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Astron. Astrophys. 337, 421-432 (1998)


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Detection of pulsed MeV emission from PSR B1951+32 by COMPTEL

L. Kuiper 2, W. Hermsen 2, K. Bennett 4, A. Carramiñana 5, A. Lyne 6, M. McConnell 3 and V. Schönfelder 1

1 Max-Planck Institut für Extraterrestrische Physik, D-8046 Garching, Germany
2 SRON-Utrecht, Sorbonnelaan 2, 3584 CA Utrecht, The Netherlands
3 Space Science Centre, University of New Hampshire, Durham, NH 03824, USA
4 Astrophysics Division, European Space Research and Technology Centre, 2200 AG Noordwijk, The Netherlands
5 I.N.A.O.E., Apartado Postal 51 y 216, Puebla 72000, Puebla, México
6 University of Manchester, NRAL, Jodrell Bank, Cheshire SK11 9DL, UK

Received 23 January 1998 / Accepted 3 June 1998

Abstract

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 100 MeV. The overall significance is [FORMULA]. 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 [FORMULA] can explain the non-detection. In addition, evidence for the presence of the pulsar in the skymaps for energies above 3 MeV is found for all Cycles of the CGRO mission. Below 3 MeV the skymaps are dominated by the strong, soft-spectrum gamma-ray source Cyg X-1, located at only [FORMULA] from PSR B1951+32. The flux [FORMULA] measured by COMPTEL between 0.75 and 30 MeV is consistent with the measured EGRET spectrum. A single power-law fit to the combined EGRET-COMPTEL data (0.75 MeV - 30 GeV) gives an good fit with spectral index [FORMULA]. Furthermore, a break in the pulsar spectrum at MeV energies appears to be required to reconcile the COMPTEL flux with upper limits reported below 1 MeV by OSSE and RXTE.

Key words: gamma rays: observations – pulsars: individual: PSR B1951+32

Send offprint requests to: e-mail: L.Kuiper[FORMULA]sron.ruu.nl

© European Southern Observatory (ESO) 1998

Online publication: August 17, 1998

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