Pulsar Death at an Advanced Age

Abstract

I summarize the theory of acceleration of non-neutral particle beams by starvation electric fields along the polar magnetic field lines of rotation powered pulsars, including the effect of dragging of inertial frames which dominates the acceleration of a space charge limited beam. I apply these results to a new calculation of the radio pulsar death line, under the hypotheses that pulsar ``death'' corresponds to cessation of pair creation over the magnetic poles {\it and} that the magnetic field has a locally dipolar topology. The frame dragging effect in star centered dipole geometry does improve comparison of the theory with observation, but an unacceptably large conflict between observation and theory still persists. Offsetting the dipole improves the comparison, but a fully satisfactory theory requires incorporating magnetic conversion of inverse Compton gamma rays, created by scattering thermal photons from the surface of old neutron stars ($t > 10^8 $ years) kept warm ($T \geq 10^5$ K) by friction between the rotating core and the crust. The result is a ``death valley'' for pulsars; offsets of the dipole center from the stellar center in the oldest stars $\sim (0.7-0.8) R_*$ suffice. The resulting theory predicted the existence of rotation powered pulsars with these advanced ages, a prediction confirmed by the recent discovery that PSR J2144-3933 actually has a rotation period of 8.5 seconds.