Young and middle age pulsar light-curve morphology: Comparison of Fermi observations with γ-ray and radio emission geometries

¹ Nicolaus Copernicus Astronomical Center, Rabiańska 8, 87-100 Toruń, Poland
e-mail: mpierba@gmail.com
² INAF−Istituto di Astrofisica Spaziale e Fisica Cosmica, 20133 Milano, Italy
³ Astrophysics Science Division, NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA
⁴ Hope College, Department of Physics, Holland MI 49423, USA
⁵ Laboratoire AIM, Université Paris Diderot/CEA-IRFU/CNRS, Service d’Astrophysique, CEA Saclay, 91191 Gif-sur-Yvette, France
⁶ Institut Universitaire de France, 75231 Paris Cedex 05, France

Received: 24 November 2015
Accepted: 18 January 2016

Abstract

Thanks to the huge amount of γ-ray pulsar photons collected by the Fermi Large Area Telescope since its launch in June 2008, it is now possible to constrain γ-ray geometrical models by comparing simulated and observed light-curve morphological characteristics. We assumed vacuum-retarded dipole (VRD) pulsar magnetic field and tested simulated and observed morphological light-curve characteristics in the framework of two pole emission geometries, Polar Cap (PC) and Slot Gap (SG), and one pole emission geometries, traditional Outer Gap (OG) and One Pole Caustic (OPC). Radio core plus cone emission was assumed for the pulsars of the simulated sample. We compared simulated and observed recurrence of class shapes and peak multiplicity, peak separation, radio-lag distributions, and trends of peak separation and radio lag as a function of observable and non-observable pulsar parameters. We studied how pulsar morphological characteristics change in multi-dimensional observable and non-observable pulsar parameter space. The PC model gives the poorest description of the LAT pulsar light-curve morphology. The OPC best explains both the observed γ-ray peak multiplicity and shape classes. The OPC and SG models describe the observed γ-ray peak-separation distribution for low- and high-peak separations, respectively. This suggests that the OPC geometry best explains the single-peak structure but does not manage to describe the widely separated peaks predicted in the framework of the SG model as the emission from the two magnetic hemispheres. The OPC radio-lag distribution shows higher agreement with observations suggesting that assuming polar radio emission, the γ-ray emission regions are likely to be located in the outer magnetosphere. Alternatively, the radio emission altitude could be higher that we assumed. We compared simulated non-observable parameters with the same parameters estimated for LAT pulsars in the framework of the same models. The larger agreement between simulated and LAT estimations in the framework of the OPC suggests that the OPC model best predicts the observed variety of profile shapes. The larger agreement obtained between observations and the OPC model predictions jointly with the need to explain the abundant 0.5 separated peaks with two-pole emission geometries, calls for thin OPC gaps to explain the single-peak geometry but highlights the need of two-pole caustic emission geometry to explain widely separated peaks.