Geminga

A powerful gamma-ray source in the constellation Gemini discovered in 1972 by the orbiting observatory SAS (Small Astronomy Satellite) 2. Weak X-rays from Geminga were detected by the Einstein Observatory and its optical counterpart appears as a twenty-fifth magnitude star. Geminga is thus a very unusual object in that it emits almost all its energy as gamma rays; the X-ray emission is a thousand times weaker and its luminosity in visible light a thousand times weaker again. It is thought to be relatively close, probably within 700 light years. Observations made by ROSAT confirmed the X-ray emission and showed it to be pulsating with a period of about a quarter of a second. Gamma-ray pulsations have also been detected by the Compton Gamma Ray Observatory. Geminga thus appears to be a gamma-ray and X-ray pulsar. Why the bulk of its radiant energy is emitted in such a high-energy form is not known.