An international team of astronomers, including Dr. Jason Hessels (ASTRON/ UvA), have used a host of radio and X-ray telescopes to catch a pulsar undergoing a radical switch in energy source. This work appears in the Sept. 26th issue of the journal Nature. The findings confirm astronomers' theory about how such stars evolve, but the observations have also left them puzzled about how this switch can be so quick.

Published by the editorial team, 25 September 2013

The cosmic drama is being played out 18,000 light-years away, in a star cluster called M28 in the constellation of Sagittarius. The pulsar (called PSR J1824-2452I) is a rapidly rotating, magnetized neutron star that is sometimes observable through its light-house-like radio pulses. This pulsar has a tiny companion star, with about a fifth the mass of the Sun. Although small, the companion is pounding the pulsar with streams of matter.

Normally the pulsar shields itself from this onslaught, its magnetized winds deflecting the matter stream into space. But sometimes the stream swells to a flood, overwhelming the pulsar's protective ‘force field'. Falling onto the pulsar's surface, the matter stream releases its kinetic energy as a blast of X-rays. Eventually the torrent slackens. Once again the pulsar re-asserts itself and deflects the companion's infalling matter.

‘We have been fortunate enough to see all stages of this process, with a range of radio and X-ray telescopes,' said Dr. Alessandro Papitto, the Nature paper's lead author and astronomer at the Catalan Institute of Space Science (IEEC) of the Spanish National Research Council, Consejo Superior de Investigaciones Científicas or CSIC. ‘We have been looking for such evidence for decades.'

The pulsar and its companion form what is called a ‘low-mass X-ray binary' system. In such a system, the matter falling from the companion blazes in X-rays and spins the pulsar up, until it becomes a ‘millisecond pulsar' that spins hundreds of times a second and emits radio waves. The process takes about a billion years, astronomers think.

In its current state the pulsar is exhibiting behaviour typical of both kinds of systems: millisecond X-ray pulses when the companion is flooding the pulsar with matter, and radio pulses when it is not.

'We were amazed at how quickly this pulsar went from only being detectable with our X-ray telescopes to only being visible with our radio telescopes,' said Dr. Jason Hessels, who played a key role in the radio observations of the system using the Westerbork Synthesis Radio Telescope in the Netherlands and the Green Bank Telescope in the United States. `It's incredible to observe the pulsar switching its appearance so radically within just a matter of weeks,' he said.

This animation represents the evolutionary process of a pulsar as it swings between X-ray and radio emission. For an elaborate description, please see below. Credits: ESA

The pulsar was initially detected as an X-ray source with the INTEGRAL satellite. X-ray pulsations were then seen with another satellite, ESA's XMM-Newton. Further X-ray observations were made with NASA's Swift. NASA's Chandra X-ray telescope pinpointed the pulsar's position.

Radio pulsations from the pulsar were detected with ASTRON's Westerbork Synthesis Radio Telescope, NRAO's Robert C. Byrd Green Bank Telescope, and with CSIRO's Parkes radio telescope. Importantly, these showed that the pulsar had ‘revived' as a normal radio pulsar, not more than a couple of weeks after the last detection of the X-ray pulsar.

All in all, a global campaign using the world's premier radio and X-ray telescopes was needed to complete the picture of this pulsar's radical transformation. Likewise, the astronomers involved in these investigations work at institutions spanning the globe: Australia, Canada, Germany Italy, the Netherlands, Spain, Switzerland, and the USA.

For further information, please contact:

• Alessandro Papitto
Institut de Ciències de l'Espai (ICE), CSIC-IEEC (Spanish National Research Council - Institute for Space Studies of Catalonia)
Barcelona, Spain
Tel: +34 935 868355
Email: papitto@ice.csic.es
• Jason Hessels
ASTRON Netherlands Institute for Radio Astronomy
Dwingeloo, the Netherlands
Tel: +31 (0)610260062
Email: hessels@astron.nl

Information about the Nature publication: Swings between rotation and accretion power in a millisecond binary pulsar by A. Papitto et al. is published in Nature, 26 September 2013. Click here for the paper.

Click here for the ESA press release

Credits for the image above: ESA. Click here for a high res image.

Movie caption (Credit: ESA):
The animation represents the evolutionary process of a pulsar as it swings between X-ray and radio emission. The pulsar (left) is in a binary system with a low-mass star as a companion (right). The two objects orbit around their mutual centre of gravity; for clarity, this motion is not shown in the animation.

At the beginning of the animation, the pulsar spins very fast emitting two narrow beams of radio waves (shown in purple). Over several million years this rotation gradually slows down. Eventually, the gravitational pull of the pulsar 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.

During the accretion process, the high density of accreted matter damps out the radio emission and it is seen only in X-rays (shown as wide, white beams). When the accretion rate decreases, the pulsar's magnetosphere expands and pushes matter away. As a consequence, the bright X-ray emission switches off, while the radio emission again becomes detectable. The pulsar swings back and forth between the two states many times over several hundreds of millions of years until it finally slows down to become a purely radio-emitting pulsar, while its companion star has evolved into a white dwarf.

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