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ay45c5-page38 - The period is quite regular(but slowly...

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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 mass. 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 sec 1 Crab pulsar: P = 33 ms r < 150 km < 10  white dwarf radius  1 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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