We report the first results of an XMM -Newton monitoring campaign of the NGC 6231 open cluster in the Sco OB 1 association .
This first paper focuses on the massive colliding wind binary HD 152248 , which is the brightest X-ray source of the cluster .
The campaign , with a total duration of 180 ksec , was split into six separate observations , following the orbital motion of HD 152248 .
The X-ray flux from this system presents a clear , asymmetric modulation with the phase and ranges from 0.73 to 1.18 10 ^ { -12 } erg s ^ { -1 } cm ^ { -2 } in the [ 0.5-10.0 keV ] energy band .
The maximum of the emission is reached slightly after apastron .
The EPIC spectra are quite soft and peak around 0.8-0.9 keV .
We characterize their shape using several combinations of mekal models and power-law spectra and we detect significant spectral variability in the [ 0.5-2.5 keV ] energy band .

We also perform 2-D hydrodynamical simulations using different sets of parameters that closely reproduce the physical and orbital configuration of the HD 152248 system at the time of the six XMM -Newton pointings .
This allows a direct confrontation of the model predictions with the constraints deduced from the X-ray observations of the system .
We show that the observed variation of the flux can be explained by a variation of the X-ray emission from the colliding wind zone , diluted by the softer X-ray contribution of the two O-type stars of the system .
Our simulations also reveal that the interaction region of HD 152248 should be highly unstable , giving rise to shells of dense gas that are separated by low density regions .

Finally , we perform a search for short-term variability in the light curves of the system and we show that trends are present within several of the 30 ksec exposures of our campaign .
Further , most of these trends are in good agreement with the orbital motion and provide a direct constraint on the first order derivative of the flux .
In the same context , we also search for long-range correlations in the X-ray data of the system , but we only marginally detect them in the high energy tail of the signal .