Pulsed and unpulsed gamma-ray emission from millisecond pulsars

Abstract

We use the outer gap model to study the gamma-ray emission and pair production processes of millisecond pulsars. The small light cylinder of millisecond pulsars results in a large optical depth for gamma rays produced 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 spectrum is F1(Eγ) ∼ Eγ -2 for Eγ ≤ 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 high energy gamma rays. The subsequent pair production and radiation processes 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(Eγ) ∼ Eγ∼ -3/2 for most millisecond pulsars. We have calculated the theoretical spectra of PSR J0751 + 18, PSR 1821-24, and PSR 1937+21 to illustrate our model. Unpulsed gamma-ray emission from other sources, e.g., 47 Tuc, are discussed also. © 1996. The American Astronomical Society. All rights reserved.