Constraining γ-ray pulsar gap models with a simulated pulsar population^⋆

¹ Laboratoire AIM, Université Paris Diderot/CEA-IRFU/CNRS, Service d’Astrophysique, CEA Saclay, 91191 Gif-sur-Yvette, France
e-mail: marco.pierbattista@cea.fr
² François Arago Centre, APC, Université Paris Diderot, CNRS/IN2P3, CEA/Irfu, Observatoire de Paris, Sorbonne Paris Cité, 10 rue A. Domon et L. Duquet, 75205 Paris Cedex 13, France
³ Institut Universitaire de France France
⁴ Astrophysics Science Division, NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA
⁵ Hope College, Department of Physics, Holland MI, USA

Received: 29 February 2012
Accepted: 23 June 2012

Abstract

With the large sample of young γ-ray pulsars discovered by the Fermi Large Area Telescope (LAT), population synthesis has become a powerful tool for comparing their collective properties with model predictions. We synthesised a pulsar population based on a radio emission model and four γ-ray gap models (Polar Cap, Slot Gap, Outer Gap, and One Pole Caustic). Applying γ-ray and radio visibility criteria, we normalise the simulation to the number of detected radio pulsars by a select group of ten radio surveys. The luminosity and the wide beams from the outer gaps can easily account for the number of Fermi detections in 2 years of observations. The wide slot-gap beam requires an increase by a factor of ~10 of the predicted luminosity to produce a reasonable number of γ-ray pulsars. Such large increases in the luminosity may be accommodated by implementing offset polar caps. The narrow polar-cap beams contribute at most only a handful of LAT pulsars. Using standard distributions in birth location and pulsar spin-down power (Ė), we skew the initial magnetic field and period distributions in a an attempt to account for the high ĖFermi pulsars. While we compromise the agreement between simulated and detected distributions of radio pulsars, the simulations fail to reproduce the LAT findings: all models under-predict the number of LAT pulsars with high Ė, and they cannot explain the high probability of detecting both the radio and γ-ray beams at high Ė. The beaming factor remains close to 1.0 over 4 decades in Ė evolution for the slot gap whereas it significantly decreases with increasing age for the outer gaps. The evolution of the enhanced slot-gap luminosity with Ė is compatible with the large dispersion of γ-ray luminosity seen in the LAT data. The stronger evolution predicted for the outer gap, which is linked to the polar cap heating by the return current, is apparently not supported by the LAT data. The LAT sample of γ-ray pulsars therefore provides a fresh perspective on the early evolution of the luminosity and beam width of the γ-ray emission from young pulsars, calling for thin and more luminous gaps.