The statistics of gamma-ray pulsars

Abstract

We use Monte Carlo methods to simulate the properties of a Galactic population of rotation-powered pulsars in the self-consistent outer gap model proposed by Zhang & Cheng, where the initial magnetic field and spatial and velocity distributions of the neutron stars at birth are obtained from the statistical results of radio pulsars. We obtain the distance, period, age, magnetic field, and γ-ray flux distributions of the γ-ray pulsars whose γ-ray and radio fluxes are above the detectable threshold fluxes. Furthermore, we simulate the properties of Geminga-like pulsars and obtain the possible parametric region and the number of the Geminga-like pulsars. In our simulations, the different beaming effects of radio and γ-ray beams are taken into account. We predict that there may be ∼11 γ-ray pulsars that are detectable at both radio and γ-ray energies and ∼ 37 Geminga-like pulsars in the Galactic plane (| b | < 5°) if the γ-ray beaming fraction is 0.67 and the birthrate of the neutron stars is 1 per century. Compared to the flux distribution, individual spectrum, and distance distribution obtained by assuming OB star associations of our simulated Geminga-like pulsars and the unidentified EGRET sources at | b | < 5°, we conclude that the majority of the unidentified γ-ray point sources near the Galactic plane may be Geminga-like pulsars. We also suggest that the most possible radio pulsars likely to be confirmed as γ-ray pulsars in future are those with LX/Ėsd ∼ 10-3. © 1998. The American Astronomical Society. All rights reserved.