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Astron. Astrophys. 351, 119-132 (1999)


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COMPTEL detection of pulsed [FORMULA]-ray emission from PSR B1509-58 up to at least 10 MeV

L. Kuiper 2, W. Hermsen 2, J.M. Krijger 2,7, K. Bennett 3, A. Carramiñana 4, V. Schönfelder 1, M. Bailes 6 and R.N. Manchester 5

1 Max-Planck-Institut für Extraterrestrische Physik, D-8046 Garching, Germany
2 SRON-Utrecht, Sorbonnelaan 2, 3584 CA, Utrecht, The Netherlands
3 Astrophysics Division, European Space Research and Technology Centre, 2200 AG, Noordwijk, The Netherlands
4 I.N.A.O.E., Apartado Postal 51 y 216, Puebla 72000, Puebla, México
5 Australia Telescope National Facility, CSIRO, PO Box 76, Epping NSW 2121, Australia
6 Astrophysics and Supercomputing, Swinburne University of Technology, P.O. Box 218 Hawthorn, Victoria 3122, Australia
7 Astronomical Institute, Utrecht University, 3508 TA, Utrecht, The Netherlands

Received 15 January 1999 / Accepted 8 February 1999

Abstract

We report on the first firm detection of pulsed [FORMULA]-ray emission from PSR B1509-58 in the 0.75-30 MeV energy range in CGRO COMPTEL data collected over more than 6 years. The modulation significance in the 0.75-30 MeV pulse-phase distribution is [FORMULA] and the lightcurve is similar to the lightcurves found earlier between 0.7 and 700 keV: a single broad asymmetric pulse reaching its maximum 0.38 [FORMULA] 0.03 in phase after the radio peak, compared to the offset of 0.30 found in the CGRO BATSE soft gamma-ray data, and 0.27 [FORMULA] 0.01 for RXTE (2-16 keV), compatible with ASCA (0.7-2.2 keV).

Analysis in narrower energy windows shows that the single broad pulse is significantly detected up to [FORMULA] MeV. Above 10 MeV we do detect marginally significant ([FORMULA]) modulation with an indication for the broad pulse. However, imaging analysis shows the presence of a strong 5.6[FORMULA] source at the position of the pulsar. To investigate this further, we have also analysed contemporaneous CGRO EGRET data ([FORMULA] MeV) collected over a nearly 4 year period. In the 30-100 MeV energy window, adjacent to the COMPTEL 10-30 MeV range, a [FORMULA] source can be attributed to PSR B1509-58. Timing analysis in this energy window yields an insignificant signal of [FORMULA], but with a shape somewhat similar to that of the COMPTEL 10-30 MeV lightcurve. Combining the two pulse-phase distributions results in a suggestive double-peaked pulsed signal above the background level estimated in the spatial analyses, with one broad peak near phase 0.38 (aligned with the pulse observed at lower energies) and a second narrower peak near phase 0.85, which is absent for energies below 10 MeV. The modulation significance is, however, only [FORMULA] and needs confirmation.

Spectral analysis based on the excess counts in the broad pulse of the lightcurve shows that extrapolation of the OSSE power-law spectral fit with index -1.68 describes our data well up to 10 MeV. Above 10 MeV the spectrum breaks abruptly. The precise location of the break/bend between 10 and 30 MeV depends on the interpretation of the structure in the lightcurve measured by COMPTEL and EGRET above 10 MeV.

Such a break in the spectrum of PSR B1509-58 has recently been interpreted in the framework of polar cap models for the explanation of gamma-ray pulsars, as a signature of the exotic photon splitting process in the strong magnetic field of PSR B1509-58. For that interpretation our new spectrum constrains the co-latitude to [FORMULA], close to the "classical" radius of the polar cap. In the case of an outer-gap scenario, our spectrum requires a dominant synchrotron component.

Key words: gamma rays: observations – stars: pulsars: individual: PSR B1509-58

Send offprint requests to: (L.M.Kuiper@sron.nl)

© European Southern Observatory (ESO) 1999

Online publication: November 2, 1999

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