The magnetic field and the location of the TeV emitter in Cygnus X-1 and LS 5039

TL;DR

This study investigates the location and magnetic field conditions of TeV gamma-ray emitters in Cygnus X-1 and LS 5039, suggesting they are positioned at the system's borders rather than inside the binary system.

Contribution

It provides new constraints on the emitter location and magnetic field strength in these systems based on TeV observations and electromagnetic cascade modeling.

Findings

TeV emission likely originates outside the binary orbit

Magnetic field in the emitter vicinity is weaker than expected for O stars

Emitters are located above the orbital plane, at system borders

Abstract

Cygnus X-1 and LS 5039 are two X-ray binaries observed at TeV energies. Both sources are compact systems, contain jet-like (radio) structures, and harbor very luminous O stars. A TeV signal has been found around the superior conjunction of the compact object in both objects, when the highest gamma-ray opacities are expected. We investigate the implications of finding TeV emission from 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 the superior conjunction in Cygnus X-1 and LS 5039, we compute the absorbed luminosity that is caused by 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 at the borders of the binary system and well above the orbital plane. This would not agree with 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).