaction with another star, he said.

Champion's team has flagged up two likely scenarios to explain the strange behaviour.

Rewriting the rules

The first is that the system originally formed in a globular star cluster, but was spat out into space; eventually settling in our galaxy. Under this scenario, the binary system was originally the more typical pairing of an MSP and a red giant. Then, a Sun-like star collided with the pair, eventually usurping the red giant as the pulsar's companion. The energy of this celestial smash-up could have blasted the binary system out of the globular cluster, until it eventually reached our galaxy.

The other scenario is that the pair is not a binary system at all, but the remnants of a stellar threesome. In this case, the ménage à trois would centre around a normal binary system where a MSP orbits a white dwarf star – but the pair would then be circled by a more distant Sun-like star; the remaining companion observed by the Arecibo telescope.

But neither model wholly accounts for the data, at least not according to currently held theories.

So could this mean rewriting the rules to account for this mysterious object? It's possible, said Champion. "Anything that explains MSPs will have to also be able to explain this system … The 'recycling' theory is not necessarily wrong, but we need to find a new way to explain it."

"This gives a new angle on what's possible in these systems," commented pulsar expert Dick Manchester, also based at the CSIRO Australia Telescope National Facility, but not one of the study authors. "[It] will stretch the ingenuity of the people who try to explain them," he said.