Rotation Rates (2) ...
As a pulsar radiates away its energy, its spin rate decreases slowly. This change is
small, but can be measured very precisely. The rate of change in the rotational
period for a radio pulsar is important because it can be
used to estimate the strength of the magnetic field associated with the neutron star.
Pulsar Glitches
The period for pulsars generally increases at a slow but steady rate as they gradually spin more slowly.
However, in some pulsars "glitches" are observed where the spin rate suddenly jumps to a higher value and then
continues its slow decline. The fractional change in period caused by a glitch is
typically from one millionth to one billionth of the original period.
Three glitches for the Vela Pulsar are illustrated in the adjacent figure.
These glitches are evidence that some internal rearrangement of the neutron star has altered its rotation rate
by a small amount. One proposal is that "starquakes" occur in the incredibly dense crust, causing the neutron
star to contract slightly and thus to speed up by angular momentum conservation. A second theory proposes
that angular momentum stored in circulation of an internal superfluid liquid (one that
exhibits no frictional effects) is suddenly
transferred to the crust, altering the rotation rate.
Millisecond Pulsars
The fastest pulsars known have periods of milliseconds, implying rotational frequencies approaching 1000
revolutions per second. This is near the maximum rate that theory predicts for a pulsar, since
a neutron star would begin flying apart if it spun much faster than that.
However, for many of these fast pulsars there is evidence that they are old, not
young as we would expect for the fastest spin rates. This evidence consists primarily of the
rate at which the pulsar spin is slowing, and where the millisecond pulsars are found.
Spin Slowing and Age
For example, the first millisecond pulsar discovered,
PSR 1937 + 21 (the blender-speed sound file from above), is very fast but
it is spinning down very slowly. This slow spindown rate
implies that it has a weak magnetic field and is old. (Older pulsars should have weaker
fields and these should
be less effective than younger, stronger fields in braking their motion; see the
later discussion of how the Crab Pulsar slows as it
transfers its energy to the Crab Nebula.)
Also, many of the
millisecond pulsars that have been discovered since the 1980s are found in globular clusters.
As we have seen in Chapter 19, globular clusters contain an old population of stars.
Therefore, they are not likely to be
sites of recent supernova explosions that could have produced young pulsars since
Type II supernovae occur in very short-lived, massive stars.
The Spinup Mechanism
The most plausible way to explain the contradiction that the fastest pulsars also appear to be
very old pulsars is that the
millisecond pulsars have been "spun up" at some point in their history to
higher rates than they had at birth. The mechanism that has
been proposed to do this involves mass transfer
in binary systems that adds angular momentum to the neutron star.
The adjacent right animation
illustrates this idea, which is termed binary spinup. In effect, this mechanism
transfers angular momentum from the orbital angular momentum of the binary to the rotation of
the neutron star. Later in the evolution, after the neutron star has been spun up to high
rotational velocity, the primary star may become a supernova and disrupt the binary system,
leaving the rapidly spinning but old neutron star as a lone millisecond pulsar.