Entropy of a Vaidya black hole - art. no. 104001

In the case of the brick wall model for a static black hole, it is shown th at the Bekenstein-Hawking entropy with the geometric character comes from t he contributions of fields in the vicinity of the horizon. According to thi s idea, the entropy of a Vaidya black hole as the simplest nonstatic case i s calculated by using the assumption of local equilibrium near the horizon. The difference from the static hole is that the result proportional to the area of the horizon relies on a time-dependent cutoff. We analyze the noti on of the local equilibrium in black hole thermodynamics and find that it i s maintained for the holes with a mass far greater than the Planck scale.