Can the gamma-ray source 3EG J2033+4118 be produced by the stellar system Cygnus OB2 No. 5?

Abstract

Abstract. We discuss the possibility that the stellar system Cyg OB2 No. 5 can be a gamma-ray source in the light of recent EGRET and radio data. This system is formed by an O7 Ia + Ofpe/WN9 contact binary. A third star, probably a B0 V star also associated with the system, is located at ∼1700 AU from the primary. We estimate the expected gamma-ray luminosity from the colliding winds region, the terminal shock of the wind, and the unstable zone at the base of the wind, and conclude that, under very reasonable assumptions, Cyg OB2 No. 5 can generate about a half of the gamma-ray flux detected from the positionally coincident source 3EG J2033+4118. We suggest, then, that other O stars belonging to the association, also placed within the 95% probability EGRET location contour, could contribute to the observed gamma-ray flux.

On the Multiplicity of the O-Star Cyg OB2 #8a and its Contribution to the γ-ray Source 3EG J2033+4118

“…A surface magnetic field of ∼ 100 G would give a L IC a factor 10 4 lower. For comparison, Benaglia et al (2001) derive a value of about 8 × 10 34 erg s −1 for Cyg OB2 #5. We also estimated the maximum energy that can be reached by the relativistic electrons under these conditions, using the relation given by Benaglia & Romero (2003).…”

Massive binaries and colliding winds

“…Romero et al (1999) found a significant correlation (at the 4 σ level) of unidentified EGRET sources with OB associations. Their study further provides evidence that several individual massive binaries, including WR 140 and the multiple system V 729 Cyg, are probable counterparts of γ-ray sources (see also Chen et al 1996;Benaglia et al 2001). …”

“…Because of the strong UV radiation field of luminous early-type stars, relativistic electrons are expected to dissipate most of their energy through the inverse-Compton scattering mechanism (Mücke & Pohl 2001) and the expected luminosities are of the order of a few times 10 34 erg s −1 (Benaglia et al 2001). …”

Gamma rays from star-forming regions

“…The clear detection of non-thermal radio emission from the colliding wind region in some systems like WR140, WR146, WR 147, and Cyg OB2 #5 indicate that electrons are been accelerated up to relativistic energies in these binaries. These electrons are located relatively close to the secondary star, so they should have strong inverse Compton losses which might result in a detectable gamma-ray signal [32,33]. Protons can be in principle also accelerate in the colliding wind shock and they can interact either with the wind material [34] or they can diffuse to interact with nearby molecular clouds [35].…”