  Context : The peculiar hot star Theta Car in the open cluster IC2602 is a
blue straggler as well as a single-line binary of short period (2.2d). Aims :
Its high-energy properties are not well known, though X-rays can provide useful
constraints on the energetic processes at work in binaries as well as in
peculiar, single objects. Methods : We present the analysis of a 50ks exposure
taken with the XMM-Newton observatory. It provides medium as well as
high-resolution spectroscopy. Results : Our high-resolution spectroscopy
analysis reveals a very soft spectrum with multiple temperature components
(1--6MK) and an X-ray flux slightly below the `canonical' value
(log[L_X(0.1-10.)/L_{BOL}] ~ -7). The X-ray lines appear surprisingly narrow
and unshifted, reminiscent of those of beta Cru and tau Sco. Their relative
intensities confirm the anomalous abundances detected in the optical domain (C
strongly depleted, N strongly enriched, O slightly depleted). In addition, the
X-ray data favor a slight depletion in neon and iron, but they are less
conclusive for the magnesium abundance (solar-like?). While no significant
changes occur during the XMM-Newton observation, variability in the X-ray
domain is detected on the long-term range. The formation radius of the X-ray
emission is loosely constrained to <5 R_sol, which allows for a range of models
(wind-shock, corona, magnetic confinement,...) though not all of them can be
reconciled with the softness of the spectrum and the narrowness of the lines.
