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

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2. Propagation of VHE [FORMULA]-rays in the radiation field of a massive star

In the previous paper (Bednarek 1997), we considered the propagation of monoenergetic [FORMULA]-ray beams in the radiation field of a massive star in the case of two massive binaries: Cyg X-3 and LSI 303o+61. We determined general conditions for which VHE [FORMULA]-rays may initiate inverse Compton cascades (ICS) (see Sect. 2 in Bednarek 1997). The massive star in Cen X-3 system has also a high enough luminosity that VHE photons injected inside a few stellar radii should initiate a cascade. This expectation has been checked by computing the optical depth for [FORMULA]-ray photons as a function of distance from the surface of the massive star, the photon energies, and the angle of injection [FORMULA] measured with respect to the direction determined by the place of photon injection and the center of the massive star. The optical depth has been computed as shown in Appendix A of Bednarek (1997). We have found that for the parameters of the massive star in the Cen X-3 system, the optical depth for photons injected at angles [FORMULA] and at the distance [FORMULA] can be greater than one. If photons are injected closer to the massive star, then the optical depth can be greater than one for all injection angles (see curve for [FORMULA] in Fig 2a). The optical depth reaches maximum (characteristic cusps visible in Fig. 2a), for photons propagating in directions tangent to the massive star surface, i.e. at an angle [FORMULA], where [FORMULA] is the angle intercepted by the massive star. For higher angles [FORMULA], the optical depth is computed up to the moment of photon collision with the stellar surface and therefore it is lower than for [FORMULA].

[FIGURE] Fig. 2a and b. The optical depth for [FORMULA]-ray photons with energies [FORMULA] MeV in the soft radiation field of the massive star in Cen X-3 system is shown as a function of the angle of injection of photons [FORMULA], for different distances of the photon injection from the massive star equal to [FORMULA] radii of the star [FORMULA] a . The dependence of the optical depth on the photon energy, for fixed values of the angle [FORMULA] and the distance [FORMULA], is shown in b .

As expected the optical depth for [FORMULA]-ray photon reaches maximum at photon energies determined by characteristic energies of soft star photons (see Fig. 2b). Photons with energies below a few tens of GeV escape without significant absorption. Therefore it is expected that cascade [FORMULA]-ray spectra should show a break close to these energies.

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© European Southern Observatory (ESO) 2000

Online publication: October 24, 2000