1.24.2010

Nearest Neutron Star: PSR J0108-1431


PSR J0108-1431, the closest known pulsar to the Earth. It lies in the direction of the constellation Cetus, at a distance of about 85 parsecs (280 light years). Nevertheless, it was not discovered until 1993 due to its extremely low luminosity. It was discovered by the Danish astronomer Thomas Tauris in collaboration with a team of Australian and European astronomers using the Parkes 64-meter radio telescope. The pulsar is 1000 times weaker than an average radio pulsar and thus this pulsar may represent the tip of an iceberg of a population of more than half a million such dim pulsars crowding our Milky Way.

The composite image on the left 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 Chandra source in the center of the image is the ancient pulsar PSR J0108-1431 (J0108 for short), located only 770 light years from us. The elongated object immediately to its upper right is a background galaxy that is unrelated to the pulsar. Since J0108 is located a long way from the plane of our galaxy, many distant galaxies are visible in the larger-scale optical image.

The position of the pulsar seen by Chandra in this image from early 2007 is slightly different from the radio position observed in early 2001, implying that the pulsar is moving at a velocity of about 440,000 miles per hour, in the direction shown by the white arrow. The detection of this motion allowed an estimate of where J0108 should be located in the VLT image taken in 2000. The faint blue star just above the galaxy is a possible optical detection of the pulsar.

The artist's impression on the right shows 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. Both of these effects are expected to generate X-ray emission. Most of the surface of the neutron star is expected to be too cool to produce X-rays, but it should produce optical and ultraviolet radiation. Thus, multiwavelength observations are important for providing a complete picture of these exotic objects.

At an age of about 200 million years, this pulsar is the oldest isolated pulsar ever detected in X-rays. Among isolated pulsars - ones that have not been spun-up in a binary system - it is over 10 times older than the previous record holder with an X-ray detection. This pulsar is slowing down as it ages and converting some of the energy that is being lost into X-rays. The efficiency of this process for J0108 is found to be higher than for any other known pulsar.


Fast Facts for PSR J0108-1431:
Credit 
X-ray: NASA/CXC/Penn State/G.Pavlov et al.; Optical: ESO/VLT/UCL/R.Mignani et al.; Illustration: NASA/CXC/M.Weiss.
Scale 
Left panel is 1.3 arcmin across.
Category 
Neutron Stars/X-ray Binaries
Coordinates (J2000) 
RA 01h 08m 08.30s | Dec -14° 31' 48.50"
Constellation 
Cetus
Observation Date 
February 5, 2007
Observation Time
8 hours
Obs. ID 
7576
Color Code 
X-ray (Purple); Optical (Red, Blue, White)
Instrument 
ACIS
References 
Pavlov, G., et al., 2009, Astrophysical Journal, 691, 458
Distance Estimate 
About 770 light years
Release Date 
February 26, 2009



Chandra X-ray Image of PSR J0108-1431


The oldest isolated pulsar ever detected in X-rays has been found with NASA's Chandra X-ray Observatory.This very old and exotic object turns out to be surprisingly active.




The pulsar, PSR J0108-1431 (J0108 for short) is about 200 million years old. Among isolated pulsars -- ones that have not been spun-up in a binary system -- it is over 10 times older than the previous record holder with an X-ray detection. At a distance of 770 light years, it is one of the nearest pulsars known.

Pulsars are born when stars that are much more massive than the Sun collapse in supernova explosions, 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. As the rotating beams of their radiation are seen as pulses by distant observers, similar to a lighthouse beam, astronomers call them "pulsars".

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 one revolution per second.

The surprise came when a team of astronomers led by George Pavlov of Penn State University observed J0108 in X-rays with Chandra. They found that it glows much brighter in X-rays than was expected for a pulsar of such advanced years.

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.

"This pulsar is pumping out high-energy radiation much more efficiently than its younger cousins," said Pavlov. "So, although it's clearly fading as it ages, it is still more than holding its own with the younger generations."

It's 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 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 objects, so observing objects like J0108 helps astronomers see a more complete range of behavior. At its advanced age, J0108 is close to the so-called "pulsar death line," where 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 and colleagues in the January 20, 2009, issue of The Astrophysical Journal. However, the extreme nature of J0108 was not fully apparent until a new distance to it was reported on February 6 in the PhD thesis of Adam Deller from Swinburne University in Australia. The new distance is both larger and more accurate than the distance used in the Chandra paper, 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 implies that the pulsar is moving at a velocity of about 440,000 miles per hour, close to a typical value for pulsars.

Currently the pulsar 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 will eventually curve back towards the plane of the Galaxy in the opposite direction.
The detection of this motion has allowed Roberto Mignani of University College London, in collaboration with Pavlov and Kargaltsev, to possibly detect J0108 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.


The team of astronomers that worked with Pavlov also included Gordon Garmire and Jared Wong at Penn State. NASA's Marshall Space Flight Center in Huntsville, Ala., manages the Chandra program for NASA's Science Mission Directorate in Washington. The Smithsonian Astrophysical Observatory controls Chandra's science and flight operations from Cambridge, Mass.



Media contacts:
Kimberly Newton
Marshall Space Flight Center, Huntsville, Ala.
256-544-0371
kimberly.d.newton@nasa.gov

Megan Watzke
Chandra X-ray Center, Cambridge, Mass.
617-496-7998
cxcpress@cfa.harvard.edu




Discovery of PSR J0108-1431, the closest known neutron star

T. M. Tauris (Institute of Physics and Astronomy, Aarhus University, Denmark),
L. Nicastro (IRA - CNR, Bologna, Italy),
S. Johnston (RCfTA, University of Sydney, Australia),
M. Bailes (ATNF, CSIRO, Australia),
R. N. Manchester (ATNF, CSIRO, Australia),
A. G. Lyne (NRAL, Jodrell Bank, University of Manchester, UK),
N. D'Amico (University of Palermo and IRA - CNR, Bologna, Italy),
J. Glowacki (Parkes Observatory, ATNF, CSIRO, Australia),
J. F. Bell (MSSSO, Australian National University, Australia),
D. R. Lorimer (NRAL, Jodrell Bank, University of Manchester, UK),
P. A. Harrison (NRAL, Jodrell Bank, University of Manchester, UK)

(1994) ApJ, 428, L53-55

Abstract

There are about 600 known radio pulsars in our Galaxy, typically at distances of a few kpc, as indicated by the amount of dispersion of the pulses in the ionized component of the interstellar medium. Here we report the discovery of PSR J0108-1431, a pulsar which has the lowest known dispersion measure, 1.83 pc cm^-3. Reasonable models of the interstellar electron density distribution indicate that its distance is less than 100 pc, making it the closest known radio pulsar and probably the closest known neutron star. Furthermore this pulsar has the lowest radio luminosity of any known pulsar by more than an order of magnitude. Such a weak pulsar in the solar neighbourhood implies a large population of active but low-luminosity pulsars in the galactic halo. X-ray observations of PSR J0108-1431 may determine whether or not there is significant decay of the surface magnetic field in isolated neutron stars and distinguish between different cooling and heating models.

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