  The classical alpha-disc model assumes that the turbulent stress scales
linearly with -- and responds instantaneously to -- the pressure. It is likely,
however, that the stress possesses a non-negligible relaxation time and will
lag behind the pressure on some timescale. To measure the size of this lag we
carry out unstratified 3D magnetohydrodynamic shearing box simulations with
zero-net-magnetic-flux using the finite-volume code PLUTO. We impose thermal
oscillations of varying periods via a cooling term, which in turn drives
oscillations in the turbulent stress. Our simulations reveal that the stress
oscillations lag behind the pressure by $\sim 5$ orbits in cases where the
oscillation period is several tens of orbits or more. We discuss the
implication of our results for thermal and viscous overstability in discs
around compact objects.
