  Our dedicated XMM-Newton monitoring, as well as archival Chandra and Swift
datasets, were used to examine the behaviour of the WN5h+O3V binary WR21a at
high energies. For most of the orbit, the X-ray emission exhibits few
variations. However, an increase in strength of the emission is seen before
periastron, following a 1/D relative trend, where D is the separation between
both components. This increase is rapidly followed by a decline due to strong
absorption as the Wolf-Rayet (WR) comes in front. The fitted local absorption
value appears to be coherent with a mass-loss rate of about 1x10^{-5} M_sol/yr
for the WR component. However, absorption is not the only parameter affecting
the X-ray emission at periastron as even the hard X-ray emission decreases,
suggesting a possible collapse of the colliding wind region near to or onto the
photosphere of the companion just before or at periastron. An eclipse may
appear as another potential scenario, but it would be in apparent contradiction
with several lines of evidence, notably the width of the dip in the X-ray light
curve and the absence of variations in the UV light curve. Afterwards, the
emission slowly recovers, with a strong hysteresis effect. The observed
behaviour is compatible with predictions from general wind-wind collision
models although the absorption increase is too shallow.
