Pulsar Polar Cap Heating and Surface Thermal X‐Ray Emission. II. Inverse Compton Radiation Pair Fronts

We investigate the production of electron-positron pairs by inverse Comptonscattered (ICS) photons above a pulsar polar cap (PC) and surface heating byreturning positrons. This paper is a continuation of our self-consistenttreatment of acceleration, pair dynamics and electric field screening abovepulsar PCs. We calculate the altitude of the inverse Compton pair formationfronts, the flux of returning positrons and present the heating efficienciesand X-ray luminosities. We revise pulsar death lines implying cessation of pairformation, and present them in surface magnetic field-period space. We findthat virtually all known radio pulsars are capable of producing pairs byresonant and non-resonant ICS photons radiated by particles accelerated abovethe PC in a pure star-centered dipole field, so that our ICS pair death linecoincides with empirical radio pulsar death. Our calculations show that ICSpairs are able to screen the accelerating electric field only for high neutronstar surface temperatures and magnetic fields. We argue that such screening atICS pair fronts occurs locally, slowing but not turning off acceleration ofparticles until screening can occur at a curvature radiation (CR) pair front athigher altitude. In the case where no screening occurs above the PC surface, weanticipate that the pulsar gamma-ray luminosity will be a substantial fractionof its spin-down luminosity. The X-ray luminosity resulting from PC heating byICS pair fronts is significantly lower than the PC heating luminosity from CRpair fronts, which dominates for most pulsars. PC heating from ICS pair frontsis highest in millisecond pulsars, which cannot produce CR pairs, and mayaccount for observed thermal X-ray components in the spectra of these oldpulsars.Comment: 29 pages, 10 figures, accepted for publication in Ap