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**Session 50 - Pulsars in the UV and Visible.**

*Topical, Oral session, Wednesday, June 10*

*Friars, *

## [50.10] Theory of Braking Indices

*A. Melatos (Astronomy Dept, UC Berkeley)*
A variant of the vacuum-dipole theory of pulsar spin-down is
presented that correctly predicts the braking indices of the
Crab pulsar, PSR B0540-69 and PSR B1509-58 as measured
from absolute pulse numbering. In the theory, the neutron
star and its inner magnetosphere are modeled
phenomenologically as a single unit, a conducting sphere of
radius r_v rotating rigidly in vacuo. The `vacuum
radius' r_v is chosen to correspond to the point in
the magnetosphere where the outflowing plasma becomes
three-dimensional and cross-field currents can flow. For
young, Crab-like pulsars, one finds r_\ast \ll r_v;
the model therefore differs from the standard vacuum
rotator, which has radius r_\ast and is treated as
point-like.

Given three observable pulsar parameters --- the rotation
frequency ømega, its time derivative \dotømega, and
the angle \alpha between the rotation and magnetic axes
--- and with zero free parameters, the theory yields braking
index values n=ømega\ddotømega/\dotømega^2 for the
above three pulsars that agree with timing data to 4 per
cent. The theory also makes testable predictions regarding
the second deceleration parameter m [e.g.\ m < n(2n-1)]
and implies that n approaches 3.0 for every pulsar as it
ages (with consequences for the P-\dotP diagram). The
success of the theory has several implications for
magnetospheric structure, e.g.\ the role of charge
separation in the inner magnetosphere, and the dominance of
displacement current over conduction current beyond r_
v.

**Program
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