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Astron. Astrophys. 362, 646-654 (2000)

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1. Introduction

Massive binary systems are often considered as sites of high energy processes in which [FORMULA]-ray production is expected. [FORMULA]-rays in these systems may be produced in interactions of relativistic particles injected by a neutron star or a black hole (e.g. Bignami et al. 1977, Kirk et al. 1999), a shock wave created in collision of the pulsar and stellar winds (e.g. Harding & Gaisser 1990) or two stellar winds (e.g. Eichler & Usov 1993). Observations of these systems in GeV and TeV energies suggest that in fact this may be the case. For example in recent years the Compton GRO detectors have reported point sources with flat spectra coincident with some massive binaries e.g. Cyg X-3, LSI 303o+61, or Cen X-3. Some positive detections of massive binaries by Cherenkov telescopes have been also claimed, although they were frequently accompanied by many negative reports (see for a review Weekes 1992, Moskalenko 1995). In the recent review on TeV observations only Cen X-3 has been mentioned as possible massive binary active at these energies (Weekes 1999).

These theoretical predictions and positive observations have stimulated analysis of propagation of VHE [FORMULA]-rays in the anisotropic radiation fields of accretion disks around compact objects in massive binaries (Carraminana 1992, Bednarek 1993), and in the radiation fields of massive stars (e.g. Protheroe & Stanev 1987, Moskalenko et al. 1993, Moskalenko & Karakua 1994). Recently we have performed Monte Carlo simulations of cascades initiated by monoenergetic [FORMULA]-ray beams injected by a discrete source (probably a compact object) in the radiation field of a massive companion. Two massive binaries, Cyg X-3 (Lamb et al. 1977, Merck et al. 1995) and LSI 303o+61 (van Dijk et al. 1996, Hermsen et al. 1977, Thompson et al. 1995), have been discussed in the context of this problem (Bednarek 1997).

Another massive X-ray binary, Cen X-3, containing a neutron star with a 4.8 s period in a 2.09 day orbit around an O-type supergiant, has been detected above 100 MeV by the EGRET detector on the Compton Observatory (Vestrand et al. 1997). There is evidence that the [FORMULA]-ray emission at these energies is in the form of outbursts and is modulated with a 4.8 s period of the pulsar. Based on the observations in late 80's, two Cherenkov groups (Raubenheimer et al. 1989, Brazier et al. 1990, North et al. 1990) reported detection of positive signal at TeV energies from Cen X-3 at an orbital phase of [FORMULA], which is modulated with a period of the pulsar. This emission has been localized by North et al. (1991) and Raubenheimer & Smit (1997) to a relatively small region between the pulsar orbit and the surface of a massive companion which may be the accretion wake or the limb of the star. More recently the Durham group has detected a persistent flux of [FORMULA]-rays above 400 GeV on a lower level than previous reports (Chadwick et al. 1998, 1999a). No evidence of correlation with the pulsar or orbital periods has been found and no evidence of correlation with the X-ray flux has been detected (Chadwick et al. 1999a,b).

The purpose of this paper is to compute the [FORMULA]-ray light curves and [FORMULA]-ray spectra escaping from the Cen X-3 system, assuming different geometries and energy distributions for primary electrons or photons injected into the radiation field of the massive star by a compact object. The geometry of the considered picture is schematically shown in Fig. 1. The [FORMULA]-ray spectrum escaping from the system is a consequence of anisotropic cascades which initiate primary electrons or [FORMULA]-ray photons in the thermal radiation of the massive star in Cen X-3. The results of computations are discussed in the context of observations of Cen X-3 in GeV and TeV energy ranges.

[FIGURE] Fig. 1. Schematic picture of the cascade initiated by a primary gamma-ray ([FORMULA]) in the radiation field of a massive star. The gamma-ray, injected by the compact object or produced by primary electron, creates an [FORMULA] pair in the interaction with a soft star photon [FORMULA]. The secondary [FORMULA] pairs create secondary gamma-rays by scattering soft star photons (one of them is marked by [FORMULA]). The next generation of gamma-rays may create further [FORMULA] pairs; escape from the system (e.g. photon marked by [FORMULA]); or collide with the star surface.

We assumed the following parameters for the massive binary Cen X-3: the radius of the O star [FORMULA] cm, its surface temperature [FORMULA] K (Krzeminski 1974), the binary star separation [FORMULA] cm, the inclination angle [FORMULA] and a circular orbit since the known eccentricity of the binary is [FORMULA] (Fabbiano & Schreier 1977).

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© European Southern Observatory (ESO) 2000

Online publication: October 24, 2000