Journal Article

Population study for γ-ray pulsars with the outer-gap model – III. Radiation characteristics and viewing geometry

J. Takata, Y. Wang and K. S. Cheng

in Monthly Notices of the Royal Astronomical Society

Published on behalf of The Royal Astronomical Society

Volume 415, issue 2, pages 1827-1848
Published in print August 2011 | ISSN: 0035-8711
Published online July 2011 | e-ISSN: 1365-2966 | DOI: http://dx.doi.org/10.1111/j.1365-2966.2011.18832.x
Population study for γ-ray pulsars with the outer-gap model – III. Radiation characteristics and viewing geometry

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We have performed a Monte Carlo simulation of the Galactic population of pulsars and γ-ray observations. We apply a two-layer outer-gap model, which has been developed by Wang, Takata & Cheng, of the γ-ray emission process, and study the radiation characteristics as a function of the magnetic inclination angle (α) and the Earth viewing angle (ζ). In our model, the γ-ray flux and the spectral cut-off energy tend to decrease as the inclination and viewing angles deviate from 90°. The emerging spectrum above 100 MeV becomes soft with a photon index p∼ 1.8–2 for ζ→ 90○ and p∼ 1.2–1.3 for ζ≪ 90°. Our simulation predicts that pulsars with larger inclination angles (α= 70–90°) and larger viewing angles (ζ= 70–90°) have been preferentially detected by the Fermiγ-ray telescope, and hence the observed pulse profiles of the γ-ray pulsars have a double-peak structure rather than a single-peak one. In the simulation, most γ-ray millisecond pulsars are categorized as radio-quiet γ-ray pulsars, because their radio fluxes are under the sensitivities of the major radio surveys. Even if we drastically increase the radio sensitivity by a factor of 10, the number of radio-selected millisecond pulsars detected by Fermi 10-year observations is still much lower than the expected number of γ-ray-selected millisecond pulsars, indicating that radio-quiet millisecond pulsars must contribute to Fermi unidentified sources and/or γ-ray background radiation. We argue that γ-ray pulsars observed with a smaller viewing angle (ζ≪ 90°) will appear as low-efficiency γ-ray pulsars. For example, the unique radiation properties of the low-efficiency γ-ray pulsar PSR J0659+1414 can be explained by our present gap model with a viewing geometry of α∼ζ= 40°–50°.

Keywords: radiation mechanisms: non-thermal; methods: statistical; stars: neutron; pulsars: general; gamma-rays: stars

Journal Article.  10765 words.  Illustrated.

Subjects: Astronomy and Astrophysics

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