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Puzzling pulses form a star cluster's core.

Puzzling pulses from a star cluster's core

Each discovery of a spinning neutron star, or pulsar, seems to bring with it new mysteries. The latest puzzling addition to the pulsar catalog is PSR 2127+11, located near the center of the globular cluster M15 in the constellation Pegasus. This recently discovered pulsar has no nearby companion star, meaning the pulsar ought to be slowing down. Instead, measurements made by Alexander Wolszczan of the Arecibo Observatory in Puerto Rico and his colleagues reveal that the pulsar's rate of spin appears to be increasing.

Normally, astronomers would attribute such an increasing rate to the transfer of matter from a companion star to the pulsar. In the absence of such a donor, the increasing rate "2s probably the result of the pulsar being bodily accelerated in our direction by the gravitational field of the collapsed core of M15," Wolszczan sand his colleagues report in the Feb. 9 NATURE. The motion of the pulsar within the globular cluster apparently overwhelms the slowing caused by "magnetic braking" as the spinning neutron star radiates energy in the form of radio waves.

Globular clusters, found near the fringes of our own Milky Way galaxy, consist of dense aggregations of old stars. M15, about 30,000 light-years away, is particularly dense. In such an environment, the pulsar could conceivably be orbiting the cluster's massive collapsed core or under the gravitational influence of stellar neighbors. However, the measured acceleration suggests that either the core contains substantially more mass than astronomers believe or the pulsar is improbably close to its neighbors.

Precise measurements of any changes in the timing of pulses over the next few years should help clarify the situation. "If the pulsar is influenced by nearby stars rather than the central gravitational potential, then in about two years we should see the effect," Wolszczan says.

The pulsar, spinning on its axis once every 110 miliseconds, also has an unexpectedly long period. "Initially, everybody thought that in globular clusters we would find predominantly rapidly spinning, millisecond pulsars," Wolszczan says. "This pulsar may be incredibly old. Even though it started out as a millisecond pulsar, it had enough time to slow down to its presently observed period."

Another possibility is that the pulsar was originally a member of a binary system, gradually increasing its spin by gathering matter from its partner. But an encounter with a third star could have removed the companion, interrupting the pulsar's cosmic meal and leaving it with a relatively long period.
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Title Annotation:pulsar PSR 2127+11
Author:Peterson, Ivars
Publication:Science News
Date:Feb 11, 1989
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