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Chang, Z.; Zhang, S.; Ji, L.; Chen, Y. P.; Kretschmar, P.; Kuulkers, E.; Collmar, W.; Liu, C. Z.
Monthly notices of the Royal Astronomical Society, 11/2016, Volume: 463, Issue: 1Journal Article
LS 5039 is so far the best-studied γ-ray binary system at multiwavelength energies. A time-resolved study of its spectral energy distribution (SED) shows that above 1 keV its power output is changing along its binary orbit as well as being a function of energy. To disentangle the energy dependence of the power output as a function of orbital phase, we investigated in detail the orbital light curves as derived with different telescopes at different energy bands. We analysed the data from all existing International Gamma-Ray Astrophysics Laboratory (INTEGRAL)/INTEGRAL on-board Imager/INTEGRAL Soft Gamma-Ray Imager observations of the source and generated the most up-to-date orbital light curves at hard X-ray energies. In the γ-ray band, we carried out orbital phase-resolved analysis of Fermi-Large Area Telescope (LAT) data between 30 MeV and 10 GeV in five different energy bands. We found that, at ≲100 MeV and ≳1 TeV the peak of the γ-ray emission is near orbital phase 0.7, while between ∼100 MeV and ∼1 GeV it moves close to orbital phase 1.0 in an orbital anticlockwise manner. This result suggests that the transition region in the SED at soft γ-rays (below a hundred MeV) is related to the orbital phase interval of 0.5–1.0 but not to the one of 0.0–0.5, when the compact object is ‘behind’ its companion. Another interesting result is that between 3 and 20 GeV no orbital modulation is found, although Fermi-LAT significantly (∼18σ) detects LS 5039. This is consistent with the fact that at these energies, the contributions to the overall emission from the inferior conjunction phase region (INFC, orbital phase 0.45–0.9) and from the superior conjunction phase region (orbital phase 0.9–0.45) are equal in strength. At TeV energies the power output is again dominant in the INFC region and the flux peak occurs at phase ∼0.7.
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