An international team of astronomers has spotted a binary pulsar that appears to behave very differently than most known binary systems in our galaxy. The most likely explanation, says the team, is that the formation of the pulsar involved three stars rather than two — the first such system to be spotted.

Pulsars are rapidly spinning neutron stars that earn their name from the beams of radiation they emit, which appear as pulses to an observer on Earth. Many pulsars orbit a companion star and such objects are called binary pulsars.

David Champion of the Australia Telescope National Facility and colleagues used the Arecibo radio telescope in Puerto Rico to study pulses coming from PSR J1903+0327, a binary pulsar located in the disk of our galaxy and discovered by the team in 2005 (Science DOI: 10.1126/science.1157580) .

Highly eccentric

The pulses arrive at Earth about once every 2.15 ms and by watching the pulsar very carefully over 1.5 years the team determined that the binary orbit is highly eccentric. This came as a surprise, because other binary pulsars with periods less than about 10 ms — called “millisecond pulsars” — have nearly perfect circular orbits. And to make matters even more confusing, the companion star in all known millisecond pulsars is a white dwarf, while PSR J1903+0327 appears to involve a star much like our Sun.

Astronomers believe that a normal millisecond pulsar is formed from a binary system comprising a large star (greater than about eight solar masses) and a star about the same size as the Sun. The large star explodes in a supernova, leaving a spinning neutron star with a period greater than 10 ms and knocking the pair into a highly eccentric orbit.

As time progresses the companion star expands to become a red giant and the neutron star begins to suck in (or accrete) material from its bloated companion. This causes the neutron star to rotate more rapidly — eventually reducing its period below 10 ms — and makes the binary orbit more and more circular. Eventually, the companion becomes a white dwarf, the accretion stops, and the pair settle down into a millisecond binary pulsar with a nearly perfect circular orbit.

Three possible explanations

However, PSR J1903+0327 does not fit this mould and the team have put forth three possible explanations. The first and least satisfactory explanation is that object is simply a very young — yet fast spinning — binary pulsar. However, the age of a pulsar can be determined by the rate at which the its period is increasing, and such measurements suggest that this binary is much too old for this explanation.

The second possibility is that the pulsar was formed in a conventional manner in a globular cluster — a region with a relatively high density of stars — where the gravitational pull of other nearby objects caused the spinning neutron star to be ejected from the cluster, taking a Sun-like companion with it. However, there is no evidence of a nearby globular cluster and Champion believes that there is at most a 10% chance that this is what happened.

The most likely explanation, according to Champion, is that the pulsar was born within a trio of stars. Two of these stars formed a millisecond pulsar, while the gravitational field of a third — and more distant Sun-like star — conspired to make the orbit highly eccentric. A variation on this explanation is that the white-dwarf companion has since been completely destroyed by the neutron star, leaving the neutron star in a highly eccentric orbit around the Sun-like star.

The team now plan to make further observations to get a better understanding of the system. One crucial measurement that must be made, according to Champion, is whether the spinning neutron star is orbiting the Sun-like star, or another object such as a white dwarf. This requires the use of a large optical telescope such as Gemini or the VLT.