Modeling TeV [gamma]-rays from LS 5039: an active OB star at the extreme

• Published in: Proceedings of the International Astronomical Union 2010-07, Vol.6 (S272), p.587
• Main Authors: Owocki, Stan P, Okazaki, Atsuo T, Romero, Gustavo
• Format: Article
• Language: English
• Subjects: Accretion disks; Astrophysics; Double stars; Gamma ray astronomy; Mathematical models
• ISSN: 1743-9213; 1743-9221
• DOI: 10.1017/S1743921311011471

Abstract Perhaps the most extreme examples of "Active OB stars" are the subset of high-mass X-ray binaries - consisting of an OB star plus compact companion - that have recently been observed by Fermi and ground-based Cerenkov telescopes like HESS to be sources of very high energy (VHE; up to 30 TeV!) γ-rays. This paper focuses on the prominent γ-ray source, LS5039, which consists of a massive O6.5V star in a 3.9-day-period, mildly elliptical (e [approximate] 0.24) orbit with its companion, assumed here to be a black-hole or unmagnetized neutron star. Using 3-D SPH simulations of the Bondi-Hoyle accretion of the O-star wind onto the companion, we find that the orbital phase variation of the accretion follows very closely the simple Bondi-Hoyle-Lyttleton (BHL) rate for the local radius and wind speed. Moreover, a simple model, wherein intrinsic emission of γ-rays is assumed to track this accretion rate, reproduces quite well Fermi observations of the phase variation of γ-rays in the energy range 0.1-10 GeV. However for the VHE (0.1-30 TeV) radiation observed by the HESS Cerenkov telescope, it is important to account also for photon-photon interactions between the γ-rays and the stellar optical/UV radiation, which effectively attenuates much of the strong emission near periastron. When this is included, we find that this simple BHL accretion model also quite thus making it a strong alternative to the pulsar-wind-shock models commonly invoked to explain such VHE γ-ray emission in massive-star binaries. [PUBLICATION ABSTRACT]