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Astron. Astrophys. 364, 165-169 (2000)

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5. Modulation

We have searched for modulation of the gamma ray signal at the orbital period of the binary system. The orbital phase of each of our observations has been calculated using the ephemeris of Kelley et al. 1983. The results are shown in Fig. 3. We conclude that there is no significant modulation of the VHE gamma ray emission at the orbital period.

[FIGURE] Fig. 3. The measured VHE gamma ray rate during each observation of Cen X-3 plotted as a function of orbital phase.

We have also looked for modulation in the VHE data at the pulsar period. The total dataset has been subjected to a Rayleigh test for periodicity at a small range of periods around the pulsar period found in the contemporary BATSE data. Phase coherence between observations was not assumed. No significant periodicity was detected, leading to a 3[FORMULA] upper limit of [FORMULA] to the flux pulsed at the pulsar period for an energy threshold of 400 GeV.

We have also looked for modulation at the pulsar period in each individual night's observations by searching for periodicity at the contemporary BATSE pulsar period, and so looking for VHE emission originating from the same site as the X-rays. No such emission was found.

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© European Southern Observatory (ESO) 2000

Online publication: December 15, 2000