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 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.