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

COMPTEL detection of pulsed -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 <sup>5

1</sup> 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 -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 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 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 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 MeV. Above 10 MeV we do detect marginally significant () modulation with an indication for the broad pulse. However, imaging analysis shows the presence of a strong 5.6 source at the position of the pulsar. To investigate this further, we have also analysed contemporaneous CGRO EGRET data ( MeV) collected over a nearly 4 year period. In the 30-100 MeV energy window, adjacent to the COMPTEL 10-30 MeV range, a source can be attributed to PSR B1509-58. Timing analysis in this energy window yields an insignificant signal of , 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 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 , 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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