Pulsed Gamma Rays from Geminga

The pulsed gamma ray spectrum is obtained by analyzing COS-B data of observation no. 14, 39, 54 and 64. We find that the pulsed spectrum differs significantly from that of previous reports in which the pulsed component and steady component are not separated. A clear spectral break of the pulsed spectrum occurs at ~ 100 MeV and the majority of the steady emission is concentrated in the lower energy band. We fit the pulsed spectrum by both the outer gap model and the polar gap model and conclude that only the outer gap model can explain the observed spectrum. Furthermore, the gamma ray spectrum suggests that Geminga is located at a distance ~ 300 pc and is a nearly orthogonal rotator.