The non-thermal, time-variable radio emission from Cyg OB2 # 5: A wind-collision region

Gisela N. Ortiz-Leon{1},
Laurent Loinard{1}, Luis F. Rodriguez{1},
Amy J. Mioduszewski{2} and Sergio A. Dzib{1}

{1}{Centro de Radiostronomia y Astrofisica, Universidad Nacional Autonoma de Mexico,
Morelia 58089, Mexico}
{2}{National Radio Astronomy Observatory, 1003 Lopezville Road, Socorro, NM 87801,

The radio emission from the well-studied massive stellar system Cyg OB2 #5 is known to
fluctuate with a period of 6.7
years between a low-flux state when the emission is entirely of free-free origin, and a
high-flux state when an additional non-thermal component (of hitherto unknown nature)
appears. In this paper, we demonstrate that the radio flux of that non-thermal component
is steady on timescales of hours, and that its morphology is arc-like.
This shows that the non-thermal emission
results from the collision between the strong wind driven by the known contact binary in the
system, and that of an unseen companion on a somewhat eccentric orbit with a 6.7-yr
period and a 5 to 10 mas semi-major axis. Together with the previously reported wind-collision
region located about 0.8" to the north-east of the contact binary, Cyg OB2 #5 appears
to be the only multiple system
known so far to harbor two radio-imaged wind-collision regions.

Reference: To appear in ApJ
Status: Manuscript has been accepted