Off-Beam Gamma-Ray Pulsars and Unidentified EGRET Sources in the Gould Belt

We investigate whether gamma-ray pulsars viewed at a large angle to theneutron star magnetic pole could contribute to the new population of galacticunidentified EGRET sources associated with the Gould Belt. The faint, softnature of these sources is distinctly different from both the properties ofunidentified EGRET sources along the galactic plane and of the known gamma-raypulsars. We explore the possibility, within the polar cap model, that some ofthese sources are emission from pulsars seen at lines of sight that miss boththe bright gamma-ray cone beams and the radio beam. The off-beam gamma-rayscome from high-altitude curvature emission of primary particles, are radiatedover a large solid angle and have a much softer spectrum than that of the mainbeams. We estimate that the detectability of such off-beam emission is about afactor of 4-5 higher than that of the on-beam emission. At least some of theradio-quiet Gould Belt sources detected by EGRET could therefore be suchoff-beam gamma-ray pulsars. GLAST should be able to detect pulsations in mostof these sources.Comment: 5 pages, uses emulateapj.sty, accepted for publication in ApJ Letter