Although astronomers predict that the gaseous shells left over from supernova explosions should hold rapidly-spinning radio pulsars, few such stars have actually been observed within these nebulae. Astronomers from Columbia University and the California Institute of Technology propose an explanation: The expected pulsars do exist, but they are slowly spinning neutron stars invisible to radio probes and have magnetic fields a quadrillion times denser that that of our Sun -- so-called "magnetars."
Radio pulsars, the first observed in the Crab nebula in the late 1960s, are believed to be neutron stars that spin at velocities of up to 600 revolutions a second, sending a beacon of radio waves whirling across the heavens. The new observation of slower X-ray pulsars confirms the existence of slower-spinning neutron stars, which are believed to have huge magnetic fields that would sweep matter along in their wake, accounting for their slower rotational velocities.
The new work overturns a 30-year-old hypothesis, that most supernova remnants should hold a rapidly-rotating radio pulsar, and offers new insights into the formation of neutron stars. It also confirms that a slowly-rotating pulsar in the Kes 73 supernova remnant, first announced by the National Aeronautics and Space Administration in October 1997, is likely a magnetar, which would make it the first one ever observed.
Astronomers have long held that young neutron stars, born in the cataclysmic explosion of supernovae, should be formed rapidly spinning. And like the most famous radio pulsar, the Crab, young pulsars are expected to be visible across the spectrum of electromagnetic radiation, from radio waves to the highest-energy gamma rays. But searches for radio emissions from pulsars near young remnants of supernovae have failed to confirm this prediction.
New observations by Eric Gotthelf of Columbia and Gautam Vasisht of Caltech provide evidence that most neutron stars are born and evolve in a manner very different than that of the Crab pulsar. Their results will be presented Jan. 7 at the American Astronomical Society's annual meeting in Austin, Texas. A web version of the poster they will exhibit is available at http://www.astro.columbia.edu/~evg/newpulsars.html. (An earlier report of this research was presented to the Italian Astronomical Society meeting last year in Elba, Italy. A reprint is available at http://xxx.lanl.gov/abs/astro-ph/9809139.)
Dr. Gotthelf and his colleagues have discovered several new neutron star candidates in supernova remnants that show these pulsars are far more numerous than previously believed. At the Austin meeting, Dr. Gotthelf presents a compilation of the latest results from these and other radio-quiet, slow X-ray pulsars and suggests that these objects are likely the "missing" pulsars in supernova remnants.
"The surprise is that the rapidly rotating pulsars are in fact the rare examples of young neutron stars, not the typical example, as was thought by most researchers until now," Dr. Gotthelf said.
Though confirming evidence hasn't yet been assembled for all the candidates, Dr. Gotthelf, Dr. Vasisht and others believe these objects can be distinguished from their radio-bright cousins by their enormous magnetic fields, about 1000 times stronger than expected. With such intense magnetic fields, these pulsars must have evolved very differently from young radio pulsars.
"If these pulsars are truly isolated pulsars in the hearts of supernova remnants, as the evidence suggests, then their inferred magnetic fields are enormous, unlike anything known on Earth or elsewhere in the heavens," Dr. Gotthelf said. "The strength would be on the order of 10 with the exponent 15, or a quadrillion, times stronger than our Sun's magnetic field."
The Crab nebula is the remains of a supernova explosion recorded by Chinese astronomers in 1054; the bright star out-shown the moon for several weeks. In 1968, David H. Staelin and E.C. Reifenstein of Massachusetts Institute of Technology, using radio telescopes, discovered that the Crab nebula contained a rapidly-rotating central star.
The Columbia and Caltech scientists have found that most young pulsars in supernova remnants are, in fact, not powerful radio beacons, as is the Crab pulsar, but are seen only by the light of the X-rays they give off, and in no other light. Furthermore, these so-called anomalous pulsars are found to be rotating 1000 times slower then the Crab pulsar, and slowing down 100 times faster.
"If these pulsars started out rotating as fast as a normal pulsar, it would require more time than the universe has existed for it to slow down to its present rate," Dr. Vasisht said.
The existence of highly magnetized neutron stars was postulated in 1992 by Robert Duncan of the University of Texas at Austin and Christopher Thompson of the University of North Carolina at Chapel Hill. They believed that the repetitious burst of gamma-rays from so-called soft gamma-ray repeaters, first observed in 1986, could be explained if such stars had huge magnetic fields, and so they were dubbed "magnetars." But the first compelling evidence for a magnetar was provided by the discovery in 1997 of a slow pulsar in the supernova remnant Kes 73 by Drs. Vasisht and Gotthelf. (See NASA press release 97-131 at ftp://pao.gsfc.nasa.gov/pub/pao/releases/1997/97-131.htm.)
This pulsar has the slowest known period of any isolated pulsar.
Since then, several new slow X-ray pulsars have been discovered in supernova remnants. And new observations of the soft gamma-ray repeaters, along with the discovery of a new one, show that they, too, are slow pulsars associated with supernova remnants. The researchers suggest that soft gamma-ray repeaters and other slow X-ray pulsars are related by their slow periods and comparable spectra.
"We have now discovered several new slow young pulsars at the center of supernova remnants," Dr. Gotthelf said. "Along with other recent discoveries, we are able to show that these slow pulsars now outnumber the observed census of Crab-like pulsars." According to the researchers, of the 300 or so visible supernova remnants, the latest tally finds seven with slow pulsars, while there are only four confirmed Crab-like pulsars.
Supernova explosions take place because a massive star, having exhausted its supply of hydrogen fuel, can no longer exert enough outward pressure to counter its own gravitational pull. It collapses upon itself in a thermonuclear explosion that releases more energy in an instant than in the previous 10 billion years. Some fraction of that energy blows away the star's outer envelope of gases, creating the hot shell visible from Earth as a nebula. Most of the remaining mass of the star, now in effect a giant atomic nucleus composed almost entirely of neutrons, occupies a diameter of no more than a dozen miles, about the size of Manhattan. A teaspoon of neutron star matter, if brought to Earth, would weigh more than a billion tons.
As young neutron stars condense, it is thought, they spin much more rapidly, just as an ice dancer spins faster as he brings his arms close to his body; the effect is called conservation of angular momentum. Radio pulsars rotate with such extreme velocity that they must have the extraordinary density of neutron stars, or they would fly apart. If neutron stars did actually form only in this way, there should be pulsars somewhere within most supernova remnants.
In Austin at the AAS meeting at 9 A.M. CST on Jan. 7, Victoria M. Kaspi, assistant professor of physics at MIT, presents evidence that most supernova remnants do not contain rapidly spinning radio pulsars, as had been assumed, but that the few that are seen can be explained by chance alignment. (See related press release available from Deborah Halber, firstname.lastname@example.org.)
The Columbia-Caltech results were obtained using pictures from the Advanced Satellite for Cosmology and Astrophysics, or ASCA, a joint United States-Japan project launched in 1993. The satellite allows imaging of astronomical sources of X-rays that are more energetic than those previous X-ray satellites could see. Using this facility, Drs. Vasisht and Gotthelf were able to find several new slow pulsars invisible to radio searches.
At least two more sensitive X-ray satellites are to be launched in the next few years, and Dr. Gotthelf believes they will find many more radio-quiet pulsars.
In mid-1999, NASA is due to launch the Advanced X-ray Astrophysics Facility,AXAF-I, and in early 2000, the European Space Agency will send aloft the High-Throughput X-Ray Spectroscopy Mission (XMM). Both will allow imaging and spectroscopy of a wide range of X-rays from a variety of cosmic sources.
Dr. Gotthelf's research is funded by NASA.
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