PULSED AND UNPULSED GAMMA-RAY EMISSION FROM MILLISECOND PULSARS

We use the outer gap model to study the gamma-ray emission and pair pr oduction processes of millisecond pulsars. The small light cylinder of millisecond pulsars results in a large optical depth for gamma rays p roduced inside the light cylinder. Electron/position cascade will take place until the energy of emitted gamma rays is less than a specific threshold energy (E(crit)). The model for the pulsed gamma-ray spectru m is F-1(E(gamma)) similar to E(gamma)(-2) for E(gamma) less than or e qual to E(gamma) less than or equal to E(crit). Furthermore, electrons /positrons accelerated in the outer gap can continue to interact with the low-energy gamma rays beyond the light cylinder to produce very hi gh energy gamma rays. The subsequent pair production and radiation pro cesses can produce unpulsed high-energy photons in the energy range of the EGRET detector on board the Compton Gamma Ray Observatory with a model spectrum F-2(E(gamma)) similar to E(gamma)(-3/2) for most millis econd pulsars. We have calculated the theoretical spectra of PSR J0751 +18, PSR 1821-24, and PSR 1937+21 to illustrate our model. Unpulsed ga mma-ray emission from other sources, e.g., 47 Tuc, are discussed also.