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-rav observations, and its po tential birthplaces are reviewed. The distance limits derived place co nstraints on the pulsar beaming factor, f(gamma), and its efficiency o f turning rotational kinetic energy into gamma-rays, eta. Using simila r constraints on four other rotation-powered neutron stars derived fro m 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(gam ma)-eta plane which is consistent with the observations, and which pro vides a method for determining the number of additional pulsars we can expect to find in the EGRET survey. A consideration of the unidentifi ed 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.