Thursday, September 26, 2013

An Ordinary Pulsar Evolving Into a Millisecond Pulsar

This animation shows an artist's impression of the evolutionary process that is believed to turn pulsars into millisecond pulsars.

The emission mechanism of pulsars transforms kinetic rotational energy into radiation: as this energy is radiated over time, the rotation is slowed down. Whilst pulsars spin rapidly at birth, they tend to rotate more slowly – with periods of up to a few seconds – as they age.

The mysterious millisecond pulsars – old but extremely quickly rotating pulsars with periods of a few thousandths of a second – are explained through a theoretical model known as the 'recycling' scenario. If a pulsar is part of a binary system and is accreting matter from a stellar companion via an accretion disc, then it may also gain angular momentum. This process can 'rejuvenate' old pulsars, boosting their rotation and making their periods as short as a few milliseconds.

Using data from INTEGRAL and XMM-Newton, astronomers have discovered IGR J18245-2452, a millisecond pulsar that within only a few weeks switched from being accretion-powered and X-ray bright to rotation-powered and bright in radio waves. As the evolutionary link between these two categories of sources, this millisecond pulsar brings conclusive evidence to the 'recycling' scenario.

The animation shows a pulsar in a binary system, with a low-mass, red star as a companion. The two objects orbit around their mutual center of gravity; for clarity, this motion is not shown in the animation.

At the beginning of the animation, the pulsar spins very fast, then its rotation gradually slows down. At this stage, the pulsar's emission is entirely supported by its rotation and results in two narrow beams of radio waves (shown in purple). The slowing down process may last several millions of years.

Eventually, the gravitational pull of the pulsar – which is a very dense object – starts drawing matter from the companion star. As the pulsar accretes matter via an accretion disc, it gains angular momentum and its rotation becomes extremely rapid again.

During the accretion process, the high density of accreted matter inhibits the acceleration of particles that cause radio emission, so the pulsar is not visible in radio waves but only in X-rays (shown as wide, white beams). When the accretion rate decreases, the magnetosphere expands and pushes matter away from the pulsar: as a consequence, the X-ray emission becomes weaker and weaker, while the radio emission intensifies.

Over a period of at least several hundreds of millions of years, the pulsar keeps swinging back and forth between the two states several times, emitting alternately X-rays and radio waves. When the accretion process stops, the pulsar becomes a purely rotation-powered, radio-emitting millisecond pulsar, while its companion star has evolved into a white dwarf.

Video credit: ESA; text credit: ESA

Note: For more information, see Volatile Pulsar Reveals Millisecond Missing Link.

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