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From: dietz%usc-cse%USC-ECL%SRI-NIC@sri-unix.UUCP
Newsgroups: net.space
Subject: The millisecond pulsar
Message-ID: <12885@sri-arpa.UUCP>
Date: Fri, 21-Oct-83 13:20:00 EDT
Article-I.D.: sri-arpa.12885
Posted: Fri Oct 21 13:20:00 1983
Date-Received: Wed, 26-Oct-83 04:57:21 EDT
Lines: 36

Last year the pulsar PSR 1937+214 was discovered.  It has a period of
1.5578 milliseconds (642 rps);  its equator moves at 20% of the speed
of light!

The November Astronomy magazine has a piece on proposed theories for
the origin of the pulsar.  The object appears to have a very small
magnetic field, is not associated with a supernova remnant and is
spinning down very slowly.  These would be the signs of an old pulsar
-- except it spins much too fast!  In addition it appears to be alone
(not a member of a multiple star system).  If it were in a multiple
star system its motion could be detected by changes in the arrival
times of its pulses.

Earlier theories involved a pulsar forming in a binary system;  the
pulsar would then spin down and lose its magnetic field; then, its
companion would expand and matter would accrete onto the pulsar,
spinning it up again.  The companion would then itself explode,
disrupting the system.  Unfortunately the numbers don't come out
correctly; the companion would have to be so heavy it would supernova
too quickly to spin up the pulsar.

The new theory is this:  the system was originally a binary system with
two similar large stars.  They supernovaed near the same time, forming
two close neutron stars.  If the stars are close enough the orbit will
decay by the emmision of gravitational radiation.  At least one such
system is known today; the orbits are indeed decaying at precisely the
rate predicted by general relativity.

After some time (hundreds of millions of years) the two neutron stars
will be orbiting each other almost in contact with a period of about
one millisecond.  Tidal forces then destroy the smaller star,
transfering matter to the larger one.  At some point the smaller
neutron star is too small to remain a neutron star: it reverts to
normal electronic matter and torn apart.  Theory predicts the remaining
star emits gravity waves until its period is about 1.5 milliseconds,
which is nicely confirmed by observation.