SUB-LUMINOUS γ -RAY PULSARS

Most pulsars observed by the Fermi Large Area Telescope have γ-ray luminosities scaling with spin-down power ˙ E as L γ ≈ (˙ E × 10 33 erg s −1) 1/2. However, there exist one detection and several upper limits an order of magnitude or more fainter than this trend. We describe these " sub-luminous " γ-ray pulsars and discuss the case for this being an orientation effect. Of the 12 known young radio pulsars with ˙ E > 10 34 erg s −1 and d 2 kpc several are substantially sub-luminous. The limited available geometrical constraints favor aligned geometries for these pulsars, although no one case for alignment is compelling. In this scenario GeV emission detected from such sub-luminous pulsars can be due to a lower altitude, lower-power accelerator gap.