  We study the time evolution of the local magnetization in the critical Ising
chain in a transverse field after a sudden change of the parameters at a
defect. The relaxation of the defect magnetization is algebraic and the
corresponding exponent, which is a continuous function of the defect
parameters, is calculated exactly. In finite chains the relaxation is
oscillating in time and its form is conjectured on the basis of precise
numerical calculations.
