The 2.35 year itch of Cyg OB2 #9 - I. Optical and X-ray monitoring

TL;DR

This study combines optical and X-ray monitoring to refine the orbit and confirm wind-wind collision in the massive binary Cyg OB2 #9, revealing unique adiabatic collision behavior at periastron.

Contribution

It provides the first detailed optical and X-ray analysis of Cyg OB2 #9, confirming wind-wind collision and refining its orbital parameters with new observational data.

Findings

Cyg OB2 #9 is an O+O binary with a long period and high eccentricity.

X-ray luminosity varies inversely with distance, indicating adiabatic wind collision.

Wind-wind collision remains adiabatic even at periastron, a first for such systems.

Abstract

Context: Nonthermal radio emission in massive stars is expected to arise in wind-wind collisions occurring inside a binary system. One such case, the O-type star Cyg OB2 #9, was proven to be a binary only four years ago, but the orbital parameters remained uncertain. The periastron passage of 2011 was the first one to be observable under good conditions since the discovery of binarity. Aims: In this context, we have organized a large monitoring campaign to refine the orbital solution and to study the wind-wind collision. Methods: This paper presents the analysis of optical spectroscopic data, as well as of a dedicated X-ray monitoring performed with Swift and XMM. Results: In light of our refined orbital solution, Cyg OB2 #9 appears as a massive O+O binary with a long period and high eccentricity; its components (O5-5.5I for the primary and O3-4III for the secondary) have similar masses and similar luminosities. The new data also provide the first evidence that a wind-wind collision is present in the system. In the optical domain, the broad Ha line varies, displaying enhanced absorption and emission components at periastron. X-ray observations yield the unambiguous signature of an adiabatic collision because, as the stars approach periastron, the X-ray luminosity closely follows the 1/D variation expected in that case. The X-ray spectrum appears, however, slightly softer at periastron, which is probably related to winds colliding at slightly lower speeds at that time. Conclusions: It is the first time that such a variation has been detected in O+O systems, and the first case where the wind-wind collision is found to remain adiabatic even at periastron passage.