Population study for gamma-ray pulsars: (III) Radiation characteristics and viewing geometry

Abstract

We have performed a Monte-Calro simulation for Galactic population of pulsars and for the $\gamma$-ray observations. We apply outer gap model for the $\gamma$-ray emission process, and study the radiation characteristics as a function of the magnetic inclination angle $(\alpha )$ and the Earth viewing angle ($\zeta$). In our model, the $\gamma$-ray flux and the spectral cut-off energy tend to decrease as the inclination and viewing angles deviate from $90^{\circ }$. The emerging spectrum above 100 MeV becomes soft with a photon index $p\sim 1.8-2$ for $\zeta \to 90^{\circ }$ and $p\sim 1.2-1.3$ for $\zeta \ll 90^{\circ }$. Our simulation predicts that the pulsars with larger inclination angles ($\alpha =70-90^{\circ }$) and larger viewing angles ($\zeta =70-90^{\circ }$) have been preferentially detected by the $Fermi$ $\gamma$-ray telescope, and hence the observed pulse profiles of the $\gamma$-ray pulsars have the double peak structure rather than single peak. In the simulation, most $\gamma$-ray millisecond pulsars are categorized as the radio-quiet $\gamma$-ray pulsars, because its radio fluxes are under the sensitivities of the major radio surveys. Even we drastically increase the radio sensitivity by a factor of ten, the number of radio-selected millisecond pulsars detected by the $Fermi$ ten years observations is still much less than the expected $\gamma$-ray-selected millisecond pulsars, indicating the radio-quiet millisecond pulsars must contribute to the $Fermi$ unidentified sources and/or the $\gamma$-ray background radiations. The Unique radiation properties of the low-efficient $\gamma$-ray pulsar, PSR J0659+1414, can be explained by the present our gap model with a viewing geometry of $\alpha \sim \zeta =40^{\circ }-50^{\circ }$.

41page, 22 figures, accepted for publication in MNRAS

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• X-ray/GeV emissions from Crab-like pulsars in LMC
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• Modeling the non-recycled Fermi gamma-ray pulsar population
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