field must be used. Chiang and Romani (1994) and Romani and Yadigaroglu (1995) assumed the entire locus of points on the last closed surface bounded by the null surface and the light cylinder as giving emission in an outer gap. In our model, the emission-producing outer gaps are limited along both radial and the $\phi$-directions. For example, the extension of the outer gap on the $\phi$-direction is about 160$^{\circ}$ for Crab parameters with $\alpha=65^{\circ}$. We describe the new photon emission morphologies below.

High-energy photons will be emitted nearly tangent to the magnetic field lines in the corotating frame because of the relativistic $1/\gamma$ beaming inherent in high energy processes unless $|\bf{E}\times\bf{B}|\sim B^2$. Then following Romani and Yadigaroglu (1995), we assume relativistic charged particles in the open zone radiate in their direction of propagation, i.e. along the magnetic field lines in the corotating frame. For each location within the open zone the direction of emission expressed as ($\zeta$,$\Phi$) is calculated, where $\zeta$ is the polar angle from the rotation axis and $\Phi$ is the phase of rotation of the star. Effects of the time of flight and aberration are taken into account. A photon with velocity ${\bf{u}}=(u_x,u_y,u_z)$ along a magnetic field 