Hadronic photoabsorption and pair production in pulsars

Abstract

THERE are two existing models for the acceleration of positive charged particles near the magnetic polar caps of a pulsar. In the first, the work function for a ⁵⁶Fe ion bound in the lattice at the stellar surface is assumed, following exhaustive calculations of Flowers et al.¹, to be too large for the occurrence of ion emission. Electron–positron pairs are created through magnetic conversion of curvature photons²; the electrons are accelerated inwards to the stellar surface, and the positrons outwards along open lines of magnetic flux to the light cylinder. In the second, the work function is assumed to be negligible and ion emission limited only by space charge effects. In the corotating frame of reference, all components of the surface electric field are zero and inertial ion acceleration^3–5 occurs. For surface magnetic flux densities B≳10¹² G, we shall attempt to show here that the nature and mode of acceleration of the plasma moving outwards along open magnetic flux lines are determined, not by these processes, but by the occurrence of hadronic photoabsorption reactions in the electromagnetic showers produced by ultra-relativistic electrons incident on the stellar surface. This work was prompted by the comment⁴ that magnetic conversion of backward moving photons from these showers may be an important source of pairs. The consequences of hadronic photoabsorption do not seem to have been considered previously. Here, it is shown that photons of the energy necessary for conversion in a magnetic flux density of 10¹²G are emitted with high probability in hadronic photoabsorption reactions, and an equation is obtained for the acceleration potential difference in a one-dimensional model of electron–positron pair creation at the magnetic polar cap. The properties of the accelerated plasma are important owing to its probable connection with radio emission and with the phenomenon of subpulse drift occurring in certain pulsars (see, for example, refs 6, 7).