Young pulsars and unidentified gamma-ray sources at the Galactic plane

Abstract

We present a statistical study on possible association between the unidentified γ-ray point sources at the Galactic plane (|b| ≤ 5°) and the Galactic young pulsars with ages of less than 106 yr. Based on the possible positional coincidence of the unidentified EGRET sources in the third catalog with the Galactic objects given by Romero et al. (1999) as well as the outer gap model of γ-ray pulsars proposed by Zhang & Cheng (1997), we have made a statistical analysis on whether most unidentified EGRET point sources in the Galactic plane have pulsar origin. In our analysis, we use an evolving γ-ray solid angle which is the function of pulsar's parameters such as period, magnetic field and magnetic inclination angle. The simulated distributions of the distance and γ-ray energy flux are compared to observed data using Kolmogorov-Smirnov (KS) test. Our results suggest that young pulsars may account for the majority of unidentified EGRET sources that are positionally associated with supernova remnants and OB star associations, in such a way that two thirds are young radio quiet pulsars and one third are young radio pulsars. We have studied the variability of these unidentified γ-ray sources and found that most of them may be non-variable. Furthermore, we expect that GLAST may detect ∼ 80Ṅ100 radio pulsars and ∼ 1100Ṅ100 unidentified γ-ray point sources in the Galactic plane with |b| ≤ 5°, where Ṅ100 is the birth rate of neutron stars in units of 100 years. This model predicts that γ-ray point sources located at small distances are more than those predicted by a γ-ray pulsar model with a constant beaming solid angle.