1998A&A...337..421K

We report on the detection at MeV energies of the radio pulsar PSR B1951+32 by the Compton telescope COMPTEL aboard the COMPTON Gamma Ray Observatory (CGRO). Folding the event arrival times with the radio ephemerides, gives for the data collected during CGRO-mission Cycles I+II a pulsar-phase distribution with two peaks, consistent in phase with the pulses measured by EGRET for energies above 100MeV. The overall significance is >4.1σ. For Cycle III-VI data, with similar exposure, no indication was found in the phase distribution. Assuming that the source is not variable, simulations show that fluctuations in the dominating background distribution at significance levels ≲3σ can explain the non-detection. In addition, evidence for the presence of the pulsar in the skymaps for energies above 3MeV is found for all Cycles of the CGRO mission. Below 3MeV the skymaps are dominated by the strong, soft-spectrum gamma-ray source Cyg X-1, located at only ∼2.6deg from PSR B1951+32. The flux (7.7±4.6)x10^–7ph/cm².s.MeV measured by COMPTEL between 0.75 and 30MeV is consistent with the measured EGRET spectrum. A single power-law fit to the combined EGRET-COMPTEL data (0.75MeV-30GeV) gives an good fit with spectral index -1.89±0.07. Furthermore, a break in the pulsar spectrum at MeV energies appears to be required to reconcile the COMPTEL flux with upper limits reported below 1MeV by OSSE and RXTE.