OSSE observations of the Vela and Geminga pulsars

The Oriented Scintillation Spectrometer Experiment (OSSE) on board the Compton Gamma Ray Observatory detected the Vela pulsar (PSR B0833 -45) during 1991 August-September, 1992 April-May, and 1993 August. Observed light curves have a two-peak pulse profile similar to that observed at higher energies, although the second peak may be wider in the OSSE light curve. Pulsed emission in the first gamma-ray peak was detected with 4.6 sigma statistical significance in the 0.07-0.6 MeV band in the sum of all three observing periods. The second gamma-ray peak was detected at no more than 3 sigma significance in the same band. Due to the low statistical significance of the observations, little can be said concerning longer term temporal variability. The spectrum is hard at lower energies and, in combination with higher energy data, appears to require a break. in the 20 MeV region. OSSE also observed Geminga during 1992 July, 1993 December, and 1994 July. No significant pulsed emission was observed on any occasion. Upper limits to the pulsed emission suggest, but do not require, a break from the extrapolation of the spectrum measured at higher energies.