Population study for γ-ray pulsars with the outer gap model - II. Millisecond pulsars

Abstract

The population of γ-ray emitting millisecond pulsars (MSPs) was studied by using Monte Carlo techniques. We simulated the Galactic distributions of the MSPs, and applied the outer gap model for the γ-ray emission from each simulated MSP. We took into account the dependence of the observed γ-ray flux on the viewing angle and on the inclination angle, which is the angle between the rotation axis and the magnetic axis, respectively. Using the sensitivity of the 6-month long observation of the Fermi telescope and radio sensitivities of existing pulsar surveys, 9-13 radio-selected and 22-35 γ-ray-selected pulsars are detected within our simulation. The statistical properties of the simulated population are consistent with the Fermi observations. Scaling the observed sensitivity, where T is the length of observation time, the present model predicts that over the 5-year mission Fermi would detect 15-22 radio-selected γ-ray MSPs, and 95-152 γ-ray-selected MSPs. Our simulation also predicts that about 100 (or 200-300) γ-ray MSPs with a flux larger than F≥ 10 -11ergcm -2s -1 (or 5 × 10 -12ergcm -2s -1) irradiate the Earth. With the present sensitivities of radio surveys, most of them are categorized as γ-ray-selected pulsars, indicating that most of the γ-ray MSPs have been missed by the present Fermi observations. We argue that the Galactic Fermi unidentified sources located at high latitudes should be dominated by MSPs, whereas the sources in the Galactic plane are dominated by radio-quiet canonical pulsars. We want to emphasize that the predicted number of radio-loud γ-ray MSPs depends on the sensitivities of radio surveys and that it can be increased, for example, from 15-22 to 26-37 if the radio sensitivity is improved by a factor of 2. © 2011 The Authors Monthly Notices of the Royal Astronomical Society © 2011 RAS.