Jan. 27, 1998
Scientists have announced the discovery of a superdense star spinning at more than 60 times per second, and calculate it could have been spinning as fast as 150 times per second or more when it formed some 4,000 years ago. Most astronomers had not previously believed this class of star, called a pulsar, could form with such a rapid spin.
"This shatters the glass ceiling," said astrophysicist John Middleditch of the U.S. Department of Energy's Los Alamos National Laboratory in New Mexico. "This is the fastest high-energy pulsar of its type we know about."
"The pulsar is spinning twice as fast as any young pulsar that we have seen before," adds Dr. Frank Marshall of NASA's Goddard Space Flight Center, Greenbelt, MD, who led the team making the discovery. "To put it in perspective, this pulsar is spinning more than 6 million times as rapidly as the Earth."
The newly discovered pulsar establishes a link between fast-spinning pulsars with relatively weak magnetic fields and slow-spinning ones with strong fields, suggesting there may be a natural continuum between the two known types. The pulsar was found by Dr. Marshall and his colleagues Drs. William Zhang and Eric Gotthelf of Goddard, and Middleditch, by examining X-ray emissions recorded by NASA's Rossi X-ray Timing Explorer spacecraft in 1996, and confirmed with observations using the joint Japanese/U.S. Advanced Satellite for Cosmology and Astrophysics (ASCA) spacecraft.
Pulsars get their name because their emissions appear to turn on and off, or pulse, very rapidly. Astronomers believe the stars channel some of their energy into a beam of radiation, and as the star spins the beam sweeps through space like a lighthouse beacon. By counting how rapidly the beam flashes at Earth, scientists can calculate a pulsar's rate of spin. When a star explodes as a supernova it leaves behind a lingering core about 15 miles across but packed with as much matter as in Earth's Sun. The star is so dense that neutrons are the only form of matter that exist in the star, thus earning the name "neutron star." Those whose rapid spin can be observed are called "pulsars."
The team identified the pulsar as most likely being associated with the remnant of a supernova (catalogued N157B by astronomers) that exploded in the Large Magellanic Cloud, a companion to our Milky Way galaxy, about 4,000 years ago. (The age estimate comes from other X-ray and visible observations of the spreading, tattered gas cloud from the supernova blast and is in agreement with that predicted by theoretical models.) Data from both the Rossi and ASCA satellites were used to calculate the rate at which the pulsar's spin is slowing, which in turn provides an estimate of its age: 5,000 years old, a close match to the age estimate for the supernova remnant.
The other well-known high energy pulsar, in the Crab Nebula, spins just under 30 times per second, and is generally thought to have been spinning at only 60 times a second at its birth in 1054 AD. Since the Crab pulsar's discovery in 1968, astronomers have spotted pulsars spinning as fast as hundreds of times per second. These so-called "millisecond pulsars" (because their spin periods are only a few thousandths of a second) have magnetic fields a thousand times weaker than the Crab pulsar.
Most astronomers believe that the weak-field, millisecond pulsars were born with a slow spin and were "spun up" after sucking in gaseous material from an orbiting stellar companion, but astronomers have not located enough suitable binary star systems to account for the large numbers of millisecond pulsars being discovered.
The pulsar found in N157B, whose magnetic field is only a few times weaker than the Crab pulsar's, suggests an evolutionary link between the strong-field, slower-spinning energetic pulsars and the weak-field millisecond pulsars. Its discovery confirms a prediction published by Gotthelf and Dr. Q. Daniel Wang of Northwestern University.
"This is a fantastic confirmation of our hypotheses; that the central source of X-ray light from N157B is a fast pulsar associated with a supernova remnant, like that seen in the Crab nebula," commented Gotthelf.
"Now, clearly, it seems that the weaker the magnetic field, the faster the pulsar will spin at birth -- possibly all the way down to one- or two-millisecond periods (corresponding to spin rates of 1,000 to 500 times per second) for fields of the strength measured for the weak-field pulsars," Middleditch said.
Astronomers continue to search for a pulsar at the heart of SN1987A, a supernova that appeared in the southern skies Feb. 23, 1987. Most astronomers who study this supernova expect that a rapidly spinning, weak-field pulsar should eventually reveal itself for observation, which would provide another link in theories of how fast pulsars are born. Marshall and his team encouraged other researchers to study N157B at other regions of the spectrum to see if its pulsations are observable there, too.
The team announced their discovery last week through a circular distributed by the International Astronomical Union.
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