Gamma-Ray Spectra & Variability of the Crab Nebula Emission Observed by BATSE

Abstract

We report ~ 600 days of BATSE earth-occultation observations of the total gamma-ray (30 keV to 1.7 MeV) emission from the Crab nebula, between 1991 May 24 (TJD 8400) and 1994 October 2 (TJD 9627). Lightcurves from 35-100, 100-200, 200-300, 300-400, 400-700, and 700-1000 keV, show that positive fluxes were detected by BATSE in each of these six energy bands at significances of approximately 31, 20, 9.2, 4.5, 2.6, and 1.3 sigma respectively per day. We also observed significant flux and spectral variations in the 35-300 keV energy region, with time scales of days to weeks. The spectra below 300 keV, averaged over typical CGRO viewing periods of 6-13 days, can be well described by a broken power law with average indices of ~ 2.1 and ~ 2.4 varying around a spectral break at ~ 100 keV. Above 300 keV, the long-term averaged spectra, averaged over three 400 d periods (TJD 8400-8800, 8800-9200, and 9200-9628, respectively) are well represented by the same power law with index of ~ 2.34 up to ~ 670 keV, plus a hard spectral component extending from ~ 670 keV to ~ 1.7 MeV, with a spectral index of ~ 1.75. The latter component could be related to a complex structure observed by COMPTEL in the 0.7-3 MeV range. Above 3 MeV, the extrapolation of the power-law continuum determined by the low-energy BATSE spectrum is consistent with fluxes measured by COMPTEL in the 3-25 MeV range, and by EGRET from 30-50 MeV. We interpret these results as synchrotron emission produced by the interaction of particles ejected from the pulsar with the field in different dynamical regions of the nebula system, as observed recently by HST, XMM-Newton, and Chandra.Comment: To be published in the November 20, 2003, Vol 598 issue of the Astrophysical Journa