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.Comment: 7 pages, 3 Postscript figures, LaTex, accepted for publication in
Astronomy and Astrophysic