University Park, Pa. -- The oldest isolated pulsar ever detected in X-rays has been found with NASA's Chandra X-ray Observatory by a team of Penn State astronomers led by George Pavlov, professor of astronomy and astrophysics. This very old and exotic object turns out to be surprisingly active. A high-resolution composite image related to this story is on the Web at http://www.science.psu.edu/alert/Pavlov2-2009.htm.
The pulsar, PSR J0108-1431 (J0108) is about 200 million years old. Among isolated pulsars -- those whose spin has not been accelerated within a binary system -- and among pulsars detected with X-rays, it is more than 10 times older than the previous record-holder. At a distance of 770 light years, it is one of the nearest pulsars known.
Other members of the Penn State team include Gordon Garmire, Evan Pugh professor of astronomy and astrophysics, who led the conception and development of the ACIS X-ray camera that now is in Earth orbit on NASA's Chandra X-ray observatory; and undergraduate student Jared Wong.
A pulsar is born when a star that is much more massive than the Sun collapses in a supernova explosion, leaving behind a small, incredibly weighty core, known as a neutron star. At birth, these neutron stars, which contain the densest material known in the universe, are spinning rapidly, up to a hundred revolutions per second. Astronomers call them "pulsars" because the rotating beams of their radiation are seen as pulses by distant observers, similar to a lighthouse beam.
Astronomers observe a gradual slowing of the rotation of the pulsars 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 1 revolution per second.
The surprise came when a team of astronomers led by Pavlov observed J0108 in X-rays with the orbiting Chandra X-ray observatory. They found that this pulsar glows much brighter in X-rays than was expected for a pulsar of such advanced years. "This pulsar is pumping out high-energy radiation much more efficiently than its younger cousins," said Pavlov.
Some of the energy that J0108 is losing as it spins more slowly is converted into X-ray radiation. The efficiency of this process for J0108 is found to be higher than for any other known pulsar. "Although this pulsar is clearly fading as it ages, it is still more than holding its own with the younger generations," Pavlov said.
It is likely that two forms of X-ray emission are produced in J0108: 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's surface and the process by which charged particles are accelerated by the pulsar.
The younger, bright pulsars commonly detected by radio and X-ray telescopes are not representative of the full population of such objects, so observing pulsars like J0108 helps astronomers to see a more complete range of pulsar behavior. At its advanced age, J0108 is close to the so-called "pulsar death line," at which its pulsed radiation is expected to switch off and it will become much harder, if not impossible, to observe.
"We can now explore the properties of this pulsar in a regime where no other pulsar has been detected outside the radio range," said co-author 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."
The Chandra observations were reported by Pavlov, Garmire and Wong in the Jan. 20 issue of the Astrophysical Journal, but the extreme nature of J0108 was not fully apparent until a new distance to it was reported on Feb. 6 in the doctoral thesis of Adam Deller at Swinburne University in Australia. The new distance is both larger and more accurate than the distance used in the paper published in the Astrophysical Journal, showing that J0108 was brighter in X-rays than previously thought. "Suddenly this pulsar became the record-holder for its ability to make X-rays", said Pavlov, "and our result became even more interesting without us doing much extra work."
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. This change implies that the pulsar is moving at a velocity of about 440,000 miles per hour, close to a typical value for pulsars.
The pulsar now is moving south from the plane of the Milky Way galaxy, but because it is moving more slowly than the escape velocity of the galaxy, it eventually will curve back toward the plane of the galaxy. The detection of this motion has allowed Roberto Mignani, of University College London, in collaboration with Pavlov and Kargaltsev, to discover -- in an image taken in 2000 -- the optical light of an object at the position where J0108 should have been located at that time. The team is planning to make new observations in an attempt to locate optical light at the location where J0108 is predicted now to have moved. Such multi-wavelength studies of old pulsars -- involving electromagnetic radiation at X-ray, optical, and other wavelengths -- 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.
NASA's Marshall Space Flight Center, Huntsville, Alabama, manages the Chandra program for NASA's Science Mission Directorate in Washington, D.C. Chandra's ACIS X-ray camera was conceived and developed for NASA by Penn State and the Massachusetts Institute of Technology under the leadership of Garmire. Northrop Grumman, of Redondo Beach, Calif., was the prime development contractor for the observatory. The Smithsonian Astrophysical Observatory controls Chandra's science and flight operations from the Chandra X-ray Center in Cambridge, Mass.