RXTE Discoveries

Astronomers Find the Youngest Pulsar Yet in the Heart of an Exploded Star - August 2000

by Christopher Wanjek

New York, N.Y. -- A team led by astronomers at Columbia University have found the youngest pulsar yet, a hot, spinning, highly-magnetized infant no larger than Manhattan, born in a massive star explosion about 700 years ago. The pulsar's unusual properties may question how pulsars are created.

Other known pulsars, in comparison, are thousands to millions of years old. This pulsar, in the supernova remnant Kes 75, is about 300 years younger than the second-youngest pulsar, the Crab, which scientists have long considered to be the archetypal young pulsar. Yet, compared to the Crab, the Kes 75 pulsar spins ten times more slowly; it is slowing down at a rate ten times faster; and it has a magnetic field that is ten times greater -- all unexpected findings that may cause scientists to rethink the birth properties and evolution of pulsars.

"People have been searching for this pulsar for years," said Dr. Eric Gotthelf, Associate Research Scientist at Columbia's Astrophysics Laboratory and lead author of a paper submitted to The Astrophysical Letters. "We have seen bright radio waves in the core of Kes 75, telltale evidence for a pulsar. The problem was that we were expecting to find a rapidly spinning 'Crab-like' pulsar. What we found is quite different from our expectations."

rotating neutron stars(pulsars)
Artists conception of the regular flashes of light produced by a rapidly rotating neutron star(pulsar).

Dr. Gotthelf located the pulsar, 60,000 light years from Earth on the far side of the Milky Way, with NASA's Rossi X-ray Timing Explorer, a powerful observatory that has been finding pulsars, neutron stars and black holes for the past four years. His colleagues are Dr. Gautam Vasisht of NASA's Jet Propulsion Laboratory, Pasadena, Calif.; Michael Boylan-Kolchin, a student at Columbia; and Dr. Kenichi Torii of the Tsukuba Space Center in Japan.

A pulsar is a type of neutron star, the core remains of a giant star that was once at least ten times more massive than the sun. Such a star, upon depleting all of its nuclear fuel, explodes, ejecting most of its outer shell and leaving only a core, which collapses upon itself to form a hot ember about 10 miles in diameter. The ejected material forms a beautiful supernova remnant of colorful gas, visible for several millennia in visible light and at X-ray and radio wavelengths.

Pulsars have strong magnetic fields that channel electrons along the magnetic field lines away from the pulsar's polar regions. These electrons, accelerated to nearly the speed of light, radiate at the poles, particularly as radio waves, X rays and gamma-rays. Astronomers see pulsars "pulse" on and off as the beams of radiation from the rotating pulsar sweep across the Earth, like a beam from a lighthouse.

Locating the pulsar associated with a given supernova remnant is not so straightforward. When the star explodes, the shock might kick the resulting pulsar away from the site of the explosion. Other times, the pulsar's beams of light might not sweep past the direction of the Earth, so it is never seen. Or, as is suspected in recent 1987a supernova, the stellar core might collapse into a black hole.

"This pulsar is located right in the center of the supernova shell," said Gotthelf. "We only see this for a few other pulsars. So, the Kes 75 pulsar provides rather strong observational evidence that the neutron star is born in the stellar explosion that gives rise to the supernova remnant shell."

Dr. Gotthelf found the newly born pulsar, now named PSR J1846-0258, through a deep observation with the Rossi Explorer of the area around Kes 75 lasting nearly three hours. He narrowed his search to a central region within the Kes 75 supernova remnant using archived X-ray data from another satellite, the Japan-US Advanced Satellite for Cosmology and Astrophysics.

X-ray data reveals that the Kes 75 pulsar spins upon its axis once every 0.3 second, which is slow for young pulsars albeit fast compared to older ones. The pulsar's age is 700 years, consistent with the age of the supernova remnant that houses it.

The Kes 75 pulsar is also at least 10 times more magnetic than ordinary pulsars and about 10 times weaker than magnetars, a mysterious, newly identified class of highly magnetized objects. The Kes 75 pulsar may therefore be a missing link between these classes of objects. The pulsar's magnetic field is 100 trillion times that of the Earth's and would distort compass readings a half million miles away. Such a strong magnetic force is actually rapidly slowing down the spin of the pulsar.

Younger, highly magnetized, rapidly spinning pulsars provide astronomers with a wealth of information about this strange class of objects, first discovered only 33 years ago. Young pulsars are prone to starquakes, which changes the spin rate and reveals clues to their internal structure. Monitoring their change in spin rate allows astronomers to measure the rotational energy loss and compare it to the observed radiation. This provides a powerful probe of this central engine powering bright radio cores of supernova remnants.

The Rossi X-ray Timing Explorer, launched in 1995, is operated from Goddard Space Flight Center in Greenbelt, Md. The Rossi Explorer's large photon collecting area and ability to measure millisecond fluctuations in X-ray signals makes it a unique instrument to detect and study rapidly spinning objects.

Images and background information are available at http://www.gsfc.nasa.gov/gsfc/spacesci/structure/youngpulsar.htm.


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