  We study the behavior of non-equilibrium spin density and spin-orbit torque
in a topological insulator - antiferromagnet heterostructure. Unlike
ferromagnetic heterostructures where Dirac cone is gapped due to time-reversal
symmetry breaking, here the Dirac cone is preserved. We demonstrate the
existence of a staggered spin density corresponding to a damping like torque,
which is quite robust against the scalar impurity, when the transport energy is
in the topological insulator surface energy regime. We show the contribution to
the non-equilibrium spin density due to both surface and bulk topological
insulator bands. Finally, we show that the torques in topological
insulator-antiferromagnet heterostructure exhibit an angular dependence that is
consistent with the standard spin-orbit torque obtained in Rashba system with
some additional nonlinear effects arising from the interfacial coupling.
