Long-term XMM-Newton investigation of two particle-accelerating colliding-wind binaries in NGC6604: HD168112 and HD167971

The long-term (over more than one decade) X-ray emission from two massive
stellar systems known to be particle accelerators is investigated using
XMM-Newton. Their X-ray properties are interpreted taking into account recent
information about their multiplicity and orbital parameters. The two targets,
HD168112 and HD167971 appear to be overluminous in X-rays, lending additional
support to the idea that a significant contribution of the X-ray emission comes
from colliding-wind regions. The variability of the X-ray flux from HD168112 is
interpreted in terms of varying separation expected to follow the 1/D rule for
adiabatic shocked winds. For HD167971, marginal decrease of the X-ray flux in
September 2002 could tentatively be explained by a partial wind eclipse in the
close pair. No long-term variability could be demonstrated despite the
significant difference of separation between 2002 and 2014. This suggests the
colliding-wind region in the wide orbit does not contribute a lot to the total
X-ray emission, with a main contribution coming from the radiative shocked
winds in the eclipsing pair. The later result provides evidence that shocks in
a colliding-wind region may be efficient particle accelerators even in the
absence of bright X-ray emission, suggesting particle acceleration may operate
in a wide range of conditions. Finally, in hierarchical triple O-type systems,
thermal X-rays do not necessarily constitute an efficient tracer to detect the
wind-wind interaction in the long period orbit.