RXTE Discoveries

RXTE Finds "Missing Link" Pulsar - February 1998

In late January 1998, the announcement came of the discovery of a pulsar that, while presently spinning at more than 60 times a second, may have been spinning 150 times per second or more, when it formed 4000 years ago. Astronomers did not previously know that a pulsar could be born with such a rapid spin rate. The pulsar, shown in the below image, was found in the supernova remnant called N157B.

"The pulsar is spinning twice as fast as any young pulsar that we have seen before," says Dr. Frank Marshall, who works at NASA's Goddard Space Flight Center and led the discovery-making team. He adds, "To put it in perspective, this pulsar is spinning more than 6 million times as rapidly as the Earth."

A composite X-ray image of the 30 Doradus region of the Large Magellanic Cloud taken
with the Solid-state Imaging Spectrometer on-board the Advanced Satellite for Cosmology and
Astrophysics (ASCA). Credit: Wang & Gotthelf 1998, ApJ, 494, 623

This discovery is important because it might prove to be an evolutionary link between pulsars that spin very rapidly but have weak magnetic fields, and others that spin more slowly but have a strong magnetic field. This newly discovered pulsar falls right in between the two kinds, leading astronomers to theorize that there is a natural continuum from one kind to the other.

The other members of this pulsar team include Drs. William Zhang and Eric Gotthelf also of Goddard, and Dr. Middleditch of the U.S. Department of Energy's Los Alamos National Laboratory. The pulsar was discovered by examining X-ray emissions recorded by the Rossi X-ray Timing Explorer in 1996, and was confirmed by the Japanese/US Advanced Satellite for Cosmology and Astrophysics (ASCA).

Pulsars are very rapidly spinning neutron stars that appear to pulse because their emissions are channeled into a beam of radiation that we observe as it sweeps through space like a lighthouse beacon. We derive its rate of spin by observing how rapidly that bean of light flashes Earth. Pulsars are created in the aftermath of a supernova explosion, when the core of the exploding star, perhaps only 15 miles across, is packed with as much matter as a Sun. The matter is so densely packed that the atoms inside are shoved together combining all the protons and electrons so that this star is made up of only neutrons. Not a normal state for matter.

The newly discovered pulsar is most likely associated with the remnant of supernova N157B (shown in the above ROSAT image), which exploded about 4000 years ago in the Large Magellanic Cloud. This estimate of the pulsar age comes from other X-ray and visible observations of the expansion of the gas cloud from the supernova blast. This age estimate is also in agreement with predicted theoretical models. Also, data from the RXTE and ASCA were used to calculate the rate at which the pulsar's spin is slowing; this age estimate yielded a number of 5000 years, which is in close agreement with the other age estimates.

There are pulsars that spin faster than the one in N157B, called millisecond pulsars because their spin periods span only a few thousandths of a second, but these pulsars were probably born with a slow spin. While these millisecond pulsars most likely attained their fast spin rate from sucking in gaseous material from an orbiting stellar companion, they still remain a mystery because astronomers have not located enough suitable binary star systems to account for the large numbers of millisecond pulsars being discovered.

Millisecond pulsars usually have weak magnetic fields, in contrast with the slower pulsars with strong magnetic fields like the famous Crab Pulsar. The Crab Pulsar, located in the Crab Nebula, is famous because the supernova that created it was actually observed in the year 1054. This pulsar spins just under 30 times a second and is thought to have started its life with a spin rate of 60 times/second.

The pulsar in N157B may be the evolutionary link between slow spinning, strong field pulsars like the Crab pulsar and the fast weak field millisecond pulsars. It has a magnetic field only a few times weaker than the Crab Pulsar, but as Dr. Middleditch says, "This is the fastest high energy pulsar of its type we know about." This pulsar might show that there is a natural continuum from one type to the other. Middleditch says, "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." What does the future bring? Astronomers continue to search for a pulsar at the heart of 1987A, the most recently occurring supernova. Their prediction is for a rapidly spinning weak-field pulsar, which would give us another clue in the mystery of how fast pulsars are born.


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