OSSE OBSERVATIONS OF THE CRAB PULSAR

We present results of the Compton Gamma-Ray Observatory OSSE observati ons of the Crab pulsar, made during MJD 48373 - 48406 (1991 April 27-1 991 May 30) and MJD 48798 - 48804 (1992 June 25-1992 July 1). Pulsar l ight curves and spectra over the approximately 0.05 to 10 MeV range ar e presented. The arrival time of the gamma-ray peak and the radio peak agree to within 30 mus, which is better than the approximately 300 mu s accuracy of the measurements. The overall pulse phase averaged spect rum in the 0.1-10 MeV range is well-fit by a power law of the form 0.0 5 x (E/0.13 MeV)-(1.99+/-0.03) photons cm-2 s-1. The outer-gap model ( with gap parameter equal to 0.46) provided to us by Ho agrees with the data to better than 20%. The spectra of the bridge and second peak ar e slightly harder than the first peak as measured by the hardness rati o (approximately 110-220 keV)/(approximately 50-105 keV): P1 = 0.54 +/ - 0.01, P2 = 0.63 +/- 0.01, bridge = 0.68 +/- 0.03. The phase of the t wo peaks in the light curve is constant over the 50-550 keV range to w ithin the accuracy of the measurements (better than 0.02 in phase). No evidence was found for variability of the light curve on timescales f rom 2 minutes (less than a factor of 1.8) to 1 year (less than a facto r of -1.06), where these are 3 sigma upper limits. However, when we ex amine the historical database, we find, in agreement with Nolan et al. (1993), that there is evidence for a 13 year variation in the ratio o f the intensity of peak 2 to peak 1. We show that if this is interpret ed as being due to precession (which changes the relative view of the intrinsic gamma-ray pulse as seen on earth), the variation is consiste nt with models of neutron star structure. The optical data may be in c onflict with the interpretation however. We found no statistically sig nificant lines in the 50-550 keV range in the spectrum. The average 3 sigma upper limits in 10(-3) photons cm-2 s-1 for lines at 0.073, 0.07 8, 0.4, 0.44, 0.511, and 0.545 MeV are 0.3, 0.5, 0.6, 0.5, 0.5, and 0. 1. These limits are based on observations that spanned approximately 1 week or more. Our results do not corroborate previous detections, mos t of which have been at the approximately 3 sigma level. We cannot exc lude, however, the possibility of transient features with our current analysis.