Geriatric pulsar still kicking

BY DR EMILY BALDWIN

ASTRONOMY NOW

Posted: 03 March, 2009

NASA’s Chandra X-ray Observatory has spotted the oldest, most isolated pulsar ever detected in X-rays.

The pulsar, assigned the identity PSR J0108-1431 (or J0108 for short) is still surprisingly active, despite its 200 million year existence, which places it at ten times older than the previous record holder detected in X-rays. J0108 is also rather isolated, that is, it has not been spun-up in a binary system. And at a distance of 770 light years, it is one of the nearest pulsars known.

Pulsars are created when stars much more massive than the Sun collapse in supernova explosions, leaving behind a small dense core, or neutron star. These spin at the dizzying rate of up to a hundred revolutions per second. Their rotating beams of radiation are seen as pulses by distant observers, similar to a lighthouse beam.

An artist's impression of what J0108 might look like if viewed up close. Radiation from particles spiraling around magnetic fields is shown along with heated areas around the neutron star's magnetic poles. Image: NASA/CXC/M.Weiss.

Pulsars gradually slow down as they radiate energy away. Radio observations of J0108 show it to be one of the oldest and faintest pulsars known, spinning only slightly faster than one revolution per second. But a team of astronomers, led by George Pavlov of Penn State University, discovered that the pulsar glows much brighter in X-rays than was expected for such an ancient object, suggesting that the efficiency of the energy conversion is much higher than for any other known pulsar.

“This pulsar is pumping out high-energy radiation much more
efficiently than its younger cousins,” says Pavlov. “So, although it’s clearly fading as it ages, it is still more than holding its own with the younger generations.” But the pulsar is close to the so-called “pulsar death line,” where its pulsed radiation is expected to switch off, rendering it difficult or impossible to observe.

But in the meantime, studying J0108 enables astronomers to probe the extremes of the varied pulsar population and behaviour. Indeed, the astronomers think that two forms of X-ray emission are produced in this enigmatic pulsar: emission from particles spiraling around magnetic fields, and emission from heated areas around the neutron star’s magnetic poles. Measuring the temperature and size of these heated regions can provide valuable insight into the extraordinary properties of the neutron star surface and the process by which charged particles are accelerated by the pulsar.

This composite image shows an image from NASA's Chandra X-ray Observatory in purple and an optical image from the European Southern Observatory's Very Large Telescope (VLT) in red, blue and white. The pulsar is moving at a speed of 440,000 miles per hour in the direction indicated by the white arrow. Image: X-ray: NASA/CXC/Penn State/G.Pavlov et al.; Optical: ESO/VLT/UCL/R.Mignani et al.

“We can now explore the properties of this pulsar in a regime where no other pulsar has been detected outside the radio range,” says Oleg Kargaltsev of the University of Florida. “To understand the properties of ‘dying pulsars,’ it is important to study their radiation in X-rays. Our finding that a very old pulsar can be such an efficient X-ray emitter gives us hope to discover new nearby pulsars of this class via their X-ray emission.”

Initial Chandra observations were reported by Pavlov and colleagues in the 20 January 2009 issue of The Astrophysical Journal, but just weeks later, the extreme nature of the pulsar was revealed by Adam Deller from Swinburne University in Australia, whose PhD research revealed that J0108 was brighter in X-rays than previously thought. “Suddenly this pulsar became the record holder for its ability to make X-rays,” says Pavlov, “and our result became even more interesting without us doing much extra work.”

Furthermore, the position of the pulsar seen by Chandra in X-rays in early 2007 is slightly different from the radio position observed in early 2001, implying that it is moving at a velocity of about 440,000 miles per hour, close to a typical value for pulsars. Together with colleagues at University College London, the team have used the motion observations to attempt to detect the pulsar in optical light, using estimates of where it should be found in an image taken in 2000. Such a multi-wavelength study of old pulsars is critical for understanding the long-term evolution of neutron stars, such as how they cool with time, and how their powerful magnetic fields evolve.