Hard X-ray timing observation of the Crab Pulsar and Cygnus X-1

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Abstract

Results of a timing analysis of the Crab Pulsar and Cygnus X-1 are presented. These two strong Galactic sources were observed on 1985 August 5 with the POKER experiment in the energy range 15-130 keV. The spectral behavior, showing a canonical spectrum for the Crab and a low emission state for Cyg X-1, has already been reported in Ubertini et al. (1991a). The phase-averaged Crab pulsar emission is well represented by a power law of photon index GAMMA = 2.06 and a 30 keV intensity of 1.04 x 10-3 ph sq cm/s/keV. The pulsar light curve shows a wide first peak (2.4 ms FWHM) if compared with similar high statistics observations in the same energy range. For Cyg X-1 we find no indication of flaring activity nor any other strong time variability in our observation. Nevertheless, the 15-100 keV data are inconsistent with steady state emission at a statistical confidence level greater than 99%. Our data also confirm the presence of the recently reported quasi-periodic oscillations (QPO), showing different frequencies at different times and centered in our observation at 0.013 Hz and 0.040 Hz and modulating a few percent of the total flux. An upper limit of 24% on chaotic variability at 0.24-14 Hz is also set.

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