Bound Pair Creation in Polar Gaps and Gamma-Ray Emission from Radio Pulsars

In a strong magnetic field, B ≳ 0.1Bcr = 4.4 x 1012 G, gamma rays emitted nearly along curved field lines adiabatically convert into bound pairs (positronium) rather than decaying into free pairs. This partially prevents the screening of the vacuum electric field by free pairs and increases the height of the gap in polar gap models of pulsars. A modified polar gap model for high-B pulsars is presented. Provided that the surface of the star is cool enough to prevent both free escape of charges from the surface and ionization of bound pairs, which should be the case except for the two young pulsars (the Crab and PSR 0540-69), the model implies a power in primary particles, and hence in X-rays and/or gamma rays, that can be a substantial fraction of the spin-down power. It is suggested that such a model can account for the very high efficiency of gamma-ray emission inferred for a small fraction of radio pulsars.