First orbital solution for the non-thermal emitter Cyg OB2 #9

TL;DR

This paper presents the first orbital solution for the non-thermal radio emitter Cyg OB2 #9, based on spectroscopic and X-ray data, revealing a highly eccentric orbit and variable X-ray emission.

Contribution

It provides the first orbital parameters for Cyg OB2 #9 using spectroscopic monitoring and analyzes X-ray data to understand its binary nature and wind interactions.

Findings

Orbital solution indicates a high eccentricity (0.7-0.75).

Mass ratio of the binary appears close to unity.

X-ray flux decreased by 10% between 2004 and 2007.

Abstract

After the first detection of its binary nature, the spectroscopic monitoring of the non-thermal radio emitter Cyg OB2 #9 (P=2.4yrs) has continued, doubling the number of available spectra of the star. Since the discovery paper of 2008, a second periastron passage has occurred in February 2009. Using a variety of techniques, the radial velocities could be estimated and a first, preliminary orbital solution was derived from the HeI5876 line. The mass ratio appears close to unity and the eccentricity is large, 0.7--0.75. X-ray data from 2004 and 2007 are also analyzed in quest of peculiarities linked to binarity. The observations reveal no large overluminosity nor strong hardness, but it must be noted that the high-energy data were taken after the periastron passage, at a time where colliding wind emission may be low. Some unusual X-ray variability is however detected, with a 10% flux decrease between 2004 and 2007. To clarify their origin and find a more obvious signature of the wind-wind collision, additional data, taken at periastron and close to it, are needed.