Speedy cannibal pulsar caught in the act

One companion star, to go

European and US space agency astronomers have spotted a particularly fast-spinning pulsar that is in the process of gobbling up its companion star.

The sighting is further supporting evidence for the hypothesis that binary pulsars evolve into isolated, fast-spinning pulsars, and actually increase their spin rate by chowing down on a nearby star. This is the first system in which the pulsar has been caught in the act of speeding itself up.

"We're getting to the point where we can look at any fast-spinning, isolated pulsar and say, 'That guy used to have a companion'," said Dr Maurizio Falanga, who led the Integral observations, at the Commissariat à l'Energie Atomique (CEA) in Saclay, France.

The researchers describe it as 'an evolutionary step' between a stately life as a binary pulsar, spinning around once or twice a second and the fast lane of being an 'X-ray millisecond pulsar'. These rotate several hundred times a second, and this particular pulsar, with a period of 1.67 milliseconds, is one of the fastest known.

The pulsar was identified by the European Space Agency's (ESA) Integral space observatory, and NASA's Rossi X-ray Timing Explorer spacecraft. It was picked up by Integral during a routine scan of the outer edges of the Milky Way. The following day, NASA observers used Rossi to work out just how fast the object was spinning.

Rossi also revealed that most of the pulsar's companion has already been swallowed - the star could now be as small as 40 Jupiter masses. The two bodies orbit each other so tightly, once every two-and-a-half hours, that both could fit neatly inside the radius of our sun.

A pulsar is a rotating neutron star, a remnant of the explosive death throes of a star at least eight times as massive as our own sun. In general, a pulsar is about the same mass as our sun, but is a mere 20km or so across. When they are born, they tend to spin very rapidly, courtesy of conservation of angular momentum, but they slow down over the course of a hundred thousand years or so.

Isolated pulsars may be condemned to a sluggish future, but those living in binary systems can halt their decline by devouring a neighbour. As the gas is torn from the companion, the pulsar spin-rate increases. As the gas from the companion crashes into the neutron star, it releases huge amounts of energy in the form of X-ray and gamma radiation.

"This object was about ten times more energetic than what is usually observed for similar sources," said Falanga. He added that "only some kind of monster emits at these energies", which correspond to a temperature of almost a billion degrees. ®