From NASANews@hq.nasa.gov Tue Jan 27 09:39:11 1998
Date: Mon, 26 Jan 1998 16:07:05 -0500 (EST)
From: NASANews@hq.nasa.gov
To: undisclosed-recipients:  ;
Subject: Fast-Spinning Pulsar Discovery Provides Evolutionary Link

Donald Savage
Headquarters, Washington, DC                     January 26, 1998
(Phone:  202/358-1547) 

Bill Steigerwald
Goddard Space Flight Center, Greenbelt, MD
(Phone:  301/286-5017)

John Gustafson
Los Alamos National Laboratory, NM
(Phone:  505/665-9197)

RELEASE:  98-14

FAST-SPINNING PULSAR DISCOVERY PROVIDES EVOLUTIONARY LINK

       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. 

                     - end -

NOTE TO EDITORS: An image to support this story may be found at:

     http://pao.gsfc.nasa.gov/gsfc/newsroom/flash/flash.htm