Astronomers have discovered a rapidly spinning pulsar in an eccentric orbit around a Sun-like star, a never before seen combination, and one that raises questions as to how such a strange system could develop.
A pulsar is a super dense stellar corpse, a neutron star, left over after a massive star exploded as a supernova. Pulsars are characterised by their strong magnetic fields which throw out lighthouse-like beams of light and radio waves as they spin. Typical pulsars spin a few times a second, but PSR J1903+0327, which was discovered in 2006 using the Arecibo radio telescope, is a ‘millisecond pulsar’, rotating 465 times a second on a highly elongated orbit that takes it around its unlikely Sun-like companion star once every 95 days. The pulsar is also unusually massive for its type.
"Our ideas about how the fastest-spinning pulsars are produced do not predict either the kind of orbit or the type of companion star this one has," said David Champion of the Australia Telescope National Facility. "We have to come up with some new scenarios to explain this weird pair.”
Comparison of the sizes and orbits of pulsar J1903+0327 and its possible Sun-like companion with the orbit of the Earth around the Sun. The sizes of the Sun and the companion star have been exaggerated by a factor of 10, the Earth by a factor of 1000, and the pulsar by a factor of 100,000. Image: Bill Saxton, NRAO/AUI/NSF.
Astronomers think most millisecond pulsars are sped up by material falling onto them from a companion star. This requires the pulsar to be in a tight orbit around its companion that becomes more and more circular with time. The orbits of some millisecond pulsars are the most perfect circles in the Universe, so the elongated orbit of the new pulsar is a mystery.
"What we have found is a millisecond pulsar that is in the wrong kind of orbit around what appears to be the wrong kind of star," says Champion. "Now we have to figure out how this strange system was produced."
The scientists are now considering several possibilities of which they favour the idea that the pulsar may in fact be part of a triple star system. In this case, the pulsar's 95-day orbit is around a neutron star or white dwarf, and the Sun-like star is in a more-distant orbit around the pulsar and its close companion. The astronomers plan to study the star further in the infrared to confirm the indications that it is similar to our Sun and that it actually is a companion to the pulsar. Additional radio observations will study the pulsar's orbit and seek to measure its motion in space in an attempt to crack the code of this puzzling pulsar system.