Brick Walls for Nonstationary Black Holes

The validity condition of the brick-wall model is analysed in nonstationary space-time. It is shown that the model holds only in thin regions, only by using tortoise coordinates, near the event horizon of a slowly varying (quasi-stationary) black hole. The reason for the use of tortoise coordinates is that the stationary state solutions of quantum field equations in other coordinates do not exist for any region in nonstationary space-time. Meanwhile, the quantum corrections to the entropy of the Vaidya-Bonner black hole due to the spin fields are calculated in terms of the brick-wall model.