Osse observations of the crab pulsar

We present results of the Compton Gamma-Ray Observatory OSSE observations of the Crab pulsar, made during MJD 48373-48406 (1991 April 27-1991 May 30) and MJD 48798-48804 (1992 June 25-1992 July 1). Pulsar light curves and spectra over the ∼0.05 to 10 MeV range are presented. The arrival time of the gamma-ray peak and the radio peak agree to within 30 μs, which is better than the ∼300 μs accuracy of the measurements. The overall pulse phase averaged spectrum in the 0.1-10 MeV range is well-fit by a power law of the form 0.05 × (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 are slightly harder than the first peak as measured by the hardness ratio (∼110-220 keV)/(∼50-105 keV): P1 = 0.54 ± 0.01, P2 = 0.63 ± 0.01, bridge = 0.68 ± 0.03. The phase of the two peaks in the light curve is constant over the 50-550 keV range to within the accuracy of the measurements (better than 0.02 in phase). No evidence was found for variability of the light curve on timescales from 2 minutes (less than a factor of 1.8) to 1 year (less than a factor of 1.06), where these are 3 σ upper limits. However, when we examine the historical database, we find, in agreement with Nolan et al. (1993), that there is evidence for a 13 year variation in the ratio of the intensity of peak 2 to peak 1. We show that if this is interpreted as being due to precession (which changes the relative view of the intrinsic gamma-ray pulse as seen on earth), the variation is consistent with models of neutron star structure. The optical data may be in conflict with the interpretation however. We found no statistically significant lines in the 50-550 keV range in the spectrum. The average 3 σ upper limits in 10^-3 photons cm^-2 s^-1 for lines at 0.073, 0.078, 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, most of which have been at the ∼3 σ level. We cannot exclude, however, the possibility of transient features with our current analysis.