On the magnetic field and the TeV emitter location in Cygnus X-1 and LS 5039
Abstract
Cygnus X-1 and LS 5039 are two X-ray binaries detected at TeV energies. Both sources are compact systems, contain jet-like (radio) structures, and harbor very luminous O stars. The two systems show TeV signal around the superior conjunction of the compact object, when the largest gamma-ray opacities are expected. We investigate the implications of the TeV detections of Cygnus X-1 and LS 5039 around the superior conjunction, since this can give information on the system magnetic field and the location of the TeV emitter. Using the very high energy spectra and fluxes observed around superior conjunction in Cygnus X-1 and LS 5039, we compute the absorbed luminosity due to pair creation in the stellar photon field for different emitter positions with respect to the star and the observer line of sight. The role of the magnetic field and electromagnetic cascading are discussed. For the case of inefficient electromagnetic cascading, the expected secondary synchrotron fluxes are compared with the observed ones at X-ray energies. We find that, in Cygnus X-1 and LS 5039, either the magnetic field in the star surroundings is much smaller than the one expected for O stars, or the TeV emitter is located at a distance >10^12 cm from the compact object. Our results strongly suggest that the TeV emitters in Cygnus X-1 and LS 5039 are located in the borders of the binary system and well above the orbital plane. This would disfavor those models for which the emitter is well inside the system, like the innermost-jet region (Cygnus X-1 and LS 5039; microquasar scenario), or the region between the pulsar and the primary star (LS 5039; standard pulsar scenario).Keywords
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