Millisecond pulsar mystery solved

BY KEITH COOPER

ASTRONOMY NOW

Posted: 22 May, 2009

Astronomers have watched a pulsar be spun up in real time by its companion star, turning it into an incredibly fast millisecond pulsar rotating at a breakneck 592 times per second. This is the first time we have ever seen the process by which millisecond pulsars are created, confirming our suspicions that a river of matter from the companion star onto the pulsar is to blame.

An artist's impression of a pulsar being spun up by an accretion disc.

Image: NASA/Dana Berry.

Pulsars are spinning neutron stars, the remnants of massive stars that have exploded as supernovae. They’re born spinning a few tens of times a second, firing out beams of radio and X-rays that flash, or pulse, in our direction every rotation. As time goes by they slow down, but the existence of older pulsars that are spinning faster than any others has always been a puzzle. The new observations made over the course of a decade have put an end to the mystery.

The pulsar was discovered in 2007 by the Green Bank Telescope in West Virginia, USA, but upon closer inspection it was realised that the same star system had been imaged on several occasions previously, first in 1998 by the Very Large Array and then as what appeared to be a Sun-like star by the Sloan Digital Sky Survey in 1999. A year later, it was seen sporting an accretion disc – a spiralling disc of gas torn from the body of a star, but when astronomers went back two years later, the disc had vanished. Now it appears to be a millisecond pulsar. What was going on?

The theory was that millisecond pulsars are spun up by gas wrapping around it from a companion star, like a spinning top. During the process of accreting the gas radio waves cannot be seen coming from the pulsar, but once the gas disappears the radio waves from the beams emerge. The fastest millisecond pulsar ever seen spins 1,112 times per second.

One of the intermediary steps before becoming a millisecond pulsar is that of a low-mass X-ray binary, which are systems usual involving a neutron star and some other small star. The companion of J1023 – the pulsar in question – is only half the mass of the Sun.

“Low mass X-ray binaries… don’t emit radio waves,” says Anne Archibald, of the McGill University in Montreal, Canada. “We’ve thought that low mass X-ray binaries probably are in the process of getting spun up, and will later emit radio waves as a pulsar.”

“It appears this thing has flipped from looking like a low mass X-ray binary to looking like a pulsar,” adds Scott Ransom of the National Radio Astronomy Observatory in the United States.

This is the first time an accretion disc has ever been seen involved with a millisecond pulsar, and the sheer rapidity of the process explains why we’ve always missed it before. It is now hoped that J1023 will become a kind of Rosetta Stone for millisecond pulsars. The research is published in the 21 May edition of the journal Science.