Unformatted text preview: The period is quite regular (but slowly decreasing, due to \spindown" of
the neutron star). The ux and pulse shape are somewhat variable, but the
long-time averages are stable. Some pulsars put out optical and x-ray pulses
as well as radio pulses.
What makes pulsars pulse? The answer is thought to be that the pulses
are from electrons trapped in the magnetic eld of the rotating neutron star.
The idea is that the pulses are thus emitted in a cone rotating with the
neutron star. Suppose the period is P (= 2=
). Then we can derive a limit
to the neutron star radius r. At the equator, the centrifugal force = 2r per
unit mass had better not exceed the gravitational force = GM=r2 per unit
to + rotation axis emission cone sweeps across
once per rotation period neutron
star charge Therefore, 2r < GM=r2 for stability. So for a given P , we require
GMP 2 1=3 1500 P 2=3 km :
r < 42
P = 33 ms r < 150 km < 10 white dwarf radius
Pulsar 1937+21: P = 2 ms r < 24 km < 100 white dwarf radius
These objects can't be white dwarf stars (periods too short), and they can't
be planets (or the rate of loss of energy would slow them too quickly), so we
infer that they are neutron stars. 150 ...
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- Fall '09
- Rotation, Supernova, White dwarf, pulsar, Neutron star