  Surface states of three-dimensional topological insulators exhibit the
phenomenon of spin-momentum locking, whereby the orientation of an electron
spin is determined by its momentum. Probing the spin texture of these states is
of critical importance for the realization of topological insulator devices,
however the main technique available so far is the spin- and angle-resolved
photoemission spectroscopy. Here we reveal a close link between the spin
texture and a new kind of magneto-resistance, which depends on the relative
orientation of the current with respect to the magnetic field as well as the
crystallographic axes, and scales linearly with both the applied electric and
magnetic fields. This bilinear magneto-electric resistance can be used to map
the spin texture of topological surface states by simple transport
measurements. For a prototypical Bi2Se3 single layer, we can map both the
in-plane and the out-of-plane components of the spin texture - the latter
arising from hexagonal warping. Theoretical calculations suggest that the
bilinear magneto-electric resistance originates from the conversion of a
non-equilibrium spin current into a charge current under the application of the
external magnetic field.
