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

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

In the previous paper (Bednarek 1997), we considered the propagation of monoenergetic -ray beams in the radiation field of a massive star in the case of two massive binaries: Cyg X-3 and LSI 303^o+61. We determined general conditions for which VHE -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 -ray photons as a function of distance from the surface of the massive star, the photon energies, and the angle of injection 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 and at the distance 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 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 , where is the angle intercepted by the massive star. For higher angles , the optical depth is computed up to the moment of photon collision with the stellar surface and therefore it is lower than for .

Fig. 2a and b. The optical depth for -ray photons with energies 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 , for different distances of the photon injection from the massive star equal to radii of the star  a . The dependence of the optical depth on the photon energy, for fixed values of the angle and the distance , is shown in b .

As expected the optical depth for -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 -ray spectra should show a break close to these energies.

© European Southern Observatory (ESO) 2000

Online publication: October 24, 2000
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