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We discuss the results of two XMM-Newton observations of the open clusterNGC6604 obtained in April and September 2002. We concentrate mainly on themultiple system HD167971 (O5-8V + O5-8V + (O8I)). The soft part of the EPICspectrum of this system is thermal with typical temperatures of about 2 10^6 to9 10^6 K. The nature (thermal vs non-thermal) of the hard part of the spectrumis not unambiguously revealed by our data. If the emission is thermal, the hightemperature of the plasma (~ 2.3 10^7 to 4.6 10^7 K) would be typical of whatshould be expected from a wind-wind interaction zone within a long periodbinary system. This emission could arise from an interaction between thecombined winds of the O5-8V + O5-8V close binary system and that of the moredistant O8I companion. Assuming instead that the hard part of the spectrum isnon-thermal, the photon index would be rather steep (~3). Moreover, a marginalvariability between our two XMM-Newton pointings could be attributed to aneclipse of the O5-8V + O5-8V system. The overall X-ray luminosity points to asignificant X-ray luminosity excess of about a factor 4 possibly due tocolliding winds. Considering HD167971 along with several recent X-ray and radioobservations, we propose that the simultaneous observation of non-thermalradiation in the X-ray (below 10.0 keV) and radio domains appears ratherunlikely. Our investigation of our XMM-Newton data of NGC6604 reveals a rathersparse distribution of X-ray emitters. Including the two bright non-thermalradio emitters HD168112 and HD167971, we present a list of 31 X-ray sources...(see paper for full abstract)Comment: 19 pages, 10 figures (8 postscript and 2 gif), accepted for  publication by A&

An XMM-Newton observation of the multiple system HD 167971 (O5-8V + O5-8V + (O8I)) and the young open cluster NGC 6604