  We discuss the results of two XMM-Newton observations of the open cluster
NGC6604 obtained in April and September 2002. We concentrate mainly on the
multiple system HD167971 (O5-8V + O5-8V + (O8I)). The soft part of the EPIC
spectrum of this system is thermal with typical temperatures of about 2 10^6 to
9 10^6 K. The nature (thermal vs non-thermal) of the hard part of the spectrum
is not unambiguously revealed by our data. If the emission is thermal, the high
temperature of the plasma (~ 2.3 10^7 to 4.6 10^7 K) would be typical of what
should be expected from a wind-wind interaction zone within a long period
binary system. This emission could arise from an interaction between the
combined winds of the O5-8V + O5-8V close binary system and that of the more
distant O8I companion. Assuming instead that the hard part of the spectrum is
non-thermal, the photon index would be rather steep (~3). Moreover, a marginal
variability between our two XMM-Newton pointings could be attributed to an
eclipse of the O5-8V + O5-8V system. The overall X-ray luminosity points to a
significant X-ray luminosity excess of about a factor 4 possibly due to
colliding winds. Considering HD167971 along with several recent X-ray and radio
observations, we propose that the simultaneous observation of non-thermal
radiation in the X-ray (below 10.0 keV) and radio domains appears rather
unlikely. Our investigation of our XMM-Newton data of NGC6604 reveals a rather
sparse distribution of X-ray emitters. Including the two bright non-thermal
radio emitters HD168112 and HD167971, we present a list of 31 X-ray sources...
(see paper for full abstract)
