Two-photon annihilation in the pair formation cascades in pulsar polar caps

The importance of the photon-photon pair production process (γ + γ′ → e+ + e−) to form pair production cascades in pulsar polar caps is investigated within the framework of the Ruderman-Sutherland vacuum gap model. It is found that this process is unimportant if the polar caps are not hot enough, but will play a non-negligible role in the pair formation cascades when the polar cap temperatures are in excess of the critical temperatures, Tcri, which are around 4×106K when P = 0.1s and will slowly increase with increasing periods. Compared with the γ −B process, it is found that the two-photon annihilation process may ignite a central spark near the magnetic pole, where γ − B sparks can not be formed due to the local weak curvatures. This central spark is large if the gap is dominated by the “resonant ICS mode”. The possible connection of these central sparks with the observed pulsar “core” emission components is discussed.