Very high energy gamma-rays from Centaurus X-3: indications and implications

Abstract

We present the results of a detailed timing analysis of observations of Cen X-3 taken by the University of Durham Mark 6 imaging atmospheric Cherenkov telescope in 1997-1999. The presence of a TeV gamma -ray signal at the overall >= 4.5sigma significance level in the ``fully cut'' image selected data, as reported earlier, is confirmed. A search for possible modulations of gamma -rays with the pulsar spin period P0~ 4.8 s was performed by the step-by-step application of image parameter cuts of gradually increasing hardness. The data of each of 23 days of observations have not revealed any statistically significant Rayleigh power peak, except for 1 day when a peak with a chance probability p=6.8x 10-7 was found in ``soft-cut'' data sets. This modulation, if real, is blue shifted by 6.6 ms (>103 km s-1) from the nominal second harmonic of the X-ray pulsar. Taking the large number of frequency trials into account, the estimated final probability of finding such a peak by chance still remains < 10-2. Bayesian statistical analysis also indicates the presence of such modulations. We show that the behaviour of the Rayleigh peak disappearing in the fully cut data set is actually quite consistent with the hypothesis of a gamma -ray origin of that peak. No modulation of the VHE gamma -ray signal with the pulsar orbital phase is found. In the second part of the paper we consider different theoretical models that could self-consistently explain the existing data from Cen X-3 in high-energy (HE, E>= 100 MeV) and very high energy (VHE, E>= 100 GeV) gamma -rays. We propose on the basis of the energetics required that all reasonable options for the gamma -ray production in Cen X-3 must be connected to jets emerging from the inner accretion disc around the neutron star. One of the principal options is a large-scale source, with R_s ~ 1013 - 1014 cm; this assumes effective acceleration of electrons up to ~ 10 TeV by shocks produced by interaction of these jets with the dense atmosphere of the binary. It is shown that such a quasi-stationary model could explain the bulk of the gamma -radiation features observed except for the gamma -ray modulations with the pulsar spin. These modulations, if genuine, would require an alternative source with R_s << 1011 cm. We consider two principal models, hadronic and leptonic, for the formation of such a compact source in the jet. Both models predict that the episodes of pulsed gamma -ray emission may be rather rare, with a typical duration not exceeding a few hours, and that generally the frequency of pulsations should be significantly shifted from the nominal frequency of the X-ray pulsar. The opportunities to distinguish between different models by means of future gamma -ray observations of this X-ray binary are also discussed.