Context : Westerlund 2 is a young , massive , obscured stellar cluster of our Galaxy .
It harbors the most massive star with well determined parameters , WR20a ( 82+83M _ { \odot } ) , a dozen very early O-type stars ( O3–7 ) , and a wealth of PMS stars .
Although of importance , this cluster is still not well known .

Aims : The high-energy properties of this cluster , especially those of its early-type stars are examined in details .
The variability of the X-ray sources is investigated .

Methods : A monitoring of the field was performed using three Chandra observations .
This dataset probes daily as well as monthly to yearly timescales and provides the deepest X-ray view of the cluster to date .

Results : The two Wolf-Rayet stars WR20a ( WN6ha+WN6ha ) and WR20b ( WN6ha ) were analyzed in detail .
They are both very luminous and display very hard spectra , but WR20b does not seem to vary .
On the contrary , WR20a , a known eclipsing , colliding-wind binary , brightens in the X-ray domain during the eclipses , i.e .
when the collision is seen face-on .
This can be explained by the properties of the wind-wind collision zone , whose high density leads to a large absorbing column ( 2 \times 10 ^ { 24 } cm ^ { -2 } ) . All twelve O-type stars previously classified spectroscopically , two eclipsing binaries previously identified and nine newly identified O-type star candidates are detected in the high energy domain ; ten of them could be analyzed spectroscopically .
Four are overluminous , but the others present typical L _ { X } / L _ { BOL } ratios , suggesting that several O-type objects are actually binaries .
Variability at the \sim 2 \sigma level was detected for a majority of the sources , of unknown origin for the putatively single objects . Faint , soft , diffuse emission pervades the entire field-of-view but no clear structure can be identified , even at the position of a blister proposed to be at the origin of the TeV source HESS J1023 - 575 . Finally , the X-ray properties of PMS objects were also investigated , in particular the brightest flaring ones .
They provided an additional argument in favor of a large distance ( \sim 8kpc ) for the cluster .

Conclusions :