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

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1. Introduction

The accreting high-mass X-ray binary (HMXRB) Cen X-3 is an accurately measured system containing a 4.8 s pulsar in a 2.1 d orbit around an O-type supergiant. It has been extensively sudied since its discovery and is one of the best characterised HMXRBs. The orbit has been well enough determined to lead to mass estimates of both the neutron star and the companion (Hutchings et al. 1979) and its distance is well known (Krzeminsky 1974; Hutchings et al. 1979). Observations of a cyclotron line have enabled the surface magnetic field to be determined (Santangelo et al. 1998; Burderi et al. 2000).

The temporal behaviour of Cen X-3 is complex. The 4.8 s pulsar and 2.1 d orbital periods are reasonably well understood. Over the 20 years of X-ray observations, the spin period shows a secular spin up trend on which fluctuations with time scales of a few years and shorter are superimposed (Tsunemi et al. 1996). The orbital period shows a long-term decay thought to be due to tidal dissipation (Nagase et al. 1992). The object has long-term variations in X-ray intensity, exhibiting X-ray high and low states on timescales of [FORMULA] d with no evidence for long term periodicity (Priedhorsky & Terrell 1983). Recently evidence has been found for kHz fluctuations in the X-ray intensity which have been interpreted as evidence for photon bubble oscillations (Jernigan et al. 2000).

Very high energy (VHE) gamma rays from Cen X-3 were first reported by the Durham group using earlier non-imaging telescopes (Carramiñana et al. 1989; Brazier et al. 1990) and by the Potchefstroom group (North et al. 1990; North et al. 1991). These early observations, which were considerably less sensitive than observations using imaging telescopes, showed evidence for sporadic outbursts of emission pulsed at the pulsar period in the [FORMULA] TeV band. These observations hinted at emission at a preferred orbital phase but were not sensitive to weak unpulsed emission. There was some evidence that the emission was not from the site of the neutron star (Bowden et al. 1993). In a recent paper, Raubenheimer & Smit (1997) have reviewed the archival evidence and have concluded that there is evidence for VHE gamma ray emission from a site in the accretion disk trailing the neutron star by [FORMULA].

Observations with the Mark 6 imaging telescope have shown evidence for a weak unpulsed signal from Cen X-3 at energies [FORMULA] GeV during observations in 1997 March and June (Chadwick et al. 1998). No episodes of pulsed emission were seen in these VHE signals.

EGRET observations of Cen X-3 have revealed one episode of sporadic emission in the 30 MeV - 10 GeV region (Vestrand et al. 1997). This emission observed during 1994 October was consistent with the gamma rays being pulsed at the contemporaneous pulsar period as deduced from BATSE X-ray measurements. No emission was detected during any other viewing period when EGRET observed Cen X-3.

We report the results of analysis of VHE data taken during 1998 March and April and 1999 February. We present the results of a search for a possible correlation between [FORMULA] GeV gamma rays recorded by the University of Durham Mark 6 telescope and X-ray emission according to measurements made with the RXTE /ASM and CGRO /BATSE experiments. We also present the results of searches for modulation of the emission at both the orbital and spin periods.

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© European Southern Observatory (ESO) 2000

Online publication: December 15, 2000
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