The Crab pulsar in the 0.75-30 MeV range as seen by CGRO COMPTEL

A coherent high-energy picture from soft X-rays up to high-energy γ-rays

¹ SRON -National Institute for Space Research, Sorbonnelaan 2, 3584 CA Utrecht, The Netherlands
² Istituto di Fisica Cosmica ed Applicazioni all'Informatica CNR, Via U. La Malfa 153, 90146 Palermo, Italy
³ Max-Planck-Institut für Extraterrestrische Physik, 85741 Garching, Germany
⁴ Astrophysics Division, European Space Research and Technology Centre, 2200 AG, Noordwijk, The Netherlands
⁵ Space Science Centre, University of New Hampshire, Durham, NH 03824, USA

Corresponding author: L. Kuiper, L.M.Kuiper@sron.nl

Received: 15 November 2000
Accepted: 4 September 2001

Abstract

We present the time-averaged characteristics of the Crab pulsar in the 0.75-30 MeV energy window using data from the imaging Compton Telescope COMPTEL aboard the Compton Gamma-Ray Observatory (CGRO) collected over its 9 year mission. Exploiting the exceptionally long COMPTEL exposure on the Crab allowed us to derive significantly improved COMPTEL spectra for the Crab nebula and pulsar emissions, and for the first time to accurately determine at low-energy γ-rays the pulse profile as a function of energy. These timing data, showing the well-known main pulse and second pulse at a phase separation of ~ with strong bridge emission, are studied together with data obtained at soft/hard X-ray energies from the ROSAT HRI, BeppoSAX LECS, MECS and PDS, at soft γ-rays from CGRO BATSE and at high-energy γ-rays from CGRO EGRET in order to obtain a coherent high-energy picture of the Crab pulsar from 0.1 keV up to 10 GeV. The morphology of the pulse profile of the Crab pulsar is continuously changing as a function of energy: the intensities of both the second pulse and the bridge emission increase relative to that of the first pulse for increasing energies up to ~ MeV. Over the COMPTEL energy range above 1 MeV an abrupt morphology change happens: the first pulse becomes again dominant over the second pulse and the bridge emission loses significance such that the pulse profile above 30 MeV is similar to the one observed at optical wavelengths. A pulse-phase-resolved spectral analysis performed in 7 narrow phase slices consistently applied over the 0.1 keV-10 GeV energy interval shows that the pulsed emission can empirically be described with 3 distinct spectral components: i) a power-law emission component (1 keV-5 GeV; photon index ), present in the phase intervals of the two pulses; ii) a curved spectral component required to describe soft ( keV) excess emission present in the same pulse-phase intervals; iii) a broad curved spectral component reflecting the bridge emission from 0.1 keV to ~ MeV. This broad spectral component extends in phase over the full pulse profile in an approximately triangular shape, peaking under the second pulse. Recent model calculations for a three-dimensional pulsar magnetosphere with outer magnetospheric gap acceleration by Cheng et al. (2000) appear at present most successful in explaining the above complex high-energy characteristics of the Crab pulsar.