The Geminga enigma: how many are there in the gamma-ray sky?

The distance to the high-energy $$\gamma$$-ray source Geminga is discussed in terms of recent optical, UV, EUV and X-ray observations, and its potential birthplaces are reviewed. The distance limits derived place constraints on the pulsar beaming factor, $$f_\gamma $$, and its efficiency of turning rotational kinetic energy into $$\gamma$$-rays, $$\eta$$. Using similar constraints on four other rotation-powered neutron stars derived from COS-B and EGRET data and the fact that 553 other known radio pulsars were not detected in the COS-B survey, I define a region of the $$f_\gamma\ - \eta$$ plane which is consistent with the observations, and which provides a method for determining the number of additional pulsars we can expect to find in the EGRET survey. A consideration of the unidentified COS-B point sources agrees with previous work suggesting that they too are rotation-powered neutron stars. Independent of their identity, however, their numbers provide additional constraints on $$\eta$$, $$f_\gamma $$, and the $$\gamma$$-ray pulsar population of the Galaxy.