  We report the discovery of a medium-strength (~0.5kG) magnetic field on the
young, massive star tauSco (B0.2V), which becomes the third-hottest magnetic
star known. Circularly polarized Zeeman signatures are clearly detected in
observations collected mostly with the ESPaDOnS spectropolarimeter, recently
installed on the 3.6-m Canada-France-Hawaii Telescope; temporal variability is
also clearly established in the polarimetry, and can be unambiguously
attributed to rotational modulation with a period close to 41d. Archival UV
spectra confirm that this modulation repeats over timescales of decades.
  By reconstructing the large-scale structure of its magnetic topology, we find
that the magnetic structure is unusually complex for a hot star. The surface
topology is dominated by a potential field, although a moderate toroidal
component is probably present. We fail to detect intrinsic temporal variability
of the magnetic structure over the 1.5-yr period of our spectropolarimetric
observations (in agreement with the stable temporal variations of the UV
spectra), and infer that any differential surface rotation must be very small.
  The topology of the extended magnetic field that we derive from the
photospheric magnetic maps is also more complex than a global dipole, and
features in particular a significantly warped torus of closed magnetic loops
encircling the star (tilted at about 90deg to the rotation axis), with
additional, smaller, networks of closed field lines. This topology appears to
be consistent with the exceptional Xray properties of tauSco and also provides
a natural explanation of the variability observed in wind-formed UV lines. We
conclude that its magnetic field is most probably a fossil remnant from the
star-formation stage.
