What looks like a case of cosmic adultery may actually be an early stage of a long and happy marriage. The faithful groom is a rapidly spinning millisecond pulsar--the collapsed remnant of an exploded star. Astronomers have seen many millisecond pulsars in long-lasting partnerships, always with a white dwarf star. So a team was surprised to discover a millisecond pulsar in a tryst with a bloated red giant star. The finding, announced today by the Hubble European Space Agency Information Centre in Garching, Germany, may help confirm the popular theory of how millisecond pulsars got revved up in the first place.
Millisecond pulsars take their name from the extremely short time it takes them to complete a rotation. Soon after their discovery 20 years ago, millisecond pulsars were found paired with white dwarves. Theorists quickly proposed an explanation of how these old pulsars got spun up to hundreds of revolutions per second. The idea is that their companion star was originally much larger and spilled part of its outer layers on the pulsar. This whipped the pulsar to spin faster and faster. Eventually the companion gave up so much mass that it became a small, compact white dwarf star and stopped feeding the pulsar with matter from its outer layers.
It was a neat theory with no direct evidence. But that may have changed with observations of PSR J1740-5340, a millisecond pulsar in the globular cluster NGC 6397. A team of astronomers from Bologna Observatory in Italy, led by Francesco Ferraro, studied the pulsar with the Parkes radio telescope in Australia and the Hubble Space Telescope. To their surprise, it turned out to be married to a red giant star. Ferraro and his colleagues think they have caught the pair before the bloated star has been emptied of gas and turned into a white dwarf. If that's true, the pulsar must have been whipped up quite recently.
Don Backer of the University of California, Berkeley, who co-discovered the first millisecond pulsar in 1982, says the new system is "a great addition to the zoo." The spin-up idea has always been considered very consistent and convincing, he says, but catching a millisecond pulsar in the process of being born helps back up the theory. "It's certainly an important step." Backer hopes that future observations of the newly formed millisecond pulsar will provide more information on how the strange spinners are created.