Phase-resolved hard X-ray emission of the high-mass binary LS 5039: a spectral hardening above 50 keV detected with INTEGRAL

Aims: LS 5039 is an enigmatic high-mass gamma-ray binary which hosts a powerful O6.5V companion, but the nature of the compact object is still to be established using multi-wavelength observations.
Methods: We analyzed phase-resolved multi-instrument spectra of nonthermal emission from LS 5039 in order to produce reliable spectral models, which can be further employed to select between various scenarios and theoretical models of the binary.
Results: The combined phase-resolved hard X-ray and MeV-range gamma-ray spectra obtained with XMM-Newton, Suzaku, NuSTAR, INTEGRAL, and COMPTEL indicate a meaningful spectral hardening above 50 keV. The spectral break observed in both major phases of the binary may indicate the presence of an upturn in the spectrum of accelerated leptons which could originate from the interaction of wind from the O6.5V companion star with the relativistic outflow from a yet unidentified compact object.